Role of SGP2, a suppressor of a gpa1 mutation, in the mating-factor signaling pathway of Saccharomyces cerevisiae.

Loss of function of GPA1, which encodes a guanine-nucleotide-binding protein, arrests the cell at the G1 phase and allows it to mate, suggesting that the gpa1 mutation spