Cofilin controls actin network identity by sorting actin binding proteins to distinct cytoskeletal structures

Proper cell physiology requires the co-assembly of multiple actin cytoskeletal networks that are tailored for specific functions. To maintain and promote the different functions of these networks, cells decorate them with distinct types of actin binding proteins (ABPs). While various models have been proposed to explain this selective sorting of ABPs, the role of actin disassembly factors is less well understood. Here, we used inducible CRISPR interference and quantitative live-cell imaging to test how disassembly factors control the ABP composition of different networks. We found that knockdown of cofilin (Cof1), a potent and highly conserved disassembly factor, disrupts the size, organization, and ABP composition of actin networks. Specifically, defects in Cof1-mediated disassembly disrupt intracellular transport due to the assembly of overgrown and disordered branched actin networks that are inappropriately decorated by tropomyosin (Tpm1). Contrary to prevailing models of ABP sorting, these networks are co-decorated by Tpm1 and fimbrin (Sac6), and their assembly is independent of formin activity. Instead, our findings support a model wherein failure to maintain the proper architecture of branched actin networks drives mis-localization of network-specific ABPs. Together, this work demonstrates that actin disassembly factors play a critical role in maintaining cytoskeletal structure and function to regulate ABP sorting across distinct networks.