The model eukaryote Saccharomyces cerevisiae is well suited to investigate the causes of metabolic disturbance. PRPP [5-phospho-D-ribosyl-1(alpha)-pyrophosphate] may be regarded as a junction of carbon and nitrogen metabolism. As a result of this central position, perturbations in its synthesis can give rise to many unexpected cellular events, such as impaired cell integrity. We have taken advantage of S. cerevisiae's genetic tractability to investigate the metabolic links responsible for connecting the biochemical intermediate PRPP to apparently unrelated cellular functions. This approach provides insight into the co-ordination of different biological processes.

The biosynthetic intermediate PRPP (phosphoribosylpyrophosphate) has a central role in cellular biochemistry since it links carbon and nitrogen metabolism. Its importance may be reflected in the fact that, in the Saccharomyces cerevisiae (yeast) genome, there are five unlinked genes, PRS1-PRS5, each of which is theoretically capable of encoding the enzyme synthesizing PRPP. Interference with the complement of PRS genes in S. cerevisiae has far-reaching consequences for yeast physiology and has uncovered unexpected metabolic links including cell wall integrity and phospholipid metabolism.

In Saccharomyces cerevisiae, PRS genes comprise a family of five paralogous genes. Previously, it has been shown that in the cell the gene products are organized into two interacting complexes, one of which is a heterodimer and the other a heterotrimer. Here, it has been demonstrated that in addition to supplying the cell with the key metabolic intermediate PRPP [5-phospho-D-ribosyl-1(alpha)-pyrophosphate], the gene products contribute to the maintenance of cell integrity. Specifically, the phosphorylation of Rlm1, one of the end points of the cell integrity signalling pathway, is significantly impaired following deletion of any one of the PRS genes, in particular PRS1 and PRS3. This is reflected in changes in the expression of the alternative 1,3-beta-glucan synthase catalytic subunit, Fks2, as measured by its promoter activity. Yeast two-hybrid analysis has shown that Prs1, specifically the non-homologous region, NHR1-1 and Prs3, and to a lesser extent Prs2 and Prs4, interact with the MAPK (mitogen-activated protein kinase) of the cell integrity pathway, Slt2. When PRS1 is lacking, the basal level of phosphorylation of Slt2 is increased. Furthermore, prs1Delta and prs3Delta strains have an increased chitin content under normal growth conditions. alpha-Factor sensitivity and Calcofluor White resistance associated with the lack of Prs1 and Prs3 corroborate the involvement of these two gene products in cell integrity signalling. It is postulated that Prs polypeptides play a significant role in the remodelling of the cell wall and may have a direct involvement in cell integrity signalling.