Acts as an actin nucleation factor, remains associated with the slow-growing pointed end of the new filament. Promotes dissociation of capu from the barbed end of actin filaments. Involved in intracellular vesicle transport along actin fibers, providing a novel link between actin cytoskeleton dynamics and intracellular transport. Required for localization of determinants within the developing oocyte to the posterior pole and to the dorsal anterior corner. Links Rho family signaling and Jnk function to the actin cytoskeleton (By similarity). Interacts with bsk, Rho1, Rac1, Cdc42 and wash. Interacts with capu (By similarity). Punctate spots in perinuclear region and cytoplasm. Binds to actin monomers via the WH2 domain. The Spir-box targets binding to intracellular membrane structures. Phosphorylated by Jnk kinase (bsk). Belongs to the spire family.