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

Cells have many types of actin structures, which must assemble from a common monomer pool. Yet, it remains poorly understood how monomers are distributed to and shared between different filament networks. Simplified model systems suggest that monomers are limited and heterogeneous, which alters actin network assembly through biased polymerization and internetwork competition. However, less is known about how monomers influence complex actin structures, where different networks competing for monomers overlap and are functionally interdependent. One example is the leading edge of migrating cells, which contains filament networks generated by multiple assembly factors. The leading edge dynamically switches between the formation of different actin structures, such as lamellipodia or filopodia, by altering the balance of these assembly factors’ activities. Here, we sought to determine how the monomer-binding protein profilin 1 (PFN1) controls the assembly and organization of actin in mammalian cells. Actin polymerization in PFN1 knockout cells was severely disrupted, particularly at the leading edge, where both Arp2/3 and Mena/VASP-based filament assembly was inhibited. Further studies showed that in the absence of PFN1, Arp2/3 no longer localizes to the leading edge and Mena/VASP is non-functional. Additionally, we discovered that discrete stages of internetwork competition and collaboration between Arp2/3 and Mena/VASP networks exist at different PFN1 concentrations. Low levels of PFN1 caused filopodia to form exclusively at the leading edge, while higher concentrations inhibited filopodia and favored lamellipodia and pre-filopodia bundles. These results demonstrate that dramatic changes to actin architecture can be made simply by modifying PFN1 availability. [Display omitted] •Actin assembly without PFN1 is severely disrupted, especially at the leading edge•PFN1 is required for Mena/VASP activity and leading-edge localization of Arp2/3•Arp2/3 and Mena/VASP internetwork dynamics are defined by PFN1 concentration•PFN1 expression level determines which leading-edge actin structures assemble How do different actin structures assemble from a common monomer pool? Skruber et al. show that the monomer-binding protein PFN1 is required for Arp2/3 and Mena/VASP network assembly at the leading edge. Altering PFN1 concentration changes how these networks compete and collaborate, driving the formation of different leading-edge actin structures.