Profilin-1 Serves as a Gatekeeper for Actin Assembly by Arp2/3-Dependent and -Independent Pathways

Cells contain multiple F-actin assembly pathways, including the Arp2/3 complex, formins, and Ena/VASP, which have largely been analyzed separately. They collectively generate the bulk of F-actin from a common pool of G-actin; however, the interplay and/or competition between these pathways remains poorly understood. Using fibroblast lines derived from an Arpc2 conditional knockout mouse, we established matched-pair cells with and without the Arp2/3 complex. Arpc2−/− cells lack lamellipodia and migrate more slowly than WT cells but have F-actin levels indistinguishable from controls. Actin assembly in Arpc2−/− cells was resistant to cytochalasin-D and was highly dependent on profilin-1 and Ena/VASP but not formins. Profilin-1 depletion in WT cells increased F-actin and Arp2/3 complex in lamellipodia. Conversely, addition of exogenous profilin-1 inhibited Arp2/3 complex actin nucleation in vitro and in vivo. Antagonism of the Arp2/3 complex by profilin-1 in cells appears to maintain actin homeostasis by balancing Arp2/3 complex-dependent and -independent actin assembly pathways. [Display omitted] •Arpc2 conditional knockout mice are used to derive cells lacking the Arp2/3 complex•Arpc2−/− and wild-type cells show similar ratios of F-actin to G-actin•Actin assembly without Arp2/3 complex relies on profilin and Ena/VASP, not formins•Profilin-1 inhibits Arp2/3 complex-dependent actin nucleation in wild-type cells Rotty et al. find that cells lacking a functional Arp2/3 complex compensate by enhancing profilin-dependent F-actin polymerization through Ena/VASP. They also present evidence that profilin-1 inhibits nucleation of actin branches by the Arp2/3 complex, suggesting that cytosolic actin polymerization is achieved, in part, by balancing Arp2/3-dependent and profilin-dependent pathways.