Spire Homolog 1 (SPIRE1)

Spire proteins, such as SPIRE1, are highly conserved between species. In vitro, Spire nucleated new filaments at a rate that is similar to that of the formin family of proteins (FMN1) but slower than in the activated Arp2/3 complex , and it remained associated with the slow-growing pointed end of the new filament. Spire contains a cluster of 4 WASP homology 2 (WH2) domains, each of which binds an actin monomer. Maximal nucleation activity requires all 4 WH2 domains along with an additional acting-binding motif, conserved among Spire proteins. Spire itself is conserved among metazoans and, together with the formin Cappuccino , is required for axis specification in oocytes and embryos, suggesting that multiple actin nucleation factors collaborate to construct essential cytoskeletal structures.