View source for F-actin

<StructureSection load='2zwh' size='350' side='right' scene='' caption='Rabbit α-actin complex with ADP and Ca+2 ion (PDB code [[2zwh]])'>
[[Image:2zwh.png|left|200px|thumb|Crystal Structure of F-actin, [[2zwh]]]]'''Filamentous actin''' ('''F-actin''') units are also referred to as [http://en.wikipedia.org/wiki/microfilament microfilaments] <ref> Microfilaments - Wikipedia, the free encyclopedia. http://en.wikipedia.org/wiki/Microfilament. Date accessed: March 16th, 2010. </ref> and are highly conserved, proteinous components found near ubiquitously in eukaryotic cytoskeletons. F-actin and other [[actin]] proteins generally have structural roles in cells.
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== Introduction ==
Actin is found in nearly all eukaryotic cells and is known primarily for its function as a structural and translocation protein. It also has an ATPase function, as it hydrolyzes ATP to ADP and P<sub>i</sub> and undergoes conformational changes with each hydrolysis. Actin belongs to the actin superfamily, which includes other proteins such as Hsp70(DnaK), Hsc70, and hexokinase, because of its nucleotide-dependent conformational change<ref name="Graceffa">PMID:12813032</ref>. Because of the similarity observed in ''Escherichia Coli'''s, Hsc70 and ATPase domain of actin, it is believed that the two proteins have a common ancestry<ref name="Holmes1">PMID:19158779</ref>. Prokaryotes are not known to have actin, but do however have an actin homologue, MreB, which also leads to the idea of possible common ancestory<ref name="Holmes2">PMID:2395461</ref>. 

Actin occurs in two forms: globular actin (G-actin), the free monomeric units of actin, and filamentous actin (F-actin) which is the polymer form. These two forms exist in a dynamic equilibrium with one another as ATP-associated polymerization and depolymerization occur continuously within the cell. The monomer units in the F-actin possess a form that is distinct from the free monomeric form and it is a result of that change that the more specific ATPase activity may be observed.

== Assembly == 
<scene name='Sandbox_154/1j6z_black_true/2'>Globular Actin (G-actin)</scene> ([http://www.rcsb.org/pdb/explore/explore.do?structureId=1J6Z 1J6Z]).

'''G-actin''' is the free monomeric form of actin which polymerizes to F-actin. The structures of globular and filamentous actin are distinct from one another in numerous ways, despite the fact that G-actin comprises F-actin. When the monomeric actin becomes polymerized into F-actin, the unit becomes flattened. Also, F-actin possesses an ATPase function which is minimal in G-actin. The domains and active site are the same in terms of constituent components and will be discussed later in terms of the F-actin monomer. 

G-actin appears to have more <scene name='Sandbox_154/1j6z_calcium/3'>calcium ion</scene> ligands in its structure, external to the active site. Only 3 of the 5 are believed to actually exist in solution and are believed to contribute to the polymerization of G-actin to F-actin<ref name="otterbein">PMID:11474115</ref>. This representation of G-actin also possesses an <scene name='Sandbox_154/1j6z_black_true_dloop/1'>alpha-helical fold in the D-loop</scene> which is observed in some actin crystalline structures but not necessarily<ref name="otterbein"/>. The observed molecule on Cys374, <scene name='Sandbox_154/1j6z_black_true_rho/1'>tetramethylrhodamine-5-maleimide</scene> was used to block polymerization activity so the crystal of G-actin could be observed<ref name="otterbein"/>


Formation of F-actin is a dynamic process of assembly and disassembly which has been termed “treadmilling”. 
The transition between G and F-actin begins with a stabilized oligomer of ATP-actin units formed through a nucleation-condensation type fold pattern<ref name="Pfaendtner">PMID:19620726</ref>. Addition of ATP-monomeric units to either end subsequently occurs, however, because of a difference in charge polarity in the two ends, there is preferential addition to what is termed the "plus (+) end" or the "barbed-end". On the opposite end, the "minus (-) end" or the "pointed end", there is preferential dissociation of actin units<ref name="Mitchinson">PMID:1493331</ref>. 

After attachment of the ATP-bound actin, hydrolysis of the ATP occurs yielding the ADP and P<sub>i</sub> bound state. Subsequent loss of a P<sub>i</sub> leaves the ADP-actin state<ref name="Chen">PMID:10637608</ref>. Because of the potential for addition or removal of monomeric units to occur at both ends, the assembly of F-actin may be described in terms of equilibrium. However, because the rate of ATP-actin association is ten-fold that of ADP-actin dissociation, the f-actin has the appearance of moving forward, or "treadmilling"<ref name="Carlier">PMID:3801442</ref>. ADP-actin monomers dissociate at the minus end and become recycled to ATP-actin so polymerization at the plus end may occur once again.

