Isolation of yeast mutants lacking proteinase B activity

Yeast carbamoyl-phosphate-synthetase--aspartate-transcarbamylase multidomain protein is phosphorylated in vitro by cAMP-dependent protein kinase

The first two steps of de novo pyrimidine synthesis in Saccharomyces cerevisiae are catalyzed by a multifunctional protein, coded by the URA2 gene and which has the carbamoyl-phosphate (CPSase) synthetase and aspartate transcarbamylase (ATCase) activities. The native enzyme purified from protease-B-deficient URA2-transformed cells, was phosphorylated in vitro using catalytic subunits of pure cAMP-dependent protein kinase. After electrophoresis under denaturing conditions, a single 240-kDa species was found to be phosphorylated. Trypsin digestion of this species gave a single, very acidic phosphopeptide upon isoelectric focussing. Purification by HPLC followed by amino acid sequencing of this peptide, showed a phosphoserine at the expected consensus sequence Arg-Arg-Phe-Ser. Knowledge of the URA2 gene sequence allowed the site to be located in the peptide link between dihydroorotase-like and ATCase domains. Such a location may explain why phosphorylation of the URA2 protein changed neither CPSase and ATCase activities nor their sensitivity to UTP, their common specific inhibitor.

Aminopeptidase yscII of yeast. Isolation of mutants and their biochemical and genetic analysis

Mutant strains of the yeast Saccharomyces cerevisiae defective in aminopeptidase yscII were isolated by screening for reduced external activity against the chromogenic substrate lysine beta-naphthylamide. One of the selected mutant strains analyzed in detail showed wild-type staining activity when tested at 23 degrees C but mutant activity after exposure to 37 degrees C, suggesting a temperature-sensitive mutation. Electrophoretic separation of mutant crude extracts on non-denaturing polyacrylamide gels and subsequent activity staining using lysine and leucine beta-naphthylamides as substrates revealed that in all strains isolated the same distinct activity band was affected, which corresponded to the aminopeptidase activity identified previously as aminopeptidase yscII [Achstetter, T., Ehmann, C. & Wolf, D. H. (1983) Arch. Biochem. Biophys. 226, 292-305]. All mutants strains isolated fell into the same complementation group. Tetrad dissection of sporulated diploids heterozygous for the wild-type and mutant allele resulted in a 2:2 segregation of mutant and wild-type phenotype indicating a single gene mutation. The characteristics of the mutations analyzed point to the gene which we called APE2 as the structural gene of aminopeptidase yscII. No vital consequences of aminopeptidase yscII deficiency on cell life and differentiation could be detected. However, the enzyme seems to be involved in the cellular supply of leucine from externally offered leucine-containing dipeptide substrates.

Purification of an acid protease from Mucor rouxii that inactivates chitin synthetase

An acid protease was purified from the mycelial form of Mucor rouxii by a method which involved salt and acid precipitation, gel filtration and anion-exchange chromatography. The enzyme had a molecular mass of 16,000 Da. Its optimum pH was 4.0, maximal activity was obtained at 50 degrees C, and it was inactivated at 70 degrees C. It was not affected by leupeptin or N alpha-p-tosyl-L-lysine chloromethyl ketone (TLCK) but diazoacetyl-DL-norleucine methyl ester (DNME) in the presence of Cu2+ and more noticeably pepstatin A, strongly inhibited the activity. This acid protease did not activate "zymogenic" chitin synthetase from the fungus, but brought about its inactivation even at low concentrations and after short periods of incubation time.

Analysis of proteinase A function in yeast

Yeast mutants lacking proteinase A were isolated. One of these mutants (Hb I) is characterized in detail. The mutation called pra1 segregates 2:2 in meiotic tetrads indicating a single gene mutation. No anti-(proteinase A) cross-reacting material can be detected. Diploids heterozygous for pra1 show gene dosage. Thus, it appears that PRA1 might be the structural gene for proteinase A. Results obtained with this mutant show that proteinase A is not a vital component of the vegetative cell cycle. The mutant exhibits normal mitotic growth under rich and poor growth conditions and shows normal mating. Enzymes subject to carbon catabolite inactivation and inactivation of NADP-dependent glutamate dehydrogenase, processes which were proposed to be of proteolytic nature, are not affected by the absence of proteinase A. However, protein degradation under sporulation conditions is about 30% reduced in proteinase A mutant cells. The differentiation process of sporulation is also disturbed leading to a 40% reduced sporulation frequency in mutant cells.

Proteolytically induced changes in the molecular form of the carbamyl phosphate synthetase-uracil-aspartate transcarbamylase complex coded for by the URA2 locus in Saccharomyces cerevisiae

When a uracil-auxotrophic yeast strain is grown under uracil-limiting conditions, the aspartate transcarbamylase activity found in crude extracts shows a variation in sensitivity to feedback inhibition by uridine 5'-triphosphate. In this study we correlated this variation with changes in the molecular form of the carbamyl phosphate synthetase-uracil-aspartate transcarbamylase complex. Carbamyl phosphate synthetase-uracil (molecular weight, 240,000) and uridine 5'-triphosphate-insensitive aspartate transcarbamylase (molecular weight, 140,000) were present separately in extracts from cells collected in the early exponential phase; this was in contrast to the presence of a single high-molecular-weight form (molecular weight, about 900,000) bearing both activities in extracts from stationary-phase cells. The lack of sensitivity to uridine 5'-triphosphate by aspartate transcarbamylase was delayed by adding uridine 5'-triphosphate before cell disruption and was prevented completely by adding phenylmethylsulfonyl fluoride. Thus, this event was attributed to a transient serine protease activity detected only in early exponential-phase cell extracts. However, even in the presence of phenylmethylsulfonyl fluoride, a sucrose density gradient analysis in the absence of uridine 5'-triphosphate revealed a change in the aggregation state of the complex which might have occurred in vivo. None of these events was observed in extracts from cells that lacked protease B activity (strain HP232-2B).

