Throughout this article, Latin indices (typically a, b, c, n) take values 1, 2, ..., 8 for the eight gluon color charges, while Greek indices (typically α, β, μ, ν) take values 0 for timelike components and 1, 2, 3 for spacelike components of four-vectors and four-dimensional spacetime tensors. In all equations, the summation convention is used on all color and tensor indices, unless the text explicitly states that there is no sum to be taken (e.g. “no sum”).
Definition
Below the definitions (and most of the notation) follow K. Yagi, T. Hatsuda, Y. Miake and Greiner, Schäfer.
are its four (coordinate-system dependent) components, that in a fixed gauge are 3×3 traceless Hermitian matrix-valued functions, while are 32 real-valued functions, the four components for each of the eight four-vector fields.
Different authors choose different signs.
Expanding the commutator gives;
Substituting and using the commutation relation for the Gell-Mann matrices (with a relabeling of indices), in which f abc are the structure constants of SU(3), each of the gluon field strength components can be expressed as a linear combination of the Gell-Mann matrices as follows:
so that:
where again a, b, c = 1, 2, ..., 8 are color indices. As with the gluon field, in a specific coordinate system and fixed gauge Gαβ are 3×3 traceless Hermitian matrix-valued functions, while Gaαβ are real-valued functions, the components of eight four-dimensional second order tensor fields.
Differential forms
The gluon color field can be described using the language of differential forms, specifically as an adjoint bundle-valued curvature 2-form (note that fibers of the adjoint bundle are the su(3) Lie algebra);
where is the gluon field, a vector potential 1-form corresponding to G and ∧ is the (antisymmetric) wedge product of this algebra, producing the structure constants f abc. The Cartan-derivative
of the field form (i.e. essentially the divergence of the field) would
be zero in the absence of the "gluon terms", i.e. those which represent the non-abelian character of the SU(3).
A more mathematically formal derivation of these same ideas (but a slightly altered setting) can be found in the article on metric connections.
The key difference between quantum electrodynamics and quantum
chromodynamics is that the gluon field strength has extra terms which
lead to self-interactions between the gluons and asymptotic freedom. This is a complication of the strong force making it inherently non-linear, contrary to the linear theory of the electromagnetic force. QCD is a non-abelian gauge theory. The word non-abelian in group-theoretical language means that the group operation is not commutative, making the corresponding Lie algebra non-trivial.
where "tr" denotes trace of the 3×3 matrix GαβGαβ, and γμ are the 4×4gamma matrices. In the fermionic term , both color and spinor indices are suppressed. With indices explicit, where are color indices and are Dirac spinor indices.
In contrast to QED, the gluon field strength tensor is not gauge
invariant by itself. Only the product of two contracted over all indices
is gauge invariant.
Equations of motion
Treated as a classical field theory, the equations of motion for the quark fields are:
which is like the Dirac equation, and the equations of motion for the gluon (gauge) fields are:
which are similar to the Maxwell equations (when written in tensor notation). More specifically, these are the Yang–Mills equations for quark and gluon fields. The color charge four-current is the source of the gluon field strength tensor, analogous to the electromagnetic four-current as the source of the electromagnetic tensor. It is given by
which is a conserved current since color charge is conserved. In other words, the color four-current must satisfy the continuity equation:
The global language system is the "ingenious pattern of connections between language groups". Dutch sociologistAbram de Swaan developed this theory in 2001 in his book Words of the World: The Global Language System
and according to him, "the multilingual connections between language
groups do not occur haphazardly, but, on the contrary, they constitute a
surprisingly strong and efficient network that ties together – directly
or indirectly – the six billion inhabitants of the earth." The global language system draws upon the world system theory to account for the relationships between the world's languages and divides them into a hierarchy consisting of four levels, namely the peripheral, central, supercentral and hypercentral languages.
Theory
Background
According
to de Swaan, the global language system has been constantly evolving
since the time period of the early 'military-agrarian' regimes.
Under these regimes, the rulers imposed their own language and so the
first 'central' languages emerged, linking the peripheral languages of
the agrarian communities via bilingual speakers to the language of the
conquerors. Then was the formation of empires, which resulted in the
next stage of integration of the world language system.
Firstly, Latin emerged from Rome. Under the rule of the Roman Empire,
which ruled an extensive group of states, the usage of Latin stretched
along the Mediterranean coast, the southern half of Europe, and more
sparsely to the North and then into the Germanic and Celtic lands. Thus,
Latin evolved to become a central language in Europe from 27 BC to 476
AD.
Secondly, there was the widespread usage of the pre-classical
version of Han Chinese in contemporary China due to the unification of
China in 221 BC by Qin Shi Huang.
Thirdly, Sanskrit started to become widely spoken in South Asia from the widespread teaching of Hinduism and Buddhism in South Asian countries.
Fourthly, the expansion of the Arabic empire also led to the
increased usage of Arabic as a language in the Afro-Eurasian land mass.
Military conquests of preceding centuries generally determine the distribution of languages today.
Supercentral languages spread by land and sea. Land-bound languages spread via marching empires: German, Russian, Arabic, Hindi, Chinese and Japanese. Languages like Bengali, Tamil, Italian and Turkish
too are less considered as land-bound languages. However, when the
conquerors were defeated and were forced to move out of the territory,
the spread of the languages receded. As a result, some of these
languages are currently barely supercentral languages and are instead
confined to their remaining state territories, as is evident from
German, Russian and Japanese.
On the other hand, sea-bound languages spread by conquests overseas: English, French, Portuguese, Spanish.
Consequently, these languages became widespread in areas settled by
European colonisers and relegated the indigenous people and their
languages to peripheral positions.
Besides, the world-systems theory
also allowed the global language system to expand further. It focuses
on the existence of the core, semi-peripheral and peripheral nations.
The core countries
are the most economically powerful and the wealthiest countries.
Besides, they also have a strong governmental system in the country,
which oversees the bureaucracies in the governmental departments. There
is also the prevalent existence of the bourgeois,
and core nations have significant influence over the non-core, smaller
nations. Historically, the core countries were found in northwestern
Europe and include countries such as England, France and the
Netherlands. They were the dominant countries that had colonized many
other nations from the early 15th century to the early 19th century.
Then is the existence of the periphery countries,
the countries with the slowest economic growth. They also have
relatively weak governments and a poor social structure and often depend
on primary industries as the main source of economic activity for the
country.
The extracting and exporting of raw materials from the peripheral
nations to core nations is the activity bringing about the most
economic benefits to the country. Much of the population that is poor
and uneducated, and the countries are also extensively influenced by
core nations and the multinational corporations found there.
Historically, peripheral nations were found outside Europe, the
continent of colonial masters. Many countries in Latin America were peripheral nations during the period of colonization, and today peripheral countries are in sub-Saharan Africa.
Lastly, the presence of the semiperiphery countries,
those in between the core and the periphery. They tend to be those
which started out as peripheral nations and are currently moving towards
industrialization and the development of more diversified labour
markets and economies. They can as well come about from declining core
countries. They are not dominant players in the international trade
market. As compared to the peripheral nations, semi-peripheries are not
as susceptible to manipulation by the core countries. However, most of
these nations have economic or political relations with the core.
Semi-peripheries also tend to exert influence and control over
peripheries and can serve to be a buffer between the core and peripheral
nations and ease political tensions. Historically, Spain and Portugal
were semi-peripheral nations after they fell from their dominant core
positions. As they still maintained a certain level of influence and
dominance in Latin America over their colonies, they could still
maintain their semi-peripheral position.
According to Immanuel Wallerstein,
one of the most well-known theorists who developed the world-systems
approach, a core nation is dominant over the non-core nations from its
economic and trade dominance. The abundance of cheap and unskilled
labour in the peripheral nations makes many large multinational corporations
(MNCs), from core countries, often outsource their production to the
peripheral countries to cut costs, by employing cheap labour. Hence, the
languages from the core countries could penetrate into the peripheries
from the setting up of the foreign MNCs in the peripheries. A
significant percentage of the population living in the core countries
had also migrated to the core countries in search of jobs with higher
wages.
The gradual expansion of the population of migrants makes the
language used in their home countries be brought into the core
countries, thus allowing for further integration and expansion of the
world language system. The semi-peripheries also maintain economic and
financial trade with the peripheries and core countries. That allows for
the penetration of languages used in the semi-peripheries into the core
and peripheral nations, with the flow of migrants moving out of the
semi-peripheral nations to the core and periphery for trade purposes.
Thus, the global language system examines rivalries and
accommodations using a global perspective and establishes that the
linguistic dimension of the world system goes hand in hand with the
political, economic, cultural and ecological aspects. Specifically, the
present global constellation of languages is the product of prior
conquest and domination and of ongoing relations of power and exchange.[1]
Q-value
is the communicative value of a language i, its potential to connect a speaker with other speakers of a constellation or subconstellation, "S". It is defined as follows:
The prevalence of language i, means the number of competent speakers in i, , divided by all the speakers, of constellation S. Centrality, is the number of multilingual speakers who speak language i divided by all the multilingual speakers in constellation S, .
