Arp2/3 and Mena/VASP Require Profilin 1 for Actin Network Assembly at the Leading Edge

Abstract

There are many different actin networks within a cell, but it is poorly understood how they assemble from a common monomer pool. One emerging concept is that monomers are limited and heterogenous, which manipulates network assembly through biased polymerization and internetwork competition. Previous work demonstrated that actin bound to the monomer-binding protein profilin favors filament assembly by formins and Mena/VASP over Arp2/3. Although, profilin can transfer monomers to Arp2/3 networks through nucleation promoting factors. It remains largely unknown how profilin influences network behavior in complex environments where multiple assembly factors are present. Here, we assess assembly factor dependence on profilin 1 (PFN1) in mammalian cells by carefully controlling PFN1 concentration. We demonstrate that PFN1 determines global actin assembly, organization, and network homeostasis. At the leading edge, PFN1 is required for both Arp2/3 and Mena/VASP activity, with discrete stages of internetwork competition and collaboration occurring at different PFN1 concentrations. These results demonstrate that the majority of actin assembly is gated by PFN1 and that dramatic changes to actin architecture can be made by modifying PFN1 availability.