Actin binding proteins that change extent and rate of actin monomer-polymer distribution by different mechanisms.

Actin binding proteins control actin assembly and disassembly by altering the critical concentration and by changing the kinetics of polymerization. All of these control mechanisms in some way or the other make use of the energy of hydrolysis of actin-bound ATP. Capping of barbed filament ends increases the critical concentration as long as ATP hydrolysis maintains a difference in the actin monomer binding constants of the two ends. A further increase in the critical concentration on adding a second cap, tropomodulin, to the other, pointed filament end also requires ATP hydrolysis as described by the model presented here. Changes in the critical concentration are amplified into much larger changes of the monomer pool by actin sequestering proteins, provided their actin binding equilibrium constants fall within a relatively narrow range around the values for the two critical concentrations of actin. Cofilin greatly speeds up treadmilling, which requires ATP hydroysis, by increasing the rate constant of depolymerization. Profilin increases the rate of elongation at the barbed filament end, coupled to a lowering of the critical concentration, only if ATP hydrolysis makes profilin binding to the barbed end independent of its binding constant for actin monomers.