The substrate specificity of proteinase B from baker's yeast

The substrate specificity of proteinase B (EC 3.4.22.9) from Baker's yeast was studied. Experiments with unblocked synthetic peptides indicated that the enzyme has no aminopeptidase activity. The proteinase cleaves trypsin substrates like Bz-Arg-OEt, Bz-Arg-pNA and Bz-Ile-Glu-Gly-Arg-pNA and chymotrypsin substrates like Ac-Tyr-OEt and Bz-Tyr-pNA. The Km value for Ac-Tyr-OEt is similar to that of chymotrypsin A, but the catalytic activity per mol proteinase B amounts to only 1/20 that of chymotrypsin A. Km and kcat for Bz-Arg-OEt are 1/50 and 1/7 as high as the corresponding values determined for trypsin. Proteinase B cleaved the oxidized insulin B chain with an initial rapid cleavage step at Leu(15)-Tyr(16) and Phe(24)-Phe(25). Slower hydrolysis was observed at Gln(4)-His(5), Leu(11)-Val(12) Tyr(16)-Leu(17), Leu(17)-Val(18), Arg(22)-Gly(23) and Phe(25)-Tyr(26). These results suggest that the specificity of proteinase B is comparable to the specificity of porcine chymotrypsin C as well as of trypsin. When the hexapeptide Leu-Trp-Met-Arg-Phe-Ala was used as a substrate for proteinase B, the enzyme preferentially attacked at Arg-Phe and more slowly at Trp-Met.

Carboxypeptidase S- and carboxypeptidase Y-deficient mutants of Saccharomyces cerevisiae

A new carboxypeptidase (carboxypeptidase S) was found in a Saccharomyces cerevisiae strain lacking carboxypeptidase Y (D. H. Wolf and U. Weiser, Eur. J. Biochem. 73:553-556, 1977). Mutants devoid of carboxypeptidase S activity were isolated from a mutant strain that was also deficient in carboxypeptidase Y. Four mutants were analyzed in detail and fell into one complementation group. The defect segregated 2:2 in meiotic tetrads. Gene dosage experiments indicated that the mutation might reside in the structural gene of carboxypeptidase S. The absence of both enzymes, carboxypeptidases Y and S, did not affect mitotic growth. Ascopore formation was only slightly affected by the absence of both carboxypeptidases. Protein degradation under conditions of nutrient deprivation and under sporulation conditions showed no obvious alteration in the absence of carboxypeptidases Y and S. When a proteinase B mutation, which led to the absence of proteinase B activity and resulted in the partial reduction of sporulation, was introduced into a mutant lacking both carboxypeptidases, the ability of diploid cells to sporulate was nearly completely lost. Mutants lacking both carboxypeptidases were unable to grow on the dipeptide benzyloxycarbonylglycyl-l-leucine as a sole nitrogen source, which indicates an additional function for carboxypeptidases Y and S in supplying nutrients from exogenous peptides. Catabolite inactivation of fructose-1,6-bisphosphatase, cytoplasmic malate dehydrogenase, and phosphoenolpyruvate carboxykinase and inactivation of nicotin-amide adenine dinucleotide phosphate-dependent, glutamate dehydrogenase, events which have been proposed to involve proteolysis in vivo, were not dependent on the presence of carboxypeptidase Y and S. In a mutant lacking both carboxypeptidases, four new proteolytic enzymes with carboxypeptidase activity were detected.

Studies on a proteinase B mutant of yeast

Yeast mutant lacking proteinase B activity have been isolated [Wolf, D. H. and Ehmann, C. (1978) FEBS Lett. 92, 121--124]. One of these mutants (HP232) is characterized in detail. Absence of the vacuolar localized enzyme is confirmed by checking for proteinase B activity in isolated mutant vacuoles. Defective proteinase B activity segregates 2:2 in meiotic tetrads. The mutation is shown to be recessive. Mutant proteinase B activity is not only absent against the synthetic substrate. Azocoll, but also against the physiological substrate pre-chitin synthetase, cytoplasmic malate dehydrogenase and fructose-1,6-bisphosphatase. The mutant shows normal vegetative growth, a phenomenon not consistent with the idea that proteinase B might be the activating principle of chitin synthetase zymogen in vivo. Fluorescence microscopy shows normal chitin insertion. Enzymes underlying carbon-catabolite inactivation in wild-type cells (a mechanism proposed to be possibly triggered by proteinase B) such as cytoplasmic malate dehydrogenase, fructose-1,6-bisphosphatase, phosphoenolpyruvate carboxykinase and isocitrate lyase, are inactivated also in the mutant. NADP-dependent glutamate dehydrogenase, which is found to be inactivated in glucose-starved wild-type cells, proceeds normally in the mutant. Mutant cells show more than 40% reduced protein degradation under starvation conditions. Sporulating diploids, homozygous for proteinase B absence, also exhibit an approximately 40% reduced protein degradation as compared to homozygous wild-type diploids or diploids heterozygous for the mutant gene. The time of the appearance of the first ascospores of diploid cells, homozygous for proteinase B deficiency, is delayed about 50% and sporulation frequency is reduced to about the same extent as compared to homozygous wild-type diploids or diploids heterozygous for the mutant gene.

Regulation of Saccharomyces cerevisiae nicotinamide adenine dinucleotide phosphate-dependent glutamate dehydrogenase by proteolysis during carbon starvation

Inactivation of the nicotinamide adenine dinucleotide phosphate-dependent glutamate dehydrogenase from Saccharomyces cerevisiae during carbon starvation occurs with a simultaneous loss of enzyme protein and enzyme activity.