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 and a complex of four to six polypeptides identified in Acanthamoeba that includes two actin‐related proteins. Here, we report the identification 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 defective for cell elongation and septation, suggesting that it is involved in multiple cortical actin‐requiring processes. Consistent with a role in actin cytoskeletal function, negative interactions have been identified 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 identified by sequencing projects in diverse species, and we have isolated a human cDNA highly related to sop2+, SOP2 Hs, which functionally complements the sop2–1 mutation. Sop2p proteins from all species contain peptide sequences identical or highly similar to two peptide sequences from an Acanthamoeba beta‐transducin repeat protein present in the profilin binding complex. Biochemical analyses demonstrate that Sop2p is 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 percentage of septating cells. Collectively these data demonstrate the interaction of Sop2p with Arp3p, profilin and actin.