Thus, the Q-value or communication value is the product of the prevalence and the centrality of language i in constellation S.
Consequently, a peripheral language has a low Q-value and the
Q-values increase along the sociology classification of languages, with
the Q-value of the hypercentral language being the highest.
De Swaan has been calculating the Q-values of the official European Union (EU) languages since 1957 to explain the acquisition of languages by EU citizens in different phases.
In 1970, when there were only four language constellations,
Q-value decreased in the order of French, German, Italian, Dutch. In
1975, the European Commission
enlarged to include Britain, Denmark and Ireland. English had the
highest Q-value followed by French and German.
In the following years, the European Commission grew, with the addition
of countries like Austria, Finland and Sweden. Q-value of English still
remained the highest, but French and German swapped places.
In EU23, which refers to the 23 official languages spoken in the European Union, the Q-values for English, German and French were 0.194, 0.045 and 0.036 respectively.
Theoretical framework
De Swaan likens the global language system to contemporary political macrosociology
and states that language constellations are a social phenomenon, which
can be understood by using social science theories. In his theory, de
Swaan uses the Political Sociology of Language and Political Economy of Language to explain the rivalry and accommodation between language groups.
Political sociology
This
theoretical perspective centres on the interconnections among the
state, nation and citizenship. Accordingly, bilingual elite groups try
to take control of the opportunities for mediation between the
monolingual group and the state. Subsequently, they use the official
language to dominate the sectors of government and administration and
the higher levels of employment. It assumes that both the established
and outsider groups are able to communicate in a shared vernacular, but
the latter groups lack the literacy skills that could allow them to
learn the written form of the central or supercentral language, which
would, in turn allow, them to move up the social ladder.
Political economy
This
perspective centres on the inclinations that people have towards
learning one language over the other. The presumption is that if given a
chance, people will learn the language that gives them more
communication advantage. In other words, a higher Q-Value.
Certain languages such as English or Chinese have high Q-values since
they are spoken in many countries across the globe and would thus be
more economically useful than to less spoken languages, such as Romanian
or Hungarian.
From an economic perspective, languages are ‘hypercollective’ goods since they exhibit properties of collective goods
and produce external network effects. Thus, the more speakers a
language has, the higher its communication value for each speaker. The
hypercollective nature and Q-Value
of languages thus help to explain the dilemma that a speaker of a
peripheral language faces when deciding whether to learn the central or
hypercentral language. The hypercollective nature and Q-value also help
to explain the accelerating spread and abandonment of various languages.
In that sense, when people feel that a language is gaining new
speakers, they would assign a greater Q-value to this language and
abandon their own native language
in place of a more central language. The hypercollective nature and
Q-value also explain, in an economic sense, the ethnic and cultural
movements for language conservation.
Specifically, a minimal Q-value of a language is guaranteed when
there is a critical mass of speakers committed to protecting it, thus
preventing the language from being forsaken.
Characteristics
The
global language system theorises that language groups are engaged in
unequal competition on different levels globally. Using the notions of a
periphery, semi-periphery and a core, which are concepts of the world system theory,
de Swaan relates them to the four levels present in the hierarchy of
the global language system: peripheral, central, supercentral and
hypercentral.
De Swaan also argues that the greater the range of potential uses
and users of a language, the higher the tendency of an individual to
move up the hierarchy in the global language system and learn a more
"central" language. Thus, de Swaan views the learning of second languages
as proceeding up rather than down the hierarchy, in the sense that they
learn a language that is on the next level up. For instance, speakers
of Catalan, a peripheral language, have to learn Spanish, a central language to function in their own society, Spain. Meanwhile, speakers of Persian, a central language, have to learn Arabic, a supercentral language, to function in their region. On the other hand, speakers of a supercentral language have to learn the hypercentral language to function globally, as is evident from the huge number of non-native English speakers.
According to de Swaan, languages exist in "constellations" and
the global language system comprises a sociological classification of languages
based on their social role for their speakers. The world's languages
and multilinguals are connected in a strongly ordered, hierarchical
pattern. There are thousands of peripheral or minority languages in the
world, each of which are connected to one of a hundred central
languages. The connections and patterns between each language is what
makes up the global language system. The four levels of language are the
peripheral, central, supercentral and hypercentral languages.
Peripheral languages
At the lowest level, peripheral languages, or minority languages,
form the majority of languages spoken in the world; 98% of the world's
languages are peripheral languages and spoken by less than 10% of the
world’s population. Unlike central languages, these are "languages of
conversation and narration rather than reading and writing, of memory
and remembrance rather than record".
They are used by native speakers within a particular area and are in
danger of becoming extinct with increasing globalisation, which sees
more and more speakers of peripheral languages acquiring more central
languages in order to communicate with others.
Central languages
The
next level constitutes about 100 central languages, spoken by 95% of
the world's population and generally used in education, media and
administration. Typically, they are the 'national' and official languages
of the ruling state. These are the languages of record, and much of
what has been said and written in those languages is saved in newspaper
reports, minutes and proceedings, stored in archives, included in
history books, collections of the 'classics', of folk talks and folk
ways, increasingly recorded on electronic media and thus conserved for
posterity.
Many speakers of central languages are multilingual
because they are either native speakers of a peripheral language and
have acquired the central language, or they are native speakers of the
central language and have learned a supercentral language.
These languages often have colonial traces and "were once imposed
by a colonial power and after independence continued to be used in
politics, administration, law, big business, technology and higher
education".
Hypercentral languages
At the highest level is the language that connects speakers of the supercentral languages. Today, English
is the only example of a hypercentral language as the standard for
science, literature, business, and law, as well as being the most widely
spoken second language.
Applications
Pyramid of languages of the world
According to David Graddol (1997), in his book titled The Future of English, the languages of the world comprise a "hierarchical pyramid", as follows:
National languages: around 80 languages serving over 180 nation states (e.g. Nepali).
Official languages within nation states (and other "safe" languages): around 600 languages worldwide (e.g. Marathi).
Local vernacular languages: the remainder of the world's 6,000+ languages.
Translation systems
The global language system is also seen in the international translation process as explained by Johan Heilbron, a historical sociologist:
"translations and the manifold activities these imply are embedded in
and dependent on a world system of translation, including both the
source and the target cultures".
The hierarchical relationship between global languages is
reflected in the global system for translations. The more "central" a
language, the greater is its capability to function as a bridge or
vehicular language to facilitate communication between peripheral and
semi-central languages.
Heilbron's version of the global system of language in translations has four levels:
Level 1: Hypercentral position —
English currently holds the largest market share of the global market
for translations; 55–60% of all book translations are from English. It
strongly dominates the hierarchical nature of book translation system.
Level 2: Central position —
German and French each hold 10% of the global translation market.
Level 3: Semi-central position —
There are 7 or 8 languages "neither very central on a global level nor very peripheral", each making up 1 to 3% of the world market (like Spanish, Italian and Russian).
Level 4: Peripheral position —
Languages from which "less than 1% of the book translations worldwide
are made", including Chinese, Hindi, Japanese, Malay, Swahili, Turkish
and Arabic. Despite having large populations of speakers, "their role in
the translation economy is peripheral as compared to more central
languages".
Acceptance
According to the Google Scholar website, de Swaan's book, Words of the world: The global language system, has been cited by 2990 other papers, as of 25 August 2021.
However, there have also been several concerns regarding the global language system:
Importance of Q-value
Van Parijs (2004)
claimed that 'frequency' or likelihood of contact is adequate as an
indicator of language learning and language spread. However, de Swaan
(2007) argued that it alone is not sufficient. Rather, the Q-value,
which comprises both frequency (better known as prevalence) and
'centrality', helps to explain the spread of (super)central languages,
especially former colonial languages in newly independent countries
where in which only the elite minority spoke the language initially.
Frequency alone would not be able to explain the spread of such
languages, but Q-value, which includes centrality, would be able to.
In another paper, Cook and Li (2009)
examined the ways to categorise language users into various groups.
They suggested two theories: one by Siegel (2006) who used
'sociolinguistic settings', which is based on the notion of dominant
language, and another one by de Swaan (2001) that used the concept of
hierarchy in the global language system. According to them, de Swaan's
hierarchy is more appropriate, as it does not imply dominance in power
terms. Rather, de Swaan's applies the concepts of geography and function
to group languages and hence language users according to the global
language system. De Swaan (2001) views the acquisition of second languages (L2) as typically going up the hierarchy.
However, Cook and Li argues that this analysis is not adequate in
accounting for the many groups of L2 users to whom the two areas of
territory and function hardly apply. The two areas of territory and
function can be associated respectively with the prevalence and
centrality of the Q-value.
This group of L2 users typically does not acquire an L2 going up the
hierarchy, such as users in an intercultural marriage or users who come
from a particular cultural or ethnic group and wish to learn its
language for identity purposes. Thus, Cook and Li argue that de Swaan's
theory, though highly relevant, still has its drawbacks in that the
concept behind Q-value is insufficient in accounting for some L2 users.
