SCHIZOSACCHAROMYCES-POMBE CDC4(-HAND PROTEIN ESSENTIAL FOR CYTOKINESIS() GENE ENCODES A NOVEL EF)

Schizosaccharomyces pombe cells divide by medial fission. One class of cell division mutants (cdc), the late septation mutants, defines four genes: cdc3, cdc4, cdc8, and cdc12 (Nurse, P., P. Thuriaux, and K. Na smyth. 1976. Mol. & Gen. Genet. 146:167-178). We have cloned and chara cterized the cdc4 gene and show that the predicted gene product, Cdc4p , is a 141-amino acid polypeptide that is similar in sequence to EF-ha nd proteins including myosin light chains, calmodulin, and troponin C. Two temperature-sensitive lethal alleles, cdc4-8 and cdc4-31, accumul ate multiple nuclei and multiple improper F-actin rings and septa but fail to complete cytokinesis. Deletion of cdc4 also results in a letha l terminal phenotype characterized by multinucleate, elongated cells t hat fail to complete cytokinesis. Sequence comparisons suggest that Cd c4p may be a member of a new class of EF-hand proteins. Cdc4p localize s to a ringlike structure in the medial region of cells undergoing cyt okinesis. Thus, Cdc4p appears to be an essential component of the F-ac tin contractile ring. We find that Cdc4 protein forms a complex with a 200-kD protein which can be cross-linked to UTP, a property common to myosin heavy chains. Together these results suggest that Cdc4p may be a novel myosin light chain.