The Cbk1p pathway is important for polarized cell growth and cell separation in Saccharomyces cerevisiae

During the early stages of budding, cell mall remodeling and polarized secr etion are concentrated at the bud tip (apical growth). The CBK1 gene, encod ing a putative serine/threonine protein kinase, was identified in a screen designed to isolate mutations that affect apical growth. Analysis of cbk1 D elta cells reveals that Cbk1p is required for efficient apical growth, prop er mating projection morphology, bipolar bud site selection in diploid cell s, and cell separation. Epitope-tagged Cbk1p localizes to both sides of the bud neck in late anaphase, just prior to cell separation. CBK1 and another gene, HYM1, were previously identified in a screen for genes involved in t ranscriptional repression and proposed to function in the same pathway. Del etion of HYM1 causes phenotypes similar to those observed in cbk1 Delta cel ls and disrupts the bud neck localization of Cbk1p. Whole-genome transcript ional analysis of cbk1 Delta suggests that the kinase regulates the express ion of a number of genes,vith cell mall-related functions, including two ge nes required for efficient cell separation: the chitinase-encoding gene CTS 1 and the glucanase-encoding gene SCW11. The Ace2p transcription factor is required for expression of CTS1 and has been shown to physically interact,v ith Cbklp, Analysis of ace2 Delta cells reveals that Ace2p is required for cell separation but not for polarized growth. Our results suggest that Cbk1 p and Hym1p function to regulate two distinct cell morphogenesis pathways: an ACE2-independent pathway that is required for efficient apical growth an d mating projection formation and an ACE2-dependent pathway that is require d for efficient cell separation following cytokinesis. Cbk1p is most closel y related to the Neurospora crassa Cot-1; Schizosaccharomyces pombe Orb6; C aenorhabditis elegans, Drosophila, and human Ndr; and Drosophila and mammal ian WARTS/LATS kinases. Many Cbk1-related kinases have been shown to regula te cellular morphology.