THE YEAST CENTRIN, CDC31P, AND THE INTERACTING PROTEIN-KINASE, KIC1P,ARE REQUIRED FOR CELL INTEGRITY

Cdc31p is the yeast homologue of centrin, a highly conserved calcium-b inding protein of the calmodulin superfamily. Previously centrins have been implicated only in microtubule-based processes. To elucidate the functions of yeast centrin, we carried out a two-hybrid screen for Cd c31p-interacting proteins and identified a novel essential protein kin ase of 1,080 residues, Kic1p (kinase that interacts with Cdc31p), Kic1 p is closely related to S. cerevisiae Ste20p and the p-21-activated ki nases (PAKs) found in a wide variety of eukaryotic organisms. Cdc31p p hysically interacts with Kic1p by two criteria; Cdc31p coprecipitated with GST-Kic1p and it bound to GST-Kic1p in gel overlay assays. Furthe rmore, GST-Kic1p exhibited in vitro kinase activity that was CDC31-dep endent. Although kid mutants were not defective for spindle pole body duplication, they exhibited a variety of mutant phenotypes demonstrati ng that Kic1p is required for cell integrity, We also found that cdc31 mutants, previously identified as defective for spindle pole body dup lication, exhibited lysis and morphological defects. The cdc31 kid dou ble mutants exhibited a drastic reduction in the range of permissive t emperature, resulting in a severe lysis defect. We conclude that Kic1p function is dependent upon Cdc31p both in vivo and in vitro. We postu late that Cdc31p is required both for SPB duplication and for cell int egrity/morphogenesis, and that the integrity/morphogenesis function is mediated through the Kic1p protein kinase.