SMK1, a developmentally regulated MAP kinase, is required for spore wall assembly in Saccharomyces cerevisiae

Mitogen-activated protein (MAP) kinases comprise a family of conserved, eukaryotic enzymes that mediate responses to a wide variety of extracellular stimuli. In yeast, different signal transduction pathways utilize distinct MAP kinase family members. We have identified a new yeast MAP kinase gene (named SMK1) that is required for the completion of sporulation. Molecular and cytologic markers indicate that meiotic development proceeds normally in homozygous smk1-Δ1 diploids through meiosis II. However, light and electron microscopy show that smk1 asci are defective in organizing spore wall assembly. Consistent with a defect in spore wall assembly, smk1-Δ1 mutant asci display enhanced sensitivities to enzymatic digestion, heat shock, and exposure to ether. SMK1 mRNA, which is not detectable in vegetative cells, is derepressed at least 200-fold just prior to prospore enclosure. We propose that the SMK1 MAP kinase participates in a developmentally regulated signal transduction pathway that coordinates cytodifferentiation events with the transcriptional program.