Sandbox 154: Difference between revisions

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_i 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<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 ancestory<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>.  

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 Pi bound state. Subsequent loss of a Pi 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.

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"/>. 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<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>pfaendtner</ref>. 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" />.

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 HIC73 (4-methyl histidine) 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_i-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="pfaendter"/><ref name="Graceffa"/>. Although the alpha-helix is not observed in this Oda model of F-actin, it is acknowkledged by Oda et. al that the experimental results could have lead to an extended alpha-helix in the model<ref name="oda"/>.  

As a result of these conformational changes, the Gln137 residue of actin is moved closer to the ATP-Ca²⁺ ligand. Gln137 holds a water molecule, and placing it in close proximity to ATP allows for the gamma-phosphate to become cleaved. Release of the inorganic phosphate occurs via the conformational change of the flexible "D-loop" into an ordered alpha-helix<ref name="Graceffa"/>.

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 strucutral 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 equlibrium 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.  

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 is the functional form of actin composing a large part of the thin filaments that function in muscle contraction<ref name="Holmes"2><ref name="Holmes3>PMID:14508495</ref>. Additionally, the structure of F-actin gives it large resistance to extensive forces, such as those experienced in muscle contraction<ref name="Mitchinson">.