Practice question

Cellular actin turnover measured by fluorescence recovery after photobleaching half time 30 seconds versus minutes in vitro requires accelerated disassembly and reassembly. Treadmilling rate defined by addition at barbed end balanced by loss at pointed end, but spontaneous off rate modest. Accessory proteins increase both fluxes dramatically. Cofilin family severs ADP rich filament segments cooperatively, generating numerous short filaments exposing many pointed ends that depolymerize rapidly ten fold higher off rate. Cofilin also promotes debranching and filament twisting destabilizing contacts. Resulting ADP G actin released bound to cofilin has low affinity for polymerization. Profilin then catalyzes nucleotide exchange by opening actin nucleotide cleft lowering ADP affinity hundred fold, converting to ATP G actin. Profilin ATP actin complex adds preferentially to barbed end, especially when delivered by formin FH1 polyproline tracts, completing cycle. Together they accelerate subunit flux 50 to 100 fold. Tubulin builds microtubules, dynein microtubule motor, myosin generates contraction not treadmilling, nebulin stabilizes muscle thin filaments, tropomyosin protects from cofilin, none accelerate both ends simultaneously like cofilin profilin pair.