Acts as an actin nucleation factor, remains associated with the slow-growing pointed end of the new filament (PubMed:21620703, PubMed:21983562). Involved in intracellular vesicle transport along actin fibers, providing a novel link between actin cytoskeleton dynamics and intracellular transport (PubMed:21983562). Required for asymmetric spindle positioning and asymmetric cell division during oocyte meiosis (PubMed:21620703). Required for normal formation of the cleavage furrow and for polar body extrusion during female germ cell meiosis (PubMed:21620703). Interacts with FMN2. Punctate spots in perinuclear region and cytoplasm, co-localized with Rab11 (PubMed:21983562). Detected at the cleavage furrow during asymmetric oocyte division and polar body extrusion (PubMed:21620703). Expressed in the developing nervous system and in the adult brain, specifically the Purkinje cells of the cerebellum, in neuronal cells in the CA1, CA2 and CA3 fields of the hippocampus and granular layer of the dentate gyrus (PubMed:15053972). Detected in oocytes (PubMed:21620703). Expressed in embryos and adults. Binds to actin monomers via the WH2 domain. The Spir-box targets binding to intracellular membrane structures. Belongs to the spire family.