Importance of free actin filament barbed ends for Arp2/3 complex function in platelets and fibroblasts.

We investigated the effect of actin filament barbed end uncapping on Arp23 complex function both in vivo and in vitro. Arp23 complex redistributes rapidly and uniformly to the lamellar edge of activated wild-type platelets and fibroblasts but clusters in marginal actin filament clumps in gelsolin-null cells. Treatment of gelsolin-null platelets with the negative dominant N-WASp C-terminal CA domain has no effect on their residual actin nucleation activity, placing gelsolin actin filament severing, capping, and uncapping function upstream of Arp23 complex nucleation. Actin filaments capped by gelsolin or the gelsolin homolog CapG fail to enhance Arp23 complex nucleation in vitro, but uncapping of the barbed ends of these actin filaments restores their ability to potentiate Arp23 complex nucleation. We conclude that Arp23 complex contribution to actin filament nucleation in platelets and fibroblasts importantly requires free barbed ends generated by severing and uncapping.