Comparison of filamin A-induced cross-linking and Arp2/3 complex-mediated branching on the mechanics of actin filaments

We compared the effects of human filamin A (FLNa) and the activated human Arp2/3 complex on mechanical properties of actin filaments. As little as 1 FLNa to 800 polymerizing actin monomers induces a sharp concentration-dependent increase in the apparent viscosity of 24 gm actin, a parameter classically defined as a gel point. The activated Arp2/3 complex, at concentrations up to 1:25 actins had no detectable actin gelation activity, even in the presence of phalloidin, to stabilize actin filaments against debranching. Increasing the activated Arp2/3 complex to actin ratio raises the FLNa concentration required to induce actin gelation, an effect ascribable to Arp2/3-mediated actin nucleation resulting in actin filament length diminution. Time lapse video microscopy of microparticles attached to actin filaments or photoactivation of fluorescence revealed actin filament immobilization by FLNa in contrast to diffusion of Arp2/3-branched actin filaments. The experimental results support theories predicting that polymer branching absent cross-linking does not lead to polymer gelation and are consistent with the observation that cells deficient in actin filament cross-linking activity have unstable surfaces. They suggest complementary roles for actin branching and cross-linking in cellular actin mechanics in vivo.