The CDK-activating kinase CAK1 can dosage suppress sporulation defects of smk1 MAP kinase mutants and is required for spore wall morphogenesis in Saccharomyces cerevisiae.

Mitogen-activated protein (MAP) kinase pathways are evolutionarily conserved kinase cascades that are required for the response of eukaryotic cells to a wide variety of environmental stimuli. MAP kinase pathways are also required for the execution of developmental and differentiative programs in a variety of cell and tissue types. SMK1 encodes a developmentally regulated MAP kinase in yeast that is required for spore wall morphogenesis. Cyclin-dependent kinase-activating kinases (CAKs) phosphorylate a conserved threonine residue in the activating loop of cyclin-dependent kinases. CAK1 encodes the major CAK activity in yeast and is required for cell cycle progression. The work presented here demonstrates that CAK1 functions positively in the spore wall morphogenesis pathway. First, CAK1 has been isolated as a dosage suppressor of a conditional smk1 mutant that is defective for spore wall morphogenesis. Second, CAK1 mRNA accumulates during spore development contemporaneously with SMK1 mRNA. Third, cak1 mutant strains have been isolated that are able to complete meiosis I and II but are specifically defective in assembly of the spore wall. These results show that cell cycle progression and morphogenetic pathways can be regulated by a single gene product and suggest mechanisms for coordinating these processes during development.