Functional analysis of RRD1 (YIL153w) and RRD2 (YPL152w), which encode two putative activators of the phosphotyrosyl phosphatase activity of PP2A in Saccharomyces cerevisiae

In the context of the cooperative project for functional analysis of novel genes uncovered during the systematic sequencing of the Saccharomyces cerevisiae genome, we deleted two paralogous ORFs: YIL153w and YPL152w. Based on the resulting phenotypes, the corresponding genes were named RRD1 and RRD2, respectively. Rrd proteins show significant similarity to the human phosphotyrosyl phosphatase activator (PTPA). Both single mutants, rrd1 Delta and rrd2 Delta, were viable. Deletion of RRD1 caused pleiotropic phenotypes under a wide range of conditions, including sensitivity to Ca2+, vanadate, ketoconazole, cycloheximide and Calcofluor white, and resistance to caffeine and rapamycin. The only phenotypes found for rrd2 Delta - resistance to caffeine and rapamycin - were weaker than the corresponding phenotypes of rrd1 Delta. The double mutant vrd1,2 Delta was inviable on rich glucose medium, but could grow in the presence of an osmotic stabilizer. The rrd1,2 Delta mutant was partially rescued by inactivation of; HOG or PBS2, suggesting an interaction between the RRD genes and the Hog1p signal transduction pathway. Introduction of slt2 Delta into the rrd1,2 Delta background improved the growth of rrd1,2 Delta on sorbitol-containing medium, indicating that the Rrd proteins also interact with the Slt2p/Mpk1p signaling pathway. Suppression of the lethal phenotype of the rrd1,2 Delta mutant by overexpression of PPH22 suggested that the products of the RRD genes function positively with catalytic subunits of PP2A. The synthetic lethality was also suppressed by the viable allele (SSD1-v1) of the SSD1 gene.