Control of actin filament length and turnover by actin depolymerizing factor (ADF/cofilin) in the presence of capping proteins and ARP2/3 complex

The effect of Arabidopsis thaliana ADF1 and human ADF on the number of fila ments in F-actin solutions has been examined using a seeded polymerization assay. ADF did not sever filaments in a catalytic fashion, but decreased th e steady-state length distribution of actin filaments in correlation with i ts effect on actin dynamics. The increase in filament number was modest as compared with the large increase in filament turnover. ADF did not decrease the length of filaments shorter than 1 mu m. ADF promoted the rapid turnov er of gelsolin-capped filaments in a manner dependent on the number of poin ted ends. To explain these results, we propose that, as a consequence of th e cooperative binding of ADF to F-actin, two populations of energetically d ifferent filaments coexist in solution pending a flux of subunits from one to the other. The ADF-decorated filaments depolymerize rapidly from their p ointed ends, while undecorated filaments polymerize. ADF also promotes rapi d turnover of gelsolin-capped filaments in the presence of the pointed end capper Arp2/3 complex. It is shown that the Arp2/3 complex steadily generat es new barbed ends in solutions of gelsolin-capped filaments, which represe nts an important aspect of its function in actin-based motility.