Acts as an actin nucleation factor, remains associated with the slow-growing pointed end of the new filament. Involved in intracellular vesicle transport along actin fibers, providing a novel link between actin cytoskeleton dynamics and intracellular transport. Required for asymmetric spindle positioning and asymmetric cell division during oocyte meiosis. Required for normal formation of the cleavage furrow and for polar body extrusion during female germ cell meiosis. Also acts in the nucleus: together with fmn2, promotes assembly of nuclear actin filaments in response to DNA damage in order to facilitate movement of chromatin and repair factors after DNA damage. Binds to actin monomers via the WH2 domain. The Spir-box targets binding to intracellular membrane structures. Belongs to the spire family.