Coordination of actin plus-end dynamics by IQGAP1, formin, and capping protein.

Cell processes require precise regulation of actin polymerization at filament plus ends to execute normal functions. The detailed mechanisms used to control filament assembly at plus ends in the presence of diverse and often opposing regulators is not clear. Here we explore and identify residues important for the plus-end related activities of IQGAP1. In multi-wavelength TIRF assays, we directly visualize dimers of IQGAP1, mDia1, and CP on filament ends alone and as a multicomponent end binding complex. IQGAP1 promotes the turnover of end-binding proteins reducing the dwell times of CP, mDia1, or mDia1-CP 'decision complexes' by 8-18-fold. Loss of these activities in cells disrupts actin filament arrays, morphology, and migration. Together, our results reveal a role for IQGAP1 in promoting protein turnover on filament ends and provide new insights into how actin assembly is regulated in cells.