Protein Degradation during Yeast Sporulation Enzyme and Cytochrome Patterns

Abstract

The levels of several enzymes were studied during sporulation of Saccharomyces cerevisiae. The specific activities of ribonuclease and aminopeptidase I raised several-fold after transfer of the cells to sporulation medium; the specific activities of phosphofructokinase [EC 2.7.1.11], glucose-6-phosphate dehydrogenase [EC 1.1.1.49], tryptophan synthase [EC 4.2.1.20] and pyruvate decarboxylase [EC 3.1.4.22] were not significantly altered. The specific activities of NAD-dependent glutamate dehydrogenase [EC 1.4.1.2], isocitrate lyase [EC 4.1.3.1], malate dehydrogenase [EC 1.1.1.37] and fructose bisphosphatase all decreased from the onset of sporulation. The inactivation of these latter enzymes was inhibited by cycloheximide and by inhibitors of energy metabolism. Hexokinase [EC 2.7.1.1], alcohol dehydrogenase [EC 1.1.1.1] and glutamate oxaloacetate transaminase [EC 2.6.1.1] were partially lost from the cells during the period of ascus maturation. None of the enzymic changes observed proved to be sporulation-specific in that it occurred exclusively in sporulating diploid yeast cells. Probably the meiotic events and the metabolic changes required for ascospore formation are under separate genetic control in this organism. During sporulation, the cellular content of cytochromes b, c and aa3 was reduced to 20% or less of that present in vegetative derepressed cells. Since the relative percentage of total to cycloheximide-insensitive mitochondrial protein synthesis was not significantly altered throughout sporulation, and the pattern of mitochondrially synthesized polypeptides was rather similar both in vegetative and sporulating cells, it appeared that not only degradation but also synthesis and therefore turnover of the mitochondrially coded polypeptides of cytochromes b and aa3 took place during sporulation. The activity ratio of cytochrome c oxidase to F1-ATPase in submitochondrial particles isolated from vegetative cells and from purified asci was almost identical. The loss of membrane-bound mitochondrial cytochromes during sporulation is probably due to a nonselective degradation of inner mitochondrial membrane proteins.