Distinct steps in yeast spore morphogenesis require distinct SMK1 MAP kinase thresholds

The SMK1 mitogen-activated protein kinase is required for spore morphogenes is in Saccharomyces cerevisiae. In contrast to the multiple aberrant spore wall assembly patterns seen even within a single smk1 null ascus, different smk1 missense mutants block in a coordinated fashion at intermediate stage s. One smk1 mutant forms asci in which the four spores are surrounded only by prospore wall-like structures, while another smk1 mutant forms asci in w hich the spores are surrounded by inner but not outer spore wall layers. St epwise increases in gene dosage of a hypomorphic smk1 allele allow for the completion of progressively later morphological and biochemical events and for the acquisition of distinct spore-resistance phenotypes. Furthermore, s mk1 allelic spore phenotypes can be recapitulated by reducing wild-type SMK 1 expression. The data demonstrate that SMK1 is required for the execution of multiple steps in spore morphogenesis that require increasing thresholds of SMK1 activity. These results suggest that quantitative changes in mitog en-activated protein kinase signaling play a role in coordinating multiple events of a single cellular differentiation program.