Choice of supercentral languages
There
is disagreement as to which languages should be considered more
central. The theory states that a language is central if it connects
speakers of "a series of central languages". Robert Phillipson questioned why Japanese is included as one of the supercentral languages but Bengali, which has more speakers, is not on the list.
Inadequate evidence for a system
Michael
Morris argued that while it is clear that there is language hierarchy
from the "ongoing interstate competition and power politics", there is
little evidence provided that shows that the "global language
interaction is so intense and systematic that it constitutes a global
language system, and that the entire system is held together by one global language,
English". He claimed that de Swaan's case studies demonstrated that
hierarchy in different regions of the world but did not show the
existence of a system within a region or across regions. The global
language system is supposed to be part of the international system but
is "notoriously vague and lacking in operational importance" and
therefore cannot be shown to exist. However, Morris believes that this
lack of evidence could be from the lack of global language data and not
negligence on de Swaan's part. Morris also believes that any theory on a
global system, if later proved, would be much more complex than what is
proposed by de Swaan. Questions on how the hypercentral language English holds together the system must also be answered by such a global language system.
Theory built on inadequate foundations
Robert
Phillipson states that the theory is based on selective theoretical
foundations. He claimed that there is a lack of consideration about the
effects of globalization,
which is especially important when the theory is about a global system:
"De Swaan nods occasionally in the direction of linguistic and cultural
capital, but does not link this to class or linguistically defined
social stratification (linguicism) or linguistic inequality" and that "key concepts in the sociology of language, language maintenance and shift, and language spread are scarcely mentioned".
On the other hand, de Swaan's work in the field of
sociolinguistics has been noted by other scholars to be focused on
"issues of economic and political sociology" and "politic and economic patterns", which may explain why he makes only "cautious references to socio-linguistic parameters".
Ribbon diagram of G-actin. ADP bound to actin's active site (multi color sticks near center of figure) as well as a complexed calcium dication (green sphere) are highlighted.
An actin protein is the monomericsubunit of two types of filaments in cells: microfilaments, one of the three major components of the cytoskeleton, and thin filaments, part of the contractile apparatus in muscle cells. It can be present as either a free monomer called G-actin (globular) or as part of a linear polymer microfilament called F-actin (filamentous), both of which are essential for such important cellular functions as the mobility and contraction of cells during cell division.
Actin participates in many important cellular processes, including muscle contraction, cell motility, cell division and cytokinesis, vesicle and organelle movement, cell signaling, and the establishment and maintenance of cell junctions and cell shape. Many of these processes are mediated by extensive and intimate interactions of actin with cellular membranes. In vertebrates, three main groups of actin isoforms, alpha, beta, and gamma
have been identified. The alpha actins, found in muscle tissues, are a
major constituent of the contractile apparatus. The beta and gamma
actins coexist in most cell types as components of the cytoskeleton, and as mediators of internal cell motility.
It is believed that the diverse range of structures formed by actin
enabling it to fulfill such a large range of functions is regulated
through the binding of tropomyosin along the filaments.
A cell's ability to dynamically form microfilaments provides the
scaffolding that allows it to rapidly remodel itself in response to its
environment or to the organism's internal signals, for example, to increase cell membrane absorption or increase cell adhesion in order to form cell tissue. Other enzymes or organelles such as cilia can be anchored to this scaffolding in order to control the deformation of the external cell membrane, which allows endocytosis and cytokinesis. It can also produce movement either by itself or with the help of molecular motors. Actin therefore contributes to processes such as the intracellular transport of vesicles and organelles as well as muscular contraction and cellular migration. It therefore plays an important role in embryogenesis, the healing of wounds, and the invasivity of cancer cells. The evolutionary origin of actin can be traced to prokaryotic cells, which have equivalent proteins.
Actin homologs from prokaryotes and archaea polymerize into different
helical or linear filaments consisting of one or multiple strands.
However the in-strand contacts and nucleotide binding sites are
preserved in prokaryotes and in archaea. Lastly, actin plays an important role in the control of gene expression.
A large number of illnesses and diseases are caused by mutations in alleles of the genes that regulate the production of actin or of its associated proteins. The production of actin is also key to the process of infection by some pathogenicmicroorganisms. Mutations in the different genes that regulate actin production in humans can cause muscular diseases, variations in the size and function of the heart as well as deafness. The make-up of the cytoskeleton is also related to the pathogenicity of intracellular bacteria and viruses, particularly in the processes related to evading the actions of the immune system.
Function
Actin's primary role in the cell is to form linear polymers called microfilaments that serve various functions in the cell's structure, trafficking networks, migration, and replication.
The multifaceted role of actin relies on a few of the microfilaments'
properties: First, the formation of actin filaments is reversible, and
their function often involves undergoing rapid polymerization and
depolymerization. Second, microfilaments are polarized – i.e. the two
ends of a filament are distinct from one another. Third, actin filaments
can bind to many other proteins, which together help modify and
organize microfilaments for their diverse functions.
In most cells actin filaments form larger-scale networks which are essential for many key functions:
Actin networks give mechanical support to cells and provide trafficking routes through the cytoplasm to aid signal transduction.
Rapid assembly and disassembly of actin network enables cells to migrate (Cell migration).
Actin is extremely abundant in most cells, comprising 1–5% of the total protein mass of most cells, and 10% of muscle cells.
The actin protein is found in both the cytoplasm and the cell nucleus. Its location is regulated by cell membrane signal transduction pathways that integrate the stimuli that a cell receives stimulating the restructuring of the actin networks in response.
Cytoskeleton
There are a number of different types of actin with slightly
different structures and functions. α-actin is found exclusively in muscle fibres,
while β- and γ-actin are found in other cells. As the latter types have
a high turnover rate the majority of them are found outside permanent
structures. Microfilaments found in cells other than muscle cells are
present in three forms:
Microfilament networks - Animal cells commonly have a cell cortex under the cell membrane that contains a large number of microfilaments, which precludes the presence of organelles. This network is connected with numerous receptors that relay signals to the outside of a cell.
Periodic actin rings - A periodic structure constructed of evenly spaced actin rings is found in axons. In this structure, the actin rings, together with spectrin tetramers that link the neighboring actin rings, form a cohesive cytoskeleton that supports the axon membrane. The structure periodicity may also regulate the sodium ion channels in axons.
Yeasts
Actin's cytoskeleton is key to the processes of endocytosis, cytokinesis, determination of cell polarity and morphogenesis in yeasts.
In addition to relying on actin, these processes involve 20 or 30
associated proteins, which all have a high degree of evolutionary
conservation, along with many signalling molecules. Together these
elements allow a spatially and temporally modulated assembly that
defines a cell's response to both internal and external stimuli.
Yeasts contain three main elements that are associated with
actin: patches, cables, and rings. Despite not being present for long,
these structures are subject to a dynamic equilibrium due to continual
polymerization and depolymerization. They possess a number of accessory
proteins including ADF/cofilin, which has a molecular weight of 16kDa
and is coded for by a single gene, called COF1; Aip1, a cofilin cofactor that promotes the disassembly of microfilaments; Srv2/CAP, a process regulator related to adenylate cyclase
proteins; a profilin with a molecular weight of approximately 14 kDa
that is related/associated with actin monomers; and twinfilin, a 40 kDa
protein involved in the organization of patches.
Plants
Plant genome studies have revealed the existence of protein isovariants within the actin family of genes. Within Arabidopsis thaliana, a model organism,
there are ten types of actin, six profilins, and dozens of myosins.
This diversity is explained by the evolutionary necessity of possessing
variants that slightly differ in their temporal and spatial expression. The majority of these proteins were jointly expressed in the tissue analysed. Actin networks are distributed throughout the cytoplasm of cells that have been cultivated in vitro.
There is a concentration of the network around the nucleus that is
connected via spokes to the cellular cortex, this network is highly
dynamic, with a continuous polymerization and depolymerization.
Even though the majority of plant cells have a cell wall
that defines their morphology, their microfilaments can generate
sufficient force to achieve a number of cellular activities, such as the
cytoplasmic currents generated by the microfilaments and myosin. Actin
is also involved in the movement of organelles and in cellular
morphogenesis, which involve cell division as well as the elongation and differentiation of the cell.
The most notable proteins associated with the actin cytoskeleton in plants include: villin, which belongs to the same family as gelsolin/severin and is able to cut microfilaments and bind actin monomers in the presence of calcium cations; fimbrin,
which is able to recognize and unite actin monomers and which is
involved in the formation of networks (by a different regulation process
from that of animals and yeasts); formins, which are able to act as an F-actin polymerization nucleating agent; myosin, a typical molecular motor that is specific to eukaryotes and which in Arabidopsis thaliana is coded for by 17 genes in two distinct classes; CHUP1, which can bind actin and is implicated in the spatial distribution of chloroplasts in the cell; KAM1/MUR3 that define the morphology of the Golgi apparatus as well as the composition of xyloglucans
in the cell wall; NtWLIM1, which facilitates the emergence of actin
cell structures; and ERD10, which is involved in the association of
organelles within membranes and microfilaments and which seems to play a role that is involved in an organism's reaction to stress.
