Fission yeast cells use Arp2/3 complex and formin to assemble diverse filamentous actin (F-actin) networks within a common cytoplasm for endocytosis, division, and polarization. Although these homeostatic F-actin networks are usually investigated separately, competition for a limited pool of actin monomers (G-actin) helps to regulate their size and density. However, the mechanism by which G-actin is correctly distributed between rival F-actin networks is not clear. Using a combination of cell biological approaches and in vitro reconstitution of competition between actin assembly factors, we found that the small G-actin binding protein profilin directly inhibits Arp2/3 complex-mediated actin assembly. Profilin is therefore required for formin to compete effectively with excess Arp2/3 complex for limited G-actin and to assemble F-actin for contractile ring formation in dividing cells.
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