== Structure == 
=== History of the structure ===
The F-actin protein was discovered by Straub in 1942<ref name="oda"/>. The structure was speculated based on a low-resolution x-ray crystallograph found in 1990 by Holmes et al. and over this time, the "Holmes model" was accepted<ref name="holmes2">PMID:2395461</ref>. In contrast, the G-actin structure has been determined independently over 30 times. A higher resolution F-actin model was only recently deposited in the PDB databank in December 2008 by Oda et al. <ref name="oda">PMID:19158791</ref>.

=== F-actin Monomer and Polymer ===
<scene name='Sandbox_154/2zwh_regions/2'>Filamentous Actin Unit (F-actin)</scene> ([[2zwh]])

==== Monomer ====
Each F-actin monomeric unit has, as part of its tertiary structure, several loops that are important to its assembly to the polymeric F-actin. These loops undergo conformational changes based on the state of the bound nucleotide or they serve as regions for adjacent monomeric actin units to bind to. The <scene name='Sandbox_154/2zwh_regions_hic73/1'>sensor loop</scene> acts as a "switch" for conformations, based on the bound nucleotide<ref name"oztug">PMID:19900461</ref>. The DNAse I-binding loop <scene name='Sandbox_154/2zwh_regions_dloop/1'>(D-loop)</scene> residues (40-50) in addition to undergoing conformational changes that impact stability bind DNAse I enzymes and are speculated to keep a hold on the DNAse I<ref name="Graceffa">PMID:12813032</ref>. The hydrophobic loop <scene name='Sandbox_154/2zwh_regions_hloop/1'>(H-loop)</scene> , spanning residues 264-273, and the <scene name='Sandbox_154/2zwh_regions_wloop/1'>W-loop</scene>, spanning residues 165-172, function as sites to which adjacent actin monomer D-loops may bind to<ref name="oztug">PMID:19900461</ref>. A similar function is noted for the <scene name='Sandbox_154/2zwh_regions_cterm/2'>C-terminus</scene> residues (374-375).

The F-actin molecule as shown here consists of 375 residues(43kDa) and two ligands, ADP and Ca<sup>2+</sup>. It has two major domains separated by a nucleotide-binding cleft<ref name="oda" />. Depending on the state of the bound nucleotide, the most stable conformation of F-actin changes. In its ATP and ADP + Pi nucleotide bound states, it has a closed binding cleft. In its ADP only bound state, it has a wider binding cleft<ref name="Pfaendtner"/>A characteristic trait of actin is that the domains remain twisted relative to one another, despite the nucleotide-state-dependent conformational changes<ref name="oda" />. 

[[Image:Actin filament atomic model.png|thumb|F-Actin polymer (based on Ken Holmes F-actin structure)<ref name="Holmes1">PMID:19158779</ref>|right]]

==== Polymer ==== 

F-actin has the appearance of two right-handed helices, with a gradual twist around one another. It is actually composed of repeats of 13 actin units for every 6 left-handed turns, spanning a length of 350 Å<ref name="Holmes2">PMID:2395461</ref>. 

=== Nucleotide-State-Dependent Conformational Changes ===
The state of the bound phosphorylated nucleotide affects what conformation the F-actin monomer undertakes. The presence of a gamma-phosphate in the active site causes the rotation of a Ser14 residue. This change leads to a methylated histidine (HIC73) becoming shifted, which alters the F-actin active site and causes a conformational change in the D-loop. The HIC73 is located in the "sensor loop", or the "switch" for linking changes in bound nucleotide to conformational changes<ref name="Graceffa"/>. In ATP-actin and ADP-P<sub>i</sub>-actin, the D-loop is unstructured. In the ADP-bound form of F-actin, an alpha helix is commonly apparent in the D-loop of the monomer<ref name="Pfaendtner"/><ref name="Graceffa"/>.  

Although the alpha-helix is not observed in this Oda model of F-actin and is not seen in some other F-actin studies<ref name="oda">PMID:19158791</ref><ref name=”Dalhaimer”>PMID:18155236</ref>, it is acknowledged by Oda et. al that the experimental results could have lead to an extended alpha-helix in the model, as opposed to an extended disordered strand as the interacting segment between F-actin monomeric units<ref name="oda"/>. 

