The synthesis and function of proteases in Saccharomyces: genetic approaches.

Genetic analysis has clarified the role of the major defined proteases in the life of yeast cells. The proteases of the vacuole are clearly involved in the massive proteolysis that occurs when cells are starved for nitrogen and in the (re)utilization of peptides. They appear not to be involved in any of the specific proteolyses that have been described. kex1, kex2, ste13, bar1 (sst1), and pep4 mutants have the characteristics expected of mutants defective in specific proteolytic events. Hence, the genetic attack on these specific proteolyses and the identification and characterization of the responsible proteases seem well under way and we can expect answers in the near future. The biggest lacuna in our understanding of proteolysis in yeast is the identity of the protease(s) or proteolytic system(s) involved in metabolically triggered proteolytic degradations and in housekeeping functions such as degradation of nonfunctional subunits and aberrant proteins. Genetic entries into these problems appear to be rare or difficult. Here lies the greatest challenge.