Fig. 4.

Sequence of steps in the transfer of actin from a proline-rich (a) or WH2 (b) domain of a membrane-associated nucleation-promoting factor onto the end of an actin filament. a A profilin-actin complex (linked circles) binds to the proline-rich (yellow) region (step 1), the bound profilin-actin complex attaches to the barbed end of an actin filament (step 2), the terminal actin monomer undergoes a rapid conformational change (step 3), and the profilin-poly-proline complex dissociates from the barbed end of the filament (step 4). In the absence of a conformational change in the terminal actin subunit, dissociation of the profilin is slow. b Monomeric actin (open circle) binds to the WH2 (red) sequence (step 1), the WH2-bound actin monomer attaches to the barbed end of a nearby filament (step 2), the terminal actin monomer undergoes a rapid conformational change (step 3), and the WH2 domain rapidly dissociates from the filament barbed end (step 4). Effective polymerase activity requires that the local density of PWCA domains be high enough to support frequent monomer transfer events