SCHIZOSACCHAROMYCES-POMBE SKP1(-KINASE RELATED TO MAMMALIAN GLYCOGEN-SYNTHASE KINASE-3 AND COMPLEMENTS A CDC14 CYTOKINESIS MUTANT() ENCODESA PROTEIN)

We report the cloning of the skp1(+) gene, a Schizosaccharomyces pombe homolog of the glycogen synthase kinase 3 (GSK-3) family whose member s in higher eukaryotes are involved in cell fate determination, nuclea r signalling, and hormonal regulation. skp1 is 67% identical to mammal ian GSK-3 beta and displays similar biochemical properties in vitro. L ike GSK-3 beta, skp1 is phosphorylated on a conserved tyrosine residue , and this phosphorylation is required for efficient activity. skp1 is also phosphorylated at a serine which has been identified as S-335. P hosphorylation at this site is likely to inhibit its function. Unlike the mammalian enzyme, skp1 both tyrosine autophosphorylates in yeast c ells and can phosphorylate other proteins on tyrosine in bacteria. The skp1(+) gene is not essential. However, cells with deletions in skp1( +) are sensitive to heat shock and exhibit defects in sporulation. Ove rexpression of wild-type skp1(+) specifically complements cdc14-118, o ne of several mutations causing a defect in cytokinesis. In addition, certain phosphorylation site mutants induce a delay or block in cytoki nesis when overexpressed. Together, these data identify novel interact ions of a fission yeast GSK-3 homolog with elements of the cytokinesis machinery.