GENETIC-ANALYSIS OF CLN CDC28 REGULATION OF CELL MORPHOGENESIS IN BUDDING YEAST

The CLN1, CLN2 and CLN3 gene family of G1-acting cyclin homologs of Sa ccharomyces cerevisiae is functionally redundant: any one of the three Cln proteins is sufficient for activation of Cdc28p protein kinase ac tivity for cell cycle START. The START event leads to multiple process es (including DNA replication and bud emergence); how Cln/Cdc28 activi ty activates these processes remains unclear. CLN3 is substantially di fferent in structure and regulation from CLN1 and CLN2, so its functio nal redundancy with CLN1 and CLN2 is also poorly understood. We have i solated mutations that alter this redundancy, making CLN3 insufficient for cell viability in the absence of ClN1 and CLN2 expression. Mutati ons causing phenotypes specific for the cell division cycle were analy zed in detail. Mutations in one gene result in complete failure of bud formation, leading to depolarized cell growth. This gene was identifi ed as BUD2, previously described as a non-essential gene required for proper bud site selection but not required for budding and viability. Bud2p is probably the GTPase-activating protein for Rsr1p/Bud1p [Park, H., Chant,I. and Herskowitz,I. (1993) Nature, 365, 269-274]; we rind t hat Rsr1p is required for the bud2 lethal phenotype. Mutations in two other genes (ERC10 and ERC19) result in a different morphogenetic defe ct: failure of cytokinesis resulting in the formation of long multinuc leate tubes. These results suggest direct regulation of diverse aspect s of bud morphogenesis by Cln/Cdc28p activity.