Protein Degradation and Proteinases during Yeast Sporulation

During ascospore formation in Saccharomyces cerevisiae, at least 60–70% of the pre‐existing vegetative protein was broken down at a rather constant rate until the time mature asci appeared. Under the same conditions in a non‐sporulating haploid derived from the same strain the rate of protein degradation, although initially comparable to that of sporulating cells, decreased much more rapidly. Proteins synthesized at different times during sporulation had approximately the same degradation rates as the vegetative proteins. Similar rates of degradation were observed for the vegetative proteins in all fractions obtained from cell homogenates by differential centrifugation.Protein breakdown after transfer to sporulation medium was blocked by uncouplers and inhibitors of energy metabolism, and was partially inhibited by cycloheximide.Polyacrylamide gel electrophoresis, in the presence of sodium dodecylsulfate, of the proteins extracted from vegetative cells and from isolated asci and ascospores revealed that ascus formation was accompanied by a shift of the cellular proteins to a lower molecular weight.From several proteinase inhibitors tested, only tosyl‐p‐lysine chloromethylketone sli