i'h¢ gen¢ of" the proteina~ y~cA inhibitor i":j, PAl3, of the yeast Saceharan)yce,~ ¢¢ecvi~ta¢ was isolated by oligonucl¢otidc screening of a genomic DNA library and sequenced. The gent codes for a single protein of 68 amino acids, The structural PAI3 gen¢ was deleted in vitro by oli$onucleotidesite.directed mutagenests. The mutated allele was introduced via homologous recombination into the 8enome of wild,type yeast and into the Itenome of a yeast mutant, which lacks the s~ond cytoplasmic proteina~-inhibitor, l°a, The deficiency of either or of both inhibitors has no effect on th© cell viability under various physiological conditions, The inhibitor mutants, ho~vever, show an increase in the lleneral in vivo protein degradation rate, The 1^3 mutant has a 2-~l.fold increased protein degradation rate in the tint 6 h aft©r a shift from rich medium onto starvation-medium, whereas the IBz mutant shows a constantly incrca~d degradation rate of 20~50~ under the same conditions. The inhibitor double null mutant has the same protein degradation rate as the [^~ null mutant. The~ results suggest an in vivo interaction b~tw©en the vacuolor endopeptidases and their cytoplasmic inhibitors.Pt~nt¢olysis; Prot¢ina,~ inhibitor; [^~; [no; Null mutant; Saccharomycts cerevisia¢
1, INTRODUCTIONThe vacuole (lysosome) of the yeast is an actdic compartment, which contains multiple hydrolases (see reviews [1][2][3][4]). It is also a storage organelle for several compounds like phosphate, amino acids and calcium. The budding yeast Saccharomyces cerevisiae contains 1-2 large vacuoles per cell, which undergo multiple fu. sion and fragmentation processes during cell growth and sporulation (for review, see [5]).The two vacuolar endopeptidases proteinase yscA and yscB are essential enzymes for protein degradation during vegetative growth and, most pronounced, under starvation conditions ([6], for review see [1][2][3][4]). Proteinase yscA is even vital for cell viability under starvetion conditions [6]. The peptidases proteinase yscA, proteinase yscB and earboxypeptidase yscY are necessary for the differentiation prbcess of sporulation [6].The cytoplasm of the yeast S:tccharomyces cerevisiae contains the peptide inhibitors, Ia3, In2 and I c, specific for the vacuolar endopeptida.,+es proteinase yscA, proteinase yscB and carbo×ypeptidase yscY, respectively [7,8] No proteinase activities can be detected in cell extracts of wild-type yeast due to the formation of the enzyme/inhibitor complexes. The proteinase act ivities can be recovered by incubation at pH 5 or in the presence of detergents [13][14][15][16]. The inhibitors were found in the cytoplasmic fraction and only traces were found in the microsomal fraction [7]. The proteinase inhibitors are small and heat stable proteins (IA~: 7.6 kDa; In2:8.5 kDa), which contain no disulphide bonds [17][18][19][20]. Little is known about the function of the cytoplasmic proteinase inhibitors.The intracellular distribution of the proteinases and their inhibitors, as well as the observation that they are ex...