==== Domains ====
<scene name='Sandbox_154/2zwh_black_domains/1'>Filamentous Actin (F-actin)</scene> ([[2zwh]])

The structure of a single unit of F-actin arises from one polypeptide chain with two domains. The nucleotide binding cleft, site of ATP hydrolysis, can be observed between the two domains. Movement of the domains allows for the open and closed F-actin conformations. 
Domain movement is made possible by rotation about the <scene name='Sandbox_154/2zwh_helix_domains_2/1'> peptide bonds of residues 141-142 and 335-336</scene>, shown in purple. According to Oda et al., during the transition from G- to F- actin, Domain 2 is believed to tilt 20&deg; and fit itself with Domain 1, thus giving a flatter conformation than the free G-actin. It is not certain whether this flattening occurs before or after ATP hydrolysis<ref name="oda" />. Holmes<ref name="Holmes2">PMID:2395461</ref> provides a simplified image of this domain movement and flattening[http://www.nature.com/nature/journal/v457/n7228/fig_tab/457389a_F2.html].

==== Stability ====
The flattened folded form of F-actin requires different stabilization mechanisms than the free monomeric G-actin form. Stability of the F-actin complex is achieved by a series of <scene name='Sandbox_154/2zwh_saltbridge/1'>salt bridge formations</scene> involving arginine 206, 183, 177 (purple); glutamate 72(blue), aspartate 187(green), 179 and 4-methyl histidine 73(yellow). Additional stability is believed to arise from a break in the interaction between residues <scene name='Sandbox_154/2zwh_leu_val/2'>108-111 and Val165 and Ile175</scene> in the same half of their respective domains to a new interaction between <scene name='Sandbox_154/2zwh_leu_thr/2'>Leu110 and Thr194</scene> where a much greater distance is observed between them<ref name="oda" />.

Once the P<sub>i</sub> is released, a conformational change on the D-loop results in the “softening” of the F-actin filament. That is, it makes the ADP-actin monomer more unstable and makes it more susceptible to cleavage <ref name=”Pfaedtner2”>PMID: 19931282</ref>

==== Active Site ====
Upon actin binding on the plus end of the actin filament, the ATPase function is activated. The conformational change from G- to F- actin promotes the catalytic activity because of the 20&deg; shift leading to a more closed binding site; this conformational change is stabilized also by the diagonal subdomain interaction between Leu110 and Thr194<ref name="oda"/>. 
As a result of these conformational changes, the <scene name='Sandbox_154/2zwh_black_domains_gln137/1'>Gln137 residue</scene> of actin is moved closer to the ATP-Ca<sup>2+</sup> ligand. Gln137 holds a water molecule, and placing it in close proximity to ATP allows for cleavage of the gamma-phosphate. Release of the inorganic phosphate occurs via the conformational change of the flexible "D-loop" into an ordered alpha-helix (though not demonstrated by this model)<ref name="Graceffa"/>.

== Function == 
F-actin performs a structural, mechanical, and enzymatic role within eukaryotic cells. These functions are not necessarily exclusive of one another. 

The dynamic functions of f-actin are heavily involved with [http://en.wikipedia.org/wiki/Cell_migration/ cell migration]<ref name="stricker">PMID:19913792</ref>.

==== [http://en.wikipedia.org/wiki/Cytoskeleton#Actin_filaments_.2F_Microfilaments Cytoskeleton] ==== 
F-actin is the most abundant component of the cytoskeleton of eukaryotes. It provides large amounts of tensile strength, considering its thin size. In cases where the flexibility is not desirable as a structural component, crosslinkages can be formed between F-actin polymers to give greater stiffness and support<ref name="Mitchinson"/>. 

Elongation of F-actin branches leads to the phenomenon of pushing of the plasma membrane forward in lamellopodial and filopodial extension<ref name="Chen"/>. This process relies on the dynamic equilibrium state in which G- and F-actin exist, as it is the continual polymerization of actin units on the leading edge that propels the membrane extension. Without the enzymatic ATPase function of F-actin, this process would not be possible.

==== [http://en.wikipedia.org/wiki/Actin#Actomyosin_filaments Actin-Myosin] ==== 
The relatively flatter shape of F-actin as compared to G-actin allows myosin to preferentially bind F-actin over G-actin. This means that F-actin, not G-actin, is the functional form of actin. It composes a large part of the thin filaments in conjunction with mysoin to give muscle contractions<ref name="Holmes2"/><ref name="Holmes3>PMID:14508495</ref>. The structure of F-actin gives it large resistance to extensive forces, such as those experienced in muscle contraction<ref name="Mitchinson"/>.
</StructureSection>
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==3D structures of actin==

[[Actin]]

==References==
<references/>