Regulates actin polymerization by stimulating the actin-nucleating activity of the Arp2/3 complex. Involved in various processes, such as mitosis and cytokinesis, via its role in the regulation of actin polymerization. Together with CDC42, involved in the extension and maintenance of the formation of thin, actin-rich surface projections called filopodia. In addition to its role in the cytoplasm, also plays a role in the nucleus by regulating gene transcription, probably by promoting nuclear actin polymerization (By similarity). 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 (By similarity). Binds actin and the Arp2/3 complex. Interacts with CDC42 (By similarity). Interacts with FCHSD1. Interacts with FCHSD2 (By similarity). Binds to SH3 domains of GRB2. Interacts with the C-terminal SH3 domain of DNMBP (By similarity). Interacts with SNX9 (By similarity). Interacts with the WW domains of PRPF40A/FBP11 (By similarity). Interacts with PTK2/FAK1 (PubMed:14676198). Interacts with PACSIN1, PACSIN2 and PACSIN3 (By similarity). Interacts with NOSTRIN. Binds to TNK2. Interacts with SNX33. Interacts with NONO (via second RRM domain); the interaction is direct. Component of a multiprotein complex with NONO and SFPQ; associates with the complex via direct interaction with NONO (By similarity). Preferentially localized in the cytoplasm when phosphorylated and in the nucleus when unphosphorylated (By similarity). Exported from the nucleus by an nuclear export signal (NES)-dependent mechanism to the cytoplasm (By similarity). Phosphorylation at Ser-239, Tyr-253, Ser-480 and Ser-481 enhances actin polymerization activity.