MAP kinases participate in signal transduction pathways that regulate myriad cellular processes, including growth and differentiation, in all eukaryotes. The SMK1 MAP kinase is required for spore morphogenesis and differentiation in Saccharomyces cerevisiae . A collection of conditional and partial-function missense mutants in smk1 has been isolated. These mutants form an allelic series which block the spore morphogenetic program at distinct intermediate stages. Stepwise increases in gene dosage of a smk1 hypomorph result in wildtype phenotypes; and acquisition of different phenotypes requires distinct allelic thresholds. Intermediate morphological blocks as seen in these missense mutants are recapitulated by reducing wildtype SMK1 expression. These results demonstrate that increasing SMK1 activity levels are required for multiple and distinct steps of spore morphogenesis. This provides a mechanism by which regulated changes in MAP kinase activity thresholds temporally coordinate events during differentiation.
CAK1 , the Cdk-activating kinase in yeast, is essential for activation of CDC28 and cell cycle progression. CAK1 was isolated as a dosage suppressor of smk1 sporulation defects. Furthermore, cak1 mutants have been identified which successfully complete meiosis but cannot coordinate spore morphogenesis. Thus CAK1 positively regulates and is required for spore morphogenesis. Dominant mutant cdc28 alleles which suppress cak1 cell cycle defects also specifically suppress spore wall defects. This indicates that CAK1 's function in spore morphogenesis is CDC28 -dependent. Thus, CDC28 positively regulates both cell cycle progression and spore morphogenesis. These findings demonstrate that Cdks can couple cell cycle events with cytodifferentiation programs in eukaryotes.
E xtragenic mutational s uppressors of s mk1 defects ( ess ), representing three distinct genetic loci, have been isolated. These mutants are recessive, indicating the existence of negative regulators of SMK1 . ess strains exhibit pleiotropic phenotypes which are hallmarks of previously, characterized Ras pathway mutants, and demonstrate an inability to respond to nutritional signals. We propose that the nutrient sensing pathways and specific meiotic cell cycle events signal SMK1 pathway activation. Characterization of the ess mutants will provide information on how multiple signals can contribute to MAP kinase regulation, and may lend insight to how Ras pathways and cell cycle are coupled cytodifferentiation programs.
