A PHOSPHORYLATION SITE MUTANT OF SCHIZOSACCHAROMYCES-POMBE CDC2P FAILS TO PROMOTE THE METAPHASE TO ANAPHASE TRANSITION

The protein kinase cdc2p is a key regulator of the G1-S and G2-M cell cycle transitions in the yeast Schizosaccharomyces pombe. Activation o f cdc2p is regulated by its phosphorylation state and by interaction w ith other proteins. We have analyzed the consequences for cell cycle p rogression of altering the conserved threonine phosphorylation site, w ithin the activation loop of cdc2p, to glutamic acid. This mutant, T16 7 E, promotes entry into mitosis, as judged by the accumulation of mit otic spindles and condensed chromosomes, despite the fact that it lack s demonstrable kinase activity both in vitro and in vivo. However, T16 7 E cannot promote the metaphase-anaphase transition. Since a componen t of the anaphase-promoting complex (APC) in S. pombe, cut9p, remains hypophosphorylated at the T167 E arrest point, the cell cycle block mi ght be due to the inability of T167 E to activate the APC. T167 E is l ethal when overexpressed, and overproduction also causes a mitotic arr est. Multicopy suppressors of the dominant negative phenotype were iso lated, and identified as cdc13(+) and suc1(+). Overexpression of suc1( +) suppresses the effects of T167 E overproduction by restoring suffic ient amounts of suc1p to the cell to allow passage through mitosis.