Practice question

Actin nucleotide state dictates polymerization competence. ATP G actin incorporates into barbed end with high affinity Kd 0.1 micromolar, ADP G actin weak Kd 1 micromolar and depolymerizes. After filament turnover ADP actin accumulates needing recharge. Profilin, 15 kilodalton ubiquitous protein present at 50 to 100 micromolar, binds actin subdomain 1 and 3 cleft with 1 to 1 stoichiometry. Structural studies show binding opens nucleotide binding pocket between subdomains 2 and 4, reducing affinity for ADP 1000 fold and accelerating its dissociation rate from 0.02 to 1 per second, allowing abundant cellular ATP 1 to 5 millimolar to bind. Resulting profilin ATP actin complex sterically inhibited from adding to pointed end due to clash with incoming filament, but adds efficiently to barbed end, upon incorporation profilin falls off. Profilin also binds poly L proline stretches via aromatic cradle, targeting complex to enabled VASP and formin FH1 for processive elongation. Thymosin beta4 sequesters ATP actin blocking all assembly, tropomodulin caps minus end, formin nucleates without exchange activity, making profilin unique catalyst of ADP to ATP exchange promoting polymerization.