Five genes but the loss of five amino acids disrupts PRPP

In spite of containing five PRS-encoding gene products Saccharomyces cerevisiae can requires only one of three possible heterodimers Prs1/Prs3, Prs2/Prs5 and Prs4/Prs5 for survival. The most important subunit is Prs1/Prs3 since in a triple deletant of PRS2 PRS4 and PRS5 there is still Prs activity approximately 20% of that found in the WT. In addition, the strain displays a phenotype consistent with impaired cell wall integrity (CWI) – temperature and caffeine sensitivities and deregulation of chitin synthesis. Prs1 and Prs5 both contain non-homologous regions that are required for the maintenance of CWI. The former binds to phosphorylated Sl2 and mutation of postulated phosphorylation residues in the latter prevents expression of Fks2 under stress condition. We have discovered that Prs3 has a potential nuclear localization site (NLS) consisting of five amino acids, ‘KKCPK’, close to its C-terminus. Elimination of this sequence results in caffeine sensitivity and a loss of Rlm1 expression in response to Slt2 activation and collapse of the Prs1/Prs3 complex as demonstrated by Western blotting. It would therefore appear that this five aa insertion in Prs3 has a significant role to play in the stability of the Prs1/Prs3 functional subunit. Plasmid shuffling has demonstrated that prs3(Δnls) is incapable of rescuing the synthetic lethality of a prs3Δ/prs5Δ strain. The results support the hypothesis that at least one of the minimal functional subunits Prs1/Prs3, in addition to supplying PRPP, plays a key role in CWI maintenance, possibly by means of nucleo-cytoplasmic shuttling of the complex.