Profilin

Profilin is an actin-binding protein involved in cytoskeleton dynamics. It is found in most eukaryotic cells and in mammalian cells two kinds of profilin have been discovered; Profilin I and II. Profilin I is expressed in most tissues except skeletal muscle, heart and brain, and profilin II expressed in brain, skeletal muscle and kidney. In general, profilin is responsible for the growth and stabilization of actin filaments and is important in the restructuring of microfilaments. This restructuring of the actin cytoskeleton is used by the cell to allow the cell to move, and to adjust its shape according to its needs.

Mechanisms of enhancing actin growth
Profilin enhances actin growth in two ways.


 * Profilin binds to monomeric actin on the plus end of the filament inducing a shape change of the actin subunit, allowing the G-actin to replace the ADP to which it is bound by ATP and form F-actin. The F-actin then forms a heterodimer which can bind to the plus end of an Actin filament. In the process of binding to the actin monomers it also stereochemically inhibits addition to the - end.


 * Secondly, following a conformational change in actin, profilin dissociates and competes for monomers with the inhibitory thymosin which sequesters the actin. Profilin is found in the cytosolic phase of the cell membrane where it can recruit monomers to grow filaments from the cortex.

Regulation
Profilin acts to regulate polymerization of actin monomers by sequestering G-actin. Profilin binds to minus ends of an actin subunit. Thymosin beta4 binds G-actin that profilin released and prevents it from binding to the plus end of a filament and thereby removing it from the concentration of the pool of free monomers that can form filaments. Thymosin beta4 can regulate concentration of ATP bound monomers and therefore the amount of actin polymerization in the cell as polymerization of these monomers requires the release of profilin from actin monomer.

Profilin can also regulate polymerization by binding to the ends of the actin filaments rather than the monomers. Once the actin/profilin complex has formed, phosphoinositides can break the complex apart, releasing the ATP bound monomer and inducing polymerization.

PIP2 has also been determined to be a regulatory mechanism on profilin, regulating the association of profilin with actin.

Effects on cofilin
Profilin acts to reverse the effects of cofilin, an actin depolymerization factor that binds to actin filaments resulting in an increase in dissociation of monomers from the polymer. The cofilin stays associated with actin monomers and prevents reassembly. When profilin binds to G-actin, the subunit will dissociate from cofilin after an exchange of ADP for ATP.