CASEIN KINASE I-LIKE PROTEIN-KINASES ENCODED BY YCK1 AND YCK2 ARE REQUIRED FOR YEAST MORPHOGENESIS

Casein kinase I is an acidotropic protein kinase class that is widely distributed among eukaryotic cell types. In the yeast Saccharomyces ce revisiae, the casein kinase I isoform encoded by the gene pair YCK1 an d YCK2 is a 60- to 62-kDa membrane-associated form. The Yck proteins p erform functions essential for growth and division; either alone suppo rts growth, but loss of function of both is lethal. We report here tha t casein kinase I-like activity is associated with a soluble Yck2-13-g alactosidase fusion protein in vitro and that thermolabile protein kin ase activity is exhibited by a protein encoded by fusion of a temperat ure-sensitive yck2 allele with lacZ. Cells carrying the yck2-2ts allel e arrest at restrictive temperature with multiple, elongated buds cont aining multiple nuclei. This phenotype suggests that the essential fun ctions of the Yck proteins include roles in bud morphogenesis, possibl y in control of cell growth polarity, and in cytokinesis or cell separ ation. Further, a genetic relationship between the yck2ts allele and d eletion of CDC55 indicates that the function of Yck phosphorylation ma y be related to that of protein phosphatase 2A activity.