Nuclear actin
Nuclear actin was first noticed and described in 1977 by Clark and Merriam. Authors describe a protein present in the nuclear fraction, obtained from Xenopus laevis
oocytes, which shows the same features as skeletal muscle actin. Since
that time there have been many scientific reports about the structure
and functions of actin in the nucleus (for review see: Hofmann 2009.)
The controlled level of actin in the nucleus, its interaction with
actin-binding proteins (ABP) and the presence of different isoforms
allows actin to play an important role in many important nuclear
processes.
Transport through the nuclear membrane
The
actin sequence does not contain a nuclear localization signal. The
small size of actin (about 43 kDa) allows it to enter the nucleus by
passive diffusion. The import of actin into the nucleus (probably in a complex with cofilin) is facilitated by the import protein importin 9.
Low levels of actin in the nucleus seems to be important, because
actin has two nuclear export signals (NES) in its sequence.
Microinjected actin is quickly removed from the nucleus to the
cytoplasm. Actin is exported at least in two ways, through exportin 1 and exportin 6.
Specific modifications, such as SUMOylation, allows for nuclear actin
retention. A mutation preventing SUMOylation causes rapid export of beta
actin from the nucleus.
Organization
Nuclear actin exists mainly as a monomer, but can also form dynamic oligomers and short polymers. Nuclear actin organization varies in different cell types. For example, in Xenopus
oocytes (with higher nuclear actin level in comparison to somatic
cells) actin forms filaments, which stabilize nucleus architecture.
These filaments can be observed under the microscope thanks to
fluorophore-conjugated phalloidin staining.
In somatic cell nuclei, however, actin filaments cannot be observed using this technique.
The DNase I inhibition assay, the only test which allows the
quantification of the polymerized actin directly in biological samples,
has revealed that endogenous nuclear actin indeed occurs mainly in a
monomeric form.
Precisely controlled level of actin in the cell nucleus, lower
than in the cytoplasm, prevents the formation of filaments. The
polymerization is also reduced by the limited access to actin monomers,
which are bound in complexes with ABPs, mainly cofilin.
Actin isoforms
Different
isoforms of actin are present in the cell nucleus. The level of actin
isoforms may change in response to stimulation of cell growth or arrest
of proliferation and transcriptional activity. Research on nuclear actin is focused on isoform beta.
However the use of antibodies directed against different actin isoforms
allows identifying not only the cytoplasmic beta in the cell nucleus,
but also alpha- and gamma-actin in certain cell types.
The presence of different isoforms of actin may have a significant
effect on its function in nuclear processes, as the level of individual
isoforms can be controlled independently.
Functions
Functions
of actin in the nucleus are associated with its ability to polymerize
and interact with various ABPs and with structural elements of the
nucleus. Nuclear actin is involved in:
Architecture of the nucleus - Interaction of actin with alpha II-spectrin and other proteins are important for maintaining proper shape of the nucleus.
Transcription – Actin is involved in chromatin reorganization, transcription initiation and interaction with the transcription complex. Actin takes part in the regulation of chromatin structure, interacting with RNA polymerase I, II and III. In Pol I transcription, actin and myosin (MYO1C, which binds DNA) act as a molecular motor.
For Pol II transcription, β-actin is needed for the formation of the
preinitiation complex. Pol III contains β-actin as a subunit. Actin can
also be a component of chromatin remodelling complexes as well as
pre-mRNP particles (that is, precursor messenger RNA bundled in proteins), and is involved in nuclear export of RNAs and proteins.
Regulation of gene activity – Actin binds to the regulatory regions of different kinds of genes.
Actin's ability to regulate gene activity is used in the molecular
reprogramming method, which allows differentiated cells return to their
embryonic state.
Translocation of the activated chromosome fragment from under
membrane region to euchromatin where transcription starts. This
movement requires the interaction of actin and myosin.
Integration of different cellular compartments. Actin is a molecule that integrates cytoplasmic and nuclear signal transduction pathways. An example is the activation of transcription in response to serum stimulation of cells in vitro.
Immune response - Nuclear actin polymerizes upon T-cell receptor stimulation and is required for cytokine expression and antibody production in vivo.
Due to its ability to undergo conformational changes and interaction
with many proteins, actin acts as a regulator of formation and activity
of protein complexes such as transcriptional complex.
Cell movement
Actin
is also involved in cell movement. A meshwork of actin filaments marks
the forward edge of a moving cell, and the polymerization of new actin
filaments pushes the cell membrane forward in protrusions called lamellipodia. These membrane protrusions then attach to the substrate, forming structures known as focal adhesions that connect to the actin network. Once attached, the rear of the cell body contracts squeezing its contents forward past the adhesion point.
Once the adhesion point has moved to the rear of the cell, the cell
disassembles it, allowing the rear of the cell to move forward.
Actin/myosin movement
In
addition to the physical force generated by actin polymerization,
microfilaments facilitate the movement of various intracellular
components by serving as the roadway along which a family of motor proteins called myosins travel.
Actin plays a particularly prominent role in muscle cells, which consist largely of repeated bundles of actin and myosin II. Each repeated unit – called a sarcomere
– consists of two sets of oppositely oriented F-actin strands ("thin
filaments"), interlaced with bundles of myosin ("thick filaments"). The
two sets of actin strands are oriented with their (+) ends embedded in
either end of the sarcomere in delimiting structures called Z-disks.
The myosin fibrils are in the middle between the sets of actin
filaments, with strands facing in both directions. When the muscle
contracts, the myosin threads move along the actin filaments towards the
(+) end, pulling the ends of the sarcomere together and shortening it
by around 70% of its length.
In order to move along the actin thread, myosin must hydrolyze ATP;
thus ATP serves as the energy source for muscle contraction.
At times of rest, the proteins tropomyosin and troponin bind to the actin filaments, preventing the attachment of myosin. When an activation signal (i.e. an action potential) arrives at the muscle fiber, it triggers the release of Ca2+ from the sarcoplasmic reticulum
into the cytosol. The resulting spike in cytosolic calcium rapidly
releases tropomyosin and troponin from the actin thread, allowing myosin
to bind, and muscle contracation to begin.
Cell division
In the final stages of cell division, many cells form a ring of actin at the cell's midpoint. This ring, aptly called the "contractile ring", uses a similar mechanism as muscle fibers where myosin II pulls along the actin ring, causing it to contract. This contraction cleaves the parent cell into two, completing cytokinesis. The contractile ring is composed of actin, myosin, anillin, and α-actinin. In the fission yeast Schizosaccharomyces pombe, actin is actively formed in the constricting ring with the participation of Arp3, the formin Cdc12, profilin, and WASp,
along with preformed microfilaments. Once the ring has been constructed
the structure is maintained by a continual assembly and disassembly
that, aided by the Arp2/3 complex and formins, is key to one of the central processes of cytokinesis.
Intracellular trafficking
Actin-myosin pairs can also participate in the trafficking of various membrane vesicles and organelles within the cell. Myosin V
is activated by binding to various cargo receptors on organelles, and
then moves along an actin filament towards the (+) end, pulling its
cargo along with it.
These nonconventional myosins use ATP hydrolysis to transport cargo, such as vesicles
and organelles, in a directed fashion much faster than diffusion.
Myosin V walks towards the barbed end of actin filaments, while myosin
VI walks toward the pointed end. Most actin filaments are arranged with
the barbed end toward the cellular membrane and the pointed end toward
the cellular interior. This arrangement allows myosin V to be an
effective motor for the export of cargos, and myosin VI to be an
effective motor for import.
Other biological processes
The traditional image of actin's function relates it to the
maintenance of the cytoskeleton and, therefore, the organization and
movement of organelles, as well as the determination of a cell's shape. However, actin has a wider role in eukaryotic cell physiology, in addition to similar functions in prokaryotes.
Apoptosis. During programmed cell death the ICE/ced-3 family of proteases (one of the interleukin-1β-converter proteases) degrade actin into two fragments in vivo;
one of the fragments is 15 kDa and the other 31 kDa. This represents
one of the mechanisms involved in destroying cell viability that form
the basis of apoptosis. The protease calpain has also been shown to be involved in this type of cell destruction; just as the use of calpain inhibitors has been shown to decrease actin proteolysis and the degradation of DNA (another of the characteristic elements of apoptosis). On the other hand, the stress-induced
triggering of apoptosis causes the reorganization of the actin
cytoskeleton (which also involves its polymerization), giving rise to
structures called stress fibers; this is activated by the MAP kinase pathway.
Cellular adhesion and development. The adhesion between cells is a characteristic of multicellular organisms that enables tissue specialization and therefore increases cell complexity. Adhesion of cell epithelia involves the actin cytoskeleton in each of the joined cells as well as cadherins acting as extracellular elements with the connection between the two mediated by catenins.
Interfering in actin dynamics has repercussions for an organism's
development, in fact actin is such a crucial element that systems of
redundant genes are available. For example, if the α-actinin or gelation factor gene has been removed in Dictyostelium individuals do not show an anomalous phenotype possibly due to the fact that each of the proteins can perform the function of the other. However, the development of double mutations that lack both gene types is affected.
