Regulates actin polymerization by stimulating the actin-nucleating activity of the Arp2/3 complex. Involved in mitosis and cytokinesis, via its role in the regulation of actin polymerization. Binds to HSF1/HSTF1 and forms a complex on heat shock promoter elements (HSE) that negatively regulates HSP90 expression. Plays a role in dendrite spine morphogenesis.