From Proteopedia

Revision as of 15:46, 26 March 2010

F-Actin

Filamentous actin (F-actin) is also referred to as microfilament ^[1] and is a highly conserved proteinous component found near ubiquitously in eukaryotic cytoskeletons. F-actin and other actin proteins generally provide a structural role to the cell.

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 --> ADP + Pi 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 nucelotide-dependent conformational change^[2]. Because of the similarity observed in E.Coli Hsc70 and the ATPase domain of actin, it is believed there was a common ancestor between the two proteins^[3]. Prokaryotes are not known to have actin, but do however have an actin homologue, MreB, which also leads to the idea of a possible common ancestor^[4].

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.

Assembly

G-actin is the free monomeric form of actin which transitions 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. G-actin appears to have more ion ligands in its structure, and also has the ligand RHO as opposed to 4-methyl histidine as found in the F-actin structure.

Formation of F-actin is a dynamic process of assembly and disassembly. The transition between G- and F-actin begins with a stabilized oligomer of actin units forms through a nucleation-condensation type fold pattern^[5]. Addition of 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^[6]. 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"^[7]. ADP-actin monomers dissociate at the minus end and become recycled to ATP-actin so polymerization at the plus end may occur once again.

According to Oda et al.^[8], there is a 20 degree tilt in one of the domains of F-actin as compared to G-actin which gives F-actin the much flatter structure as compared to G-actin.

Structure

History of the structure

The F-actin protein was discovered by Straub in 1942. The structure was speculated based on a low-resolution x-ray crystallograph found in 1990 by Holmes et al. and over this time, despite the importance of F-actin in eukaryotic cells, this speculated structure was accepted. A higher resolution structure was only recently deposited in the PDB databank in Decemeber 2008 by Oda et al. ^[9].

Polymer F-actin

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. ^[10]. Including the ADP and Ca²⁺, the F-actin molecule as shown here consists of 377 residues (43kDa), two major domains separated by a nucleotide-binding cleft^[11]. 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^[12]. A characteristic trait of actin is that the domains remain twisted relative to one another, despite the nucleotide-state-dependent conformational changes^[13].

Domains

Domain movement is made possible by rotation about the shown in purple. These occur in an alpha-helix (Ile136-Gly146) sequence that does not belong to either domain^[14].

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 salt bridge formations involving polar and charged residues, including HIC73, a methylated histidine residue. Additional stability is believed to arise from cross-subunit interactions between Leu110 and Thr194.

Function

F-actin performs a structural, mechanical, and enzymatic role within eukaryotic cells. These functions are not necessarily distinct.

Cytoskeleton

F-actin is the most abundant component of the cytoskeleton of eukaryotes. Elongation of F-actin leads to the phenomenon of "pushing" the plasma membrane forward.

Actin-Myosin

The relatively flatter shape of F-actin as compared to G-actin means that myosin preferentially binds to F-actin and not G-actin. This means that F-actin is the functional form of actin in the thin filaments of actin used in muscle contractions caused by the actin-myoglobin relationship^[15].

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Active Site

The cleavage of the gamma-phosphoryl from the bound ATP is a result of a conformational change upon binding that moves the Gln137 residue closer to the ATP-Ca2+ ligand. Release of the inorganic phosphate occurs via the conformational change of the flexible "D-loop" into an ordered alpha-helix^[16].

References

1. ↑ Microfilament - Wikipedia, the free encyclopedia. http://en.wikipedia.org/wiki/Microfilaments. Date accessed: March 16th, 2010.
2. ↑ Graceffa
3. ↑ Holmes1
4. ↑ holmes2
5. ↑ pfaendtner
6. ↑ mitchinson
7. ↑ clasier
8. ↑ Oda
9. ↑ Oda T, Iwasa M, Aihara T, Maéda Y, and Narita A. 2009. The nature of the globular-to fibrous actin transition. Nature,457(7228):441-445. PMID: 19158791
10. ↑ Holmes, K.C., Popp, D., Gebhard, W. and Kabsch, W. 1990. Atomic model of the actin filament. Nature,347(6288):44-49. PMID: 2395461
11. ↑ oda
12. ↑ pfaendtner
13. ↑ oda
14. ↑ graceffa
15. ↑ Holmes 2, Holmes et al 3
16. ↑ graceffa