Gene expression modulation. Actin's state of polymerization affects the pattern of gene expression. In 1997, it was discovered that cytocalasin D-mediated depolymerization in Schwann cells causes a specific pattern of expression for the genes involved in the myelinization of this type of nerve cell. F-actin has been shown to modify the transcriptome in some of the life stages of unicellular organisms, such as the fungus Candida albicans. In addition, proteins that are similar to actin play a regulatory role during spermatogenesis in mice and, in yeasts, actin-like proteins are thought to play a role in the regulation of gene expression.
In fact, actin is capable of acting as a transcription initiator when
it reacts with a type of nuclear myosin that interacts with RNA polymerases and other enzymes involved in the transcription process.
Stereocilia dynamics. Some cells develop fine filiform outgrowths on their surface that have a mechanosensory function. For example, this type of organelle is present in the Organ of Corti, which is located in the ear. The main characteristic of these structures is that their length can be modified. The molecular architecture of the stereocilia includes a paracrystalline
actin core in dynamic equilibrium with the monomers present in the
adjacent cytosol. Type VI and VIIa myosins are present throughout this
core, while myosin XVa is present in its extremities in quantities that
are proportional to the length of the stereocilia.
Intrinsic chirality. Actomyosin networks have been implicated in generating an intrinsic chirality in individual cells. Cells grown out on chiral surfaces can show a directional left/right bias that is actomyosin dependent.
Structure
Monomeric actin, or G-actin, has a globular structure consisting of two lobes separated by a deep cleft.
The bottom of the cleft represents the “ATPase fold”, a structure
conserved among ATP and GTP-binding proteins that binds to a magnesium
ion and a molecule of ATP. Binding of ATP or ADP is required to stabilize each actin monomer; without one of these molecules bound, actin quickly becomes denatured.
Elzinga and co-workers first determined the complete peptide sequence for this type of actin in 1973, with later work by the same author adding further detail to the model. It contains 374 amino acid residues. Its N-terminus is highly acidic and starts with an acetyledaspartate in its amino group. While its C-terminus is alkaline and is formed by a phenylalanine preceded by a cysteine, which has a degree of functional importance. Both extremes are in close proximity within the I-subdomain. An anomalous Nτ-methylhistidine is located at position 73.
Tertiary structure — domains
The tertiary structure is formed by two domains known as the large and the small, which are separated by a cleft centred around the location of the bond with ATP-ADP+Pi. Below this there is a deeper notch called a “groove”. In the native state, despite their names, both have a comparable depth.
The normal convention in topological
studies means that a protein is shown with the biggest domain on the
left-hand side and the smallest domain on the right-hand side. In this
position the smaller domain is in turn divided into two: subdomain I
(lower position, residues 1–32, 70–144, and 338–374) and subdomain II
(upper position, residues 33–69). The larger domain is also divided in
two: subdomain III (lower, residues 145–180 and 270–337) and subdomain
IV (higher, residues 181–269). The exposed areas of subdomains I and III
are referred to as the “barbed” ends, while the exposed areas of
domains II and IV are termed the “pointed" ends. This nomenclature
refers to the fact that, due to the small mass of subdomain II actin is
polar; the importance of this will be discussed below in the discussion
on assembly dynamics. Some authors call the subdomains Ia, Ib, IIa, and
IIb, respectively.
Other important structures
The most notable supersecondary structure is a five chain beta sheet
that is composed of a β-meander and a β-α-β clockwise unit. It is
present in both domains suggesting that the protein arose from gene
duplication.
The adenosine nucleotide binding site is located between two beta hairpin-shaped
structures pertaining to the I and III domains. The residues that are
involved are Asp11-Lys18 and Asp154-His161 respectively.
The divalent cation binding site is located just below that for the adenosine nucleotide. In vivo it is most often formed by Mg2+ or Ca2+ while in vitro it is formed by a chelating structure made up of Lys18 and two oxygens from the nucleotide's α-and β-phosphates. This calcium is coordinated with six water molecules that are retained by the amino acids Asp11, Asp154, and Gln137.
They form a complex with the nucleotide that restricts the movements of
the so-called "hinge" region, located between residues 137 and 144.
This maintains the native form of the protein until its withdrawal denatures
the actin monomer. This region is also important because it determines
whether the protein's cleft is in the "open" or "closed" conformation.
It is highly likely that there are at least three other centres with a lesser affinity
(intermediate) and still others with a low affinity for divalent
cations. It has been suggested that these centres may play a role in the
polymerization of actin by acting during the activation stage.
There is a structure in subdomain 2 that is called the “D-loop” because it binds with DNase I, it is located between the His40 and Gly48
residues. It has the appearance of a disorderly element in the majority
of crystals, but it looks like a β-sheet when it is complexed with
DNase I. It has been proposed that the key event in polymerization is
probably the propagation of a conformational change from the centre of
the bond with the nucleotide to this domain, which changes from a loop
to a spiral. However, this hypothesis has been refuted by other studies.
F-actin
Under various conditions, G-actin molecules polymerize into longer
threads called "filamentous-" or "F-actin". These F-actin threads are
typically composed of two helical strands of actin wound around each
other, forming a 7 to 9 nanometer wide helix that repeats every 72 nanometers (or every 14 G-actin subunits).
In F-actin threads, G-actin molecules are all oriented in the same
direction. The two ends of the F-actin thread are distinct from one
another. At one end – designated the (−) end – the ATP-binding cleft of
the terminal actin molecule is facing outward. At the opposite end –
designated (+) – the ATP-binding cleft is buried in the filament,
contacting the neighboring actin molecule.
As F-actin threads grow, new molecules tend to join at the (+) end of
an existing F-actin strand. Conversely, threads tend to shrink by
shedding actin monomers from the strand's (-) end.
Some proteins, such as cofilin
appear to increase the angle of turn, but again this could be
interpreted as the establishment of different structural states. These
could be important in the polymerization process.
There is less agreement regarding measurements of the turn radius
and filament thickness: while the first models assigned a length of 25
Å, current X-ray diffraction data, backed up by cryo-electron microscopy
suggests a length of 23.7 Å. These studies have shown the precise
contact points between monomers. Some are formed with units of the same
chain, between the "barbed" end on one monomer and the "pointed" end of
the next one. While the monomers in adjacent chains make lateral contact
through projections from subdomain IV, with the most important
projections being those formed by the C-terminus and the hydrophobic
link formed by three bodies involving residues 39–42, 201–203, and 286.
This model suggests that a filament is formed by monomers in a "sheet"
formation, in which the subdomains turn about themselves, this form is
also found in the bacterial actin homologue MreB.
The terms "pointed" and "barbed" referring to the two ends of the microfilaments derive from their appearance under transmission electron microscopy when samples are examined following a preparation technique called "decoration". This method consists of the addition of myosin S1 fragments to tissue that has been fixed with tannic acid.
This myosin forms polar bonds with actin monomers, giving rise to a
configuration that looks like arrows with feather fletchings along its
shaft, where the shaft is the actin and the fletchings are the myosin.
Following this logic, the end of the microfilament that does not have
any protruding myosin is called the point of the arrow (- end) and the
other end is called the barbed end (+ end).
A S1 fragment is composed of the head and neck domains of myosin II. Under physiological conditions, G-actin (the monomer form) is transformed to F-actin (the polymer form) by ATP, where the role of ATP is essential.
The helical F-actin filament found in muscles also contains a tropomyosin molecule, which is a 40 nanometre long protein that is wrapped around the F-actin helix.
During the resting phase the tropomyosin covers the actin's active
sites so that the actin-myosin interaction cannot take place and produce
muscular contraction. There are other protein molecules bound to the
tropomyosin thread, these are the troponins that have three polymers: troponin I, troponin T, and troponin C.
F-actin is both strong and dynamic. Unlike other polymers, such as DNA, whose constituent elements are bound together with covalent bonds, the monomers of actin filaments are assembled by weaker bonds.
The lateral bonds with neighbouring monomers resolve this anomaly,
which in theory should weaken the structure as they can be broken by
thermal agitation. In addition, the weak bonds give the advantage that
the filament ends can easily release or incorporate monomers. This means
that the filaments can be rapidly remodelled and can change cellular
structure in response to an environmental stimulus. Which, along with
the biochemical mechanism by which it is brought about is known as the "assembly dynamic".
Folding
Actin can spontaneously acquire a large part of its tertiary structure. However, the way it acquires its fully functional form from its newly synthesized
native form is special and almost unique in protein chemistry. The
reason for this special route could be the need to avoid the presence of
incorrectly folded actin monomers, which could be toxic as they can act
as inefficient polymerization terminators. Nevertheless, it is key to
establishing the stability of the cytoskeleton, and additionally, it is
an essential process for coordinating the cell cycle.
CCT is required in order to ensure that folding takes place
correctly. CCT is a group II chaperonin, a large protein complex that
assists in the folding of other proteins. CCT is formed of a double ring
of eight different subunits (hetero-octameric) and it differs from
group I chaperonins like GroEL,
which is found in Eubacteria and in eukaryotic organelles, as it does
not require a co-chaperone to act as a lid over the central catalytic cavity. Substrates bind to CCT through specific domains. It was initially thought that it only bound with actin and tubulin, although recent immunoprecipitation studies have shown that it interacts with a large number of polypeptides, which possibly function as substrates.
