The importance of the five phosphoribosyl-pyrophosphate synthetase (Prs) gene products of Saccharomyces cerevisiae in the maintenance of cell integrity and the subcellular localization of Prs1p

Phosphoribosyl-pyrophosphate synthetase (Prs) catalyses the synthesis of phosphoribosyl pyrophosphate (PRPP), an intermediate in nucleotide metabolism and the biosynthesis of the amino acids histidine and tryptophan. The Saccharomyces cerevisiae genome contains a family of five PRS genes, PRS1-PRS5. Using anti-peptide antisera directed against two different epitopes of Prs1p it was shown that Prs1p localizes to granular cytoplasmic structures. This localization was confirmed by living cell microscopy of strains expressing a functional green fluorescent protein (GFP)-tagged Prs1p. Analysis of Prs1p distribution in conditional secretory-deficient (sec) mutants suggested that the observed distribution of Prs1p is independent of the secretory pathway. Electron microscopy revealed that plasma membrane invaginations and accumulation of cytoplasmic vesicles were more frequent in strains which lack some of the PRS genes than in the wild-type. The fact that ?prs1 and ?prs3 are hypersensitive to caffeine and unable to recover from exposure to it as judged by the release of alkaline phosphatase points to a possible link between Prs and the maintenance of cell integrity.