Evidence for monomeric actin function in INO80 chromatin remodeling

Actin has well-established functions in the cytoplasm, but its roles in the nucleus remain poorly defined. Here, by studying the nuclear actin-containing yeast INO80 chromatin remodeling complex, we provide genetic and biochemical evidence for a role of monomeric actin in INO80 chromatin remodeling. In contrast to cytoplasmic actin, nuclear actin is present as a monomer in the INO80 complex and its barbed end is not accessible for polymerization. An actin mutation affecting in vivo nuclear functions is identified in subdomain 2, which reduces the chromatin remodeling activities of the INO80 complex in vitro. Importantly, the highly conserved subdomain 2 at the pointed end of actin contributes to INO80 interactions with chromatin. Our results establish an evolutionarily conserved function of nuclear actin in its monomeric form and suggest that nuclear actin can utilize a fundamentally distinct mechanism from cytoplasmic actin.