It acts through ATP-dependent conformational changes that on occasion
require several rounds of liberation and catalysis in order to complete a
reaction.
In order to successfully complete their folding, both actin and tubulin need to interact with another protein called prefoldin,
which is a heterohexameric complex (formed by six distinct subunits),
in an interaction that is so specific that the molecules have coevolved. Actin complexes with prefoldin while it is still being formed, when it is approximately 145 amino acids long, specifically those at the N-terminal.
Different recognition sub-units are used for actin or tubulin
although there is some overlap. In actin the subunits that bind with
prefoldin are probably PFD3 and PFD4, which bind in two places one
between residues 60–79 and the other between residues 170–198. The actin
is recognized, loaded, and delivered to the cytosolic chaperonin (CCT)
in an open conformation by the inner end of prefoldin's "tentacles” (see
the image and note). The contact when actin is delivered is so brief that a tertiary complex is not formed, immediately freeing the prefoldin.
The CCT then causes actin's sequential folding by forming bonds with its subunits rather than simply enclosing it in its cavity.
This is why it possesses specific recognition areas in its apical
β-domain. The first stage in the folding consists of the recognition of
residues 245–249. Next, other determinants establish contact.
Both actin and tubulin bind to CCT in open conformations in the absence
of ATP. In actin's case, two subunits are bound during each
conformational change, whereas for tubulin binding takes place with four
subunits. Actin has specific binding sequences, which interact with the
δ and β-CCT subunits or with δ-CCT and ε-CCT. After AMP-PNP is bound to
CCT the substrates move within the chaperonin's cavity. It also seems
that in the case of actin, the CAP protein is required as a possible cofactor in actin's final folding states.
The exact manner by which this process is regulated is still not
fully understood, but it is known that the protein PhLP3 (a protein
similar to phosducin) inhibits its activity through the formation of a tertiary complex.
ATPase’s catalytic mechanism
Actin is an ATPase, which means that it is an enzyme that hydrolyzes
ATP. This group of enzymes is characterised by their slow reaction
rates. It is known that this ATPase is “active”, that is, its speed
increases by some 40,000 times when the actin forms part of a filament. A reference value for this rate of hydrolysis under ideal conditions is around 0.3 s−1. Then, the Pi
remains bound to the actin next to the ADP for a long time, until it is
cooperatively liberated from the interior of the filament.
The exact molecular details of the catalytic mechanism are still
not fully understood. Although there is much debate on this issue, it
seems certain that a "closed" conformation is required for the
hydrolysis of ATP, and it is thought that the residues that are involved
in the process move to the appropriate distance. The glutamic acid Glu137 is one of the key residues, which is located in subdomain 1. Its function is to bind the water molecule that produces a nucleophilic attack on the ATP's γ-phosphate bond,
while the nucleotide is strongly bound to subdomains 3 and 4. The
slowness of the catalytic process is due to the large distance and
skewed position of the water molecule in relation to the reactant. It is
highly likely that the conformational change produced by the rotation
of the domains between actin's G and F forms moves the Glu137 closer
allowing its hydrolysis. This model suggests that the polymerization and
ATPase's function would be decoupled straight away.
The "open" to "closed" transformation between G and F forms and its
implications on the relative motion of several key residues and the
formation of water wires have been characterized in molecular dynamics and QM/MM simulations.
Assembly dynamics
Actin filaments are often rapidly assembled and disassembled, allowing them to generate force and support cell movement.
Assembly classically occurs in three steps. First, the "nucleation
phase", in which two to three G-actin molecules slowly join to form a
small oligomer that will nucleate further growth. Second, the
"elongation phase", when the actin filament rapidly grows by the
addition of many actin molecules to both ends. As the filament grows,
actin molecules are added to the (+) end of the filament around 10 times
faster than to the (−) end, and so filaments tend to primarily grow at
the (+) end.
Third, the "steady-state phase", where an equillibrium is reached as
actin molecules join and leave the filament at the same rate,
maintaining the filament's length.
While the filament's length remains constant in the steady-state phase,
new molecules are constantly being added to the (+) end and falling off
the (−) end, a phenomenon called "treadmilling" as a given actin
molecule would appear to move along the strand.
In isolation, whether a filament will grow or shrink, and how quickly,
are determined by the concentration of G-actin around the filament; however, in cells, the dynamics of actin filaments are heavily influenced by various actin-binding proteins.
Actin binding proteins
The actin cytoskeleton in vivo
is not exclusively composed of actin, other proteins are required for
its formation, continuance, and function. These proteins are called actin-binding proteins and they are involved in actin's polymerization, depolymerization, stability, and organisation. The diversity of these proteins is such that actin is thought to be the protein that takes part in the greatest number of protein-protein interactions.
The nucleation of new actin filaments – the rate-limiting step in actin polymerization – is aided by actin-nucleating proteins such as formins (like formin-2) and the Arp2/3 complex.
Formins help to nucleate long actin filaments. They bind two free
actin-ATP molecules, bringing them together. Then as the filament begins
to grow, formin moves along the (+) end of the growing filament, all
the while recruiting actin-binding proteins that promote filament
growth, and excluding capping proteins that would block filament
extension. Branches in actin filaments are typically nucleated by the Arp2/3 complex in concert with nucleation promoting factors.
Nucleation promoting factors bind two free G-actin molecules, then
recruit and activate the Arp2/3 complex. The activated Arp2/3 complex
attaches to an existing actin filament, and uses the two bound G-actin
molecules to nucleate a new actin filament branching off of the old one
at a 70° angle.
As filaments grow, the pool of available G-actin molecules is managed by G-actin-binding proteins such as profilin and thymosin β-4. Profilin
ensures a supply of available actin-ATP by binding to ADP-bound G-actin
and promoting the exchange of ADP for ATP. Profilin's binding to the
actin molecule physically blocks its addition to a filament's (−) end,
but permits it to join the (+) end. Once the actin-ATP has joined the
filament, profilin releases it.
As formins promote the nucleation and extension of new actin filaments,
they recruit profilin to the area, increasing the local concentration
of actin-ATP to boost filament growth. In contrast, thymosin β-4 binds and sequesters actin-ATP, preventing it from joining a microfilament.
Once an actin fiber is established, the dynamics of its growth or
collapse are influenced by numerous proteins. Existing strands can be
interrupted by filament cleaving proteins, such as cofilin and gelsolin.
Cofilin binds along two actin-ADP molecules in a filament, forcing a
movement that destabilizes the filament and causes it to break.
Gelsolin inserts itself between actin molecules in a filament,
disrupting the filament. After the filament breaks, gelsolin remains
attached to the new (+) end, preventing it from growing, thus forcing
its disassembly.
Other proteins bind to the ends of actin filaments, stabilizing them. These are called "capping proteins" and include CapZ and tropomodulin. CapZ binds the (+) end of a filament, preventing further addition or loss of actin from that end. Tropomodulin
binds to a filament's (−) end, again preventing addition or loss of
molecule's at that end. Tropomodulin is typically found in cells that
require extremely stable actin filaments, such as those in muscle and
red blood cells.
These actin binding proteins are typically regulated by various
cellular signals to control actin assembly dynamics in different
cellular locations. Formins, for example, are typically folded in an
inactive conformation until they're activated by the binding of the small GTPaseRho. Actin branching at the cell membrane is important for cell movement, and so the plasma membrane lipid PIP2 activates the nucleation promoting factor WASp and inhibits CapZ. WASp is also activated by the small GTPase Cdc42, while another nucleation promoting factor WAVE is activated by the GTPase Rac1.
Genetics
Although most yeasts have only a single actin gene, higher eukaryotes, in general, express several isoforms of actin encoded by a family of related genes. Mammals have at least six actin isoforms coded by separate genes, which are divided into three classes – alpha, beta, and gamma – according to their isoelectric points. In general, alpha actins are found in muscle (α-skeletal, α-aortic smooth, α-cardiac), whereas beta and gamma isoforms are prominent in non-muscle cells (β-cytoplasmic, γ1-cytoplasmic, γ2-enteric smooth). Although the amino acid sequences and in vitro properties of the isoforms are highly similar, these isoforms cannot completely substitute for one another in vivo. Plants contains more than 60 actin genes and pseudogenes.
The typical actin gene has an approximately 100-nucleotide 5' UTR, a 1200-nucleotide translated region, and a 200-nucleotide 3' UTR. The majority of actin genes are interrupted by introns,
with up to six introns in any of 19 well-characterised locations. The
high conservation of the family makes actin the favoured model for
studies comparing the introns-early and introns-late models of intron
evolution.
Evolution
Actin and closely related proteins are present in all organisms, suggesting the common ancestor of all life on Earth had actin. Actin is one of the most conserved proteins throughout the evolution of eukaryotes. The sequences of actin proteins from animals and amoebae are 80% identical despite being separated by approximately one billion years of evolution. Many unicellular
eukaryotes have a single actin gene, while multicellular eukaryotes
often have several closely related genes that serve specialized
functions. Humans have six; plants have 10 or more.
In addition to actin, eukaryotes have a large family of actin-related
proteins, or "Arps", that share a common ancestor with actin and are
called Arp1–Arp11, with Arp1 the most closely related to actin, and
Arp11 the least.
Bacteria encode three types of actin: MreB influences cell shape, FtsA cell division, and ParM separation of large plasmids. Some archaea have a bacteria-like MreB gene, while others have an actin gene that more closely resembles eukaryote actin.
The eukaryotic cytoskeleton of organisms among all taxonomic groups have similar components to actin and tubulin. For example, the protein that is coded by the ACTG2 gene in humans is completely equivalent to the homologues present in rats and mice, even though at a nucleotide level the similarity decreases to 92%. However, there are major differences with the equivalents in prokaryotes (FtsZ and MreB), where the similarity between nucleotide sequences is between 40 and 50% among different bacteria and archaea
species. Some authors suggest that the ancestral protein that gave rise
to the model eukaryotic actin resembles the proteins present in modern
bacterial cytoskeletons.
Some authors point out that the behaviour of actin, tubulin, and histone,
a protein involved in the stabilization and regulation of DNA, are
similar in their ability to bind nucleotides and in their ability of
take advantage of Brownian motion. It has also been suggested that they all have a common ancestor. Therefore, evolutionary
processes resulted in the diversification of ancestral proteins into
the varieties present today, conserving, among others, actins as
efficient molecules that were able to tackle essential ancestral
biological processes, such as endocytosis.
The bacterial cytoskeleton contains proteins that are highly similar to actin monomers and polymers. The bacterial protein MreB polymerizes into thin non-helical filaments and occasionally into helical structures similar to F-actin.
Furthermore, its crystalline structure is very similar to that of
G-actin (in terms of its three-dimensional conformation), there are even
similarities between the MreB protofilaments and F-actin. The bacterial
cytoskeleton also contains the FtsZ proteins, which are similar to tubulin.
Bacteria therefore possess a cytoskeleton with homologous elements to actin (for example, MreB, AlfA, ParM, FtsA,
and MamK), even though the amino acid sequence of these proteins
diverges from that present in animal cells. However, such proteins have a
high degree of structural
similarity to eukaryotic actin. The highly dynamic microfilaments
formed by the aggregation of MreB and ParM are essential to cell
viability and they are involved in cell morphogenesis, chromosome segregation, and cell polarity. ParM is an actin homologue that is coded in a plasmid and it is involved in the regulation of plasmid DNA. ParMs from different bacterial plasmids can form astonishingly diverse helical structures comprising two or four strands to maintain faithful plasmid inheritance.
In archaea the homologue Ta0583 is even more similar to the eukaryotic actins.
ACTA1 is the gene that codes for the α-isoform of actin that is predominant in human skeletal striated muscles, although it is also expressed in heart muscle and in the thyroid gland. Its DNA sequence consists of seven exons that produce five known transcripts. The majority of these consist of point mutations causing substitution of amino acids. The mutations are in many cases associated with a phenotype that determines the severity and the course of the affliction.
The mutation alters the structure and function of skeletal muscles producing one of three forms of myopathy: type 3 nemaline myopathy, congenital myopathy with an excess of thin myofilaments (CM) and congenital myopathy with fibre type disproportion (CMFTD). Mutations have also been found that produce core myopathies.
Although their phenotypes are similar, in addition to typical nemaline
myopathy some specialists distinguish another type of myopathy called
actinic nemaline myopathy. In the former, clumps of actin form instead
of the typical rods. It is important to state that a patient can show
more than one of these phenotypes in a biopsy. The most common symptoms consist of a typical facial morphology (myopathic facies),
muscular weakness, a delay in motor development and respiratory
difficulties. The course of the illness, its gravity, and the age at
which it appears are all variable and overlapping forms of myopathy are
also found. A symptom of nemaline myopathy is that "nemaline rods"
appear in differing places in type 1 muscle fibres. These rods are non-pathognomonic structures that have a similar composition to the Z disks found in the sarcomere.
The pathogenesis of this myopathy is very varied. Many mutations occur in the region of actin's indentation near to its nucleotide
binding sites, while others occur in Domain 2, or in the areas where
interaction occurs with associated proteins. This goes some way to
explain the great variety of clumps that form in these cases, such as
Nemaline or Intranuclear Bodies or Zebra Bodies. Changes in actin's folding occur in nemaline myopathy as well as changes in its aggregation and there are also changes in the expression of other associated proteins. In some variants where intranuclear bodies are found the changes in the folding masks the nucleus's protein exportation signal so that the accumulation of actin's mutated form occurs in the cell nucleus. On the other hand, it appears that mutations to ACTA1 that give rise to a CFTDM have a greater effect on sarcomeric function than on its structure.
Recent investigations have tried to understand this apparent paradox,
which suggests there is no clear correlation between the number of rods
and muscular weakness. It appears that some mutations are able to induce
a greater apoptosis rate in type II muscular fibres.
ACTG2 codes for the largest actin isoform, which has nine exons, one of which, the one located at the 5' end, is not translated.
It is a γ-actin that is expressed in the enteric smooth muscle. No
mutations to this gene have been found that correspond to pathologies,
although microarrays have shown that this protein is more often expressed in cases that are resistant to chemotherapy using cisplatin.
ACTA2
codes for an α-actin located in the smooth muscle, and also in vascular
smooth muscle. It has been noted that the MYH11 mutation could be
responsible for at least 14% of hereditary thoracic aortic aneurisms
particularly Type 6. This is because the mutated variant produces an
incorrect filamentary assembly and a reduced capacity for vascular
smooth muscle contraction. Degradation of the aortic media has been recorded in these individuals, with areas of disorganization and hyperplasia as well as stenosis of the aorta's vasa vasorum. The number of afflictions that the gene is implicated in is increasing. It has been related to Moyamoya disease
and it seems likely that certain mutations in heterozygosis could
confer a predisposition to many vascular pathologies, such as thoracic
aortic aneurysm and ischaemic heart disease. The α-actin found in smooth muscles is also an interesting marker for evaluating the progress of liver cirrhosis.
In heart muscle
The ACTC1
gene codes for the α-actin isoform present in heart muscle. It was
first sequenced by Hamada and co-workers in 1982, when it was found that
it is interrupted by five introns. It was the first of the six genes where alleles were found that were implicated in pathological processes.
A number of structural disorders associated with point mutations of
this gene have been described that cause malfunctioning of the heart,
such as Type 1R dilated cardiomyopathy and Type 11 hypertrophic cardiomyopathy. Certain defects of the atrial septum have been described recently that could also be related to these mutations.
Two cases of dilated cardiomyopathy have been studied involving a substitution of highly conserved amino acids belonging to the protein domains that bind and intersperse with the Z discs. This has led to the theory that the dilation is produced by a defect in the transmission of contractile force in the myocytes.
The mutations in ACTC1 are responsible for at least 5% of hypertrophic cardiomyopathies. The existence of a number of point mutations have also been found:
Mutation E101K: changes of net charge and formation of a weak electrostatic link in the actomyosin-binding site.
P166A: interaction zone between actin monomers.
A333P: actin-myosin interaction zone.
Pathogenesis appears to involve a compensatory mechanism: the mutated
proteins act like toxins with a dominant effect, decreasing the heart's
ability to contract
causing abnormal mechanical behaviour such that the hypertrophy, that
is usually delayed, is a consequence of the cardiac muscle's normal
response to stress.
ACTB is a highly complex locus. A number of pseudogenes exist that are distributed throughout the genome, and its sequence contains six exons that can give rise to up to 21 different transcriptions by alternative splicing,
which are known as the β-actins. Consistent with this complexity, its
products are also found in a number of locations and they form part of a
wide variety of processes (cytoskeleton, NuA4 histone-acyltransferase complex, cell nucleus) and in addition they are associated with the mechanisms of a great number of pathological processes (carcinomas, juvenile dystonia, infection mechanisms, nervous system malformations and tumour invasion, among others). A new form of actin has been discovered, kappa actin, which appears to substitute for β-actin in processes relating to tumours.
Three pathological processes have so far been discovered that are caused by a direct alteration in gene sequence:
Juvenile onset dystonia is a rare degenerative disease that affects the central nervous system; in particular, it affects areas of the neocortex and thalamus, where rod-like eosinophilic inclusions are formed. The affected individuals represent a phenotype with deformities on the median line, sensory hearing loss and dystonia. It is caused by a point mutation in which the amino acid tryptophan replaces arginine in position 183. This alters actin's interaction with the ADF/cofilin system, which regulates the dynamics of nerve cell cytoskeleton formation.
A dominant point mutation has also been discovered that causes neutrophil granulocyte dysfunction and recurring infections. It appears that the mutation modifies the domain responsible for binding between profilin and other regulatory proteins. Actin's affinity for profilin is greatly reduced in this allele.
The ACTG1 locus codes for the cytosolic γ-actin protein that is responsible for the formation of cytoskeletal microfilaments. It contains six exons, giving rise to 22 different mRNAs, which produce four complete isoforms whose form of expression is probably dependent on the type of tissue they are found in. It also has two different DNA promoters.
It has been noted that the sequences translated from this locus and
from that of β-actin are very similar to the predicted ones, suggesting a
common ancestral sequence that suffered duplication and genetic
conversion.
In terms of pathology, it has been associated with processes such as amyloidosis, retinitis pigmentosa, infection mechanisms, kidney diseases, and various types of congenital hearing loss.
Six autosomal-dominant point mutations in the sequence have been
found to cause various types of hearing loss, particularly sensorineural
hearing loss linked to the DFNA 20/26 locus. It seems that they affect
the stereocilia of the ciliated cells present in the inner ear's Organ of Corti.
β-actin is the most abundant protein found in human tissue, but it is
not very abundant in ciliated cells, which explains the location of the
pathology. On the other hand, it appears that the majority of these
mutations affect the areas involved in linking with other proteins,
particularly actomyosin.
Some experiments have suggested that the pathological mechanism for
this type of hearing loss relates to the F-actin in the mutations being
more sensitive to cofilin than normal.
However, although there is no record of any case, it is known
that γ-actin is also expressed in skeletal muscles, and although it is
present in small quantities, model organisms have shown that its absence can give rise to myopathies.
Other pathological mechanisms
Some infectious agents use actin, especially cytoplasmic actin, in their life cycle. Two basic forms are present in bacteria:
Listeria monocytogenes, some species of Rickettsia, Shigella flexneri and other intracellular germs escape from phagocytic vacuoles by coating themselves with a capsule of actin filaments. L. monocytogenes and S. flexneri
both generate a tail in the form of a "comet tail" that gives them
mobility. Each species exhibits small differences in the molecular
polymerization mechanism of their "comet tails". Different displacement
velocities have been observed, for example, with Listeria and Shigella found to be the fastest. Many experiments have demonstrated this mechanism in vitro.
This indicates that the bacteria are not using a myosin-like protein
motor, and it appears that their propulsion is acquired from the
pressure exerted by the polymerization that takes place near to the
microorganism's cell wall. The bacteria have previously been surrounded
by ABPs from the host, and as a minimum the covering contains Arp2/3 complex, Ena/VASP proteins, cofilin, a buffering protein and nucleation promoters, such as vinculin complex. Through these movements they form protrusions that reach the neighbouring cells, infecting them as well so that the immune system
can only fight the infection through cell immunity. The movement could
be caused by the modification of the curve and debranching of the
filaments. Other species, such as Mycobacterium marinum and Burkholderia pseudomallei,
are also capable of localized polymerization of cellular actin to aid
their movement through a mechanism that is centered on the Arp2/3
complex. In addition the vaccine virusVaccinia also uses elements of the actin cytoskeleton for its dissemination.
In addition to the previously cited example, actin polymerization is
stimulated in the initial steps of the internalization of some viruses,
notably HIV, by, for example, inactivating the cofilin complex.
The role that actin plays in the invasion process of cancer cells has still not been determined.
Applications
Actin is used in scientific and technological laboratories as a track for molecular motors
such as myosin (either in muscle tissue or outside it) and as a
necessary component for cellular functioning. It can also be used as a
diagnostic tool, as several of its anomalous variants are related to the
appearance of specific pathologies.
Nanotechnology.
Actin-myosin systems act as molecular motors that permit the transport
of vesicles and organelles throughout the cytoplasm. It is possible that
actin could be applied to nanotechnology
as its dynamic ability has been harnessed in a number of experiments
including those carried out in acellular systems. The underlying idea is
to use the microfilaments as tracks to guide molecular motors that can
transport a given load. That is actin could be used to define a circuit
along which a load can be transported in a more or less controlled and
directed manner. In terms of general applications, it could be used for
the directed transport of molecules for deposit in determined locations,
which would permit the controlled assembly of nanostructures. These attributes could be applied to laboratory processes such as on lab-on-a-chip, in nanocomponent mechanics and in nanotransformers that convert mechanical energy into electrical energy.
Actin is used as an internal control in western blots
to ascertain that equal amounts of protein have been loaded on each
lane of the gel. In the blot example shown on the left side, 75 µg of
total protein was loaded in each well. The blot was reacted with
anti-β-actin antibody (for other details of the blot see the reference )
The use of actin as an internal control is based on the assumption
that its expression is practically constant and independent of
experimental conditions. By comparing the expression of the gene of
interest to that of the actin, it is possible to obtain a relative
quantity that can be compared between different experiments,
whenever the expression of the latter is constant. It is worth pointing
out that actin does not always have the desired stability in its gene expression.
Health. Some alleles
of actin cause diseases; for this reason techniques for their detection
have been developed. In addition, actin can be used as an indirect
marker in surgical pathology: it is possible to use variations in the
pattern of its distribution in tissue as a marker of invasion in neoplasia, vasculitis, and other conditions.
Further, due to actin's close association with the apparatus of
muscular contraction its levels in skeletal muscle diminishes when these
tissues atrophy, it can therefore be used as a marker of this physiological process.
Food technology. It is possible to determine the quality of certain processed foods, such as sausages,
by quantifying the amount of actin present in the constituent meat.
Traditionally, a method has been used that is based on the detection of 3-methylhistidine in hydrolyzed
samples of these products, as this compound is present in actin and
F-myosin's heavy chain (both are major components of muscle). The
generation of this compound in flesh derives from the methylation of histidine residues present in both proteins.
History
Actin was first observed experimentally in 1887 by W.D. Halliburton, who extracted a protein from muscle that 'coagulated' preparations of myosin that he called "myosin-ferment". However, Halliburton was unable to further refine his findings, and the discovery of actin is credited instead to Brunó Ferenc Straub, a young biochemist working in Albert Szent-Györgyi's laboratory at the Institute of Medical Chemistry at the University of Szeged, Hungary.
Following up on the discovery of Ilona Banga
& Szent-Györgyi in 1941 that the coagulation only occurs in some
myosin extractions and was reversed upon the addition of ATP,
Straub identified and purified actin from those myosin preparations
that did coagulate. Building on Banga's original extraction method, he
developed a novel technique for extracting muscle protein that allowed him to isolate substantial amounts of relatively pure actin, published in 1942. Straub's method is essentially the same as that used in laboratories today. Since Straub's protein was necessary to activate the coagulation of myosin, it was dubbed actin. Realizing that Banga's coagulating myosin preparations contained actin
as well, Szent-Györgyi called the mixture of both proteins actomyosin.
The hostilities of World War II meant Szent-Gyorgyi was unable to publish his lab's work in Westernscientific journals. Actin therefore only became well known in the West in 1945, when their paper was published as a supplement to the Acta Physiologica Scandinavica. Straub continued to work on actin, and in 1950 reported that actin contains bound ATP and that, during polymerization of the protein into microfilaments, the nucleotide is hydrolyzed to ADP and inorganic phosphate
(which remain bound to the microfilament). Straub suggested that the
transformation of ATP-bound actin to ADP-bound actin played a role in
muscular contraction. In fact, this is true only in smooth muscle, and was not supported through experimentation until 2001.
The amino acid sequencing of actin was completed by M. Elzinga and co-workers in 1973. The crystal structure of G-actin was solved in 1990 by Kabsch and colleagues.
In the same year, a model for F-actin was proposed by Holmes and
colleagues following experiments using co-crystallization with different
proteins.
The procedure of co-crystallization with different proteins was used
repeatedly during the following years, until in 2001 the isolated
protein was crystallized along with ADP. However, there is still no
high-resolution X-ray structure of F-actin. The crystallization of
G-actin was possible due to the use of a rhodamine conjugate that impedes polymerization by blocking the amino acid cys-374.
Christine Oriol-Audit died in the same year that actin was first
crystallized but she was the researcher that in 1977 first crystallized
actin in the absence of Actin Binding Proteins (ABPs). However, the
resulting crystals were too small for the available technology of the
time.
Although no high-resolution model of actin's filamentous form
currently exists, in 2008 Sawaya's team were able to produce a more
exact model of its structure based on multiple crystals of actin dimers that bind in different places. This model has subsequently been further refined by Sawaya and Lorenz. Other approaches such as the use of cryo-electron microscopy and synchrotron radiation
have recently allowed increasing resolution and better understanding of
the nature of the interactions and conformational changes implicated in
the formation of actin filaments.
Research
Chemical inhibitors
A number of natural toxins that interfere with actin's dynamics are widely used in research to study actin's role in biology. Latrunculin – a toxin produced by sponges – binds to G-actin preventing it from joining microfilaments. Cytochalasin D – produced by certain fungi – serves as a capping factor, binding to the (+) end of a filament and preventing further addition of actin molecules. In contrast, the sponge toxin jasplakinolide promotes the nucleation of new actin filaments by binding and stabilzing pairs of actin molecules. Phalloidin – from the "death cap" mushroom Amanita phalloides – binds to adjacent actin molecules within the F-actin filament, stabilizing the filament and preventing its depolymerization.