Cell cycle regulation of yeast morphogenesis

Cell division is a highly regulated process in eukaryotic organisms including multicellular plants and animals and unicellular microbes. Of these diverse species, the unicellular yeast S. cerevisiae is the most well suited for genetic analyses. S. cerevisiae exhibits distinct morphological changes during each stage of cell cycle progression facilitating its study.; The yeast cell cycle, as in other eukaryotes, is controlled by cyclin dependent kinase (CDK). CDC28 is the major yeast CDK that modulates each cell cycle transition by phosphorylation of protein substrates. In turn, CDK is positively and negatively regulated by multiple mechanisms.; Although yeast normally persist as circular cells forming round colonies, delay in the onset of chromosome separation (mitosis), leads to cellular elongation. Propagation of polarized cells produces a filamentous colony appearance in which pseudohyphae extend away from the colony center. This morphological transition is regulated by environmental signals that activate the Ras2-mediated Mitogen Activated Protein Kinase (MAPK) and cAMP-Protein Kinase A (PKA) cascades. Few targets of these pathways have been identified.; We hypothesized that the cell cycle machinery may be a target in signaling of filamentous growth. Toward understanding these mechanisms, we identified cdc28 alleles that exhibit constitutive filamentation. The mutations localized throughout the three dimensional structure of Cdc28 including the cyclin and ATP binding sites suggesting multiple determinants of CDK function during mitotic entry. Many of these mutants exhibited different but specific genetic interactions with components of Ras2-mediated pathways and cell cycle machinery.; A large number of the substitutions also altered the binding site of Cks1, a highly conserved, essential CDK subunit, suggesting a role for Cks1 in morphogenesis. The phenotypes of one mutant, cdc28-E217V, that diminished Cks1 association, depended on the presence of SWE1 , a CDK inhibitory tyrosine kinase. Genetic and biochemical tests revealed that the Ras2-cAMP-PKA pathway potentiates Swe1 inhibition of mitotic Cdc28 complexes, but that Cks1 is critical for Swe1 downregulation. These mechanisms of cell cycle control may have implications for regulation of mammalian CDKs. Furthermore, detailed analyses of other cdc28 mutants may elucidate additional mechanisms of regulation.