FISSION YEAST SOP2P - A NOVEL AND EVOLUTIONARILY CONSERVED PROTEIN THAT INTERACTS WITH ARP3P AND MODULATES PROFILIN FUNCTION

Profilins bind to monomeric actin and also interact with ligands such as phosphoinositide 4,5-bisphosphate, the proline-rich protein VASP an d a complex of four to six polypeptides identified in Acanthamoeba tha t includes two actin-related proteins, Here, we report the identificat ion and characterization of an essential gene from Schizosaccharomyces pombe, sop2(+), a mutation in which rescues the temperature-sensitive lethality of a profilin mutation, cdc3-124, The sop2-1 mutant is defe ctive for cell elongation and septation, suggesting that it is involve d in multiple cortical actin-requiring processes, Consistent with a ro le in actin cytoskeletal function, negative interactions have been ide ntified between sop2-1 and act1-48, a mutant allele of actin, Sop2p is a novel 377 amino acid polypeptide with similarity to proteins of the beta-transducin repeat family. Sop2p-related proteins have been ident ified by sequencing projects in diverse species, and we have isolated a human cDNA highly related to sop2(+), SOP2 Hs, which functionally co mplements the sop2-1 mutation, Sop2p proteins from all species contain peptide sequences identical or highly similar to two peptide sequence s from an Acanthamoeba beta-transducin repeat protein present in the p rofilin binding complex, Biochemical analyses demonstrate that Sop2p i s present in a complex which also contains the actin-related protein, Arp3p, Immunofluorescence studies reveal the presence of Sop2p in (i) punctate structures distributed throughout the cell, (ii) cables that extend the length of the cell, and (iii) a medial band in a small perc entage of septating cells, Collectively these data demonstrate the int eraction of Sop2p with Arp3p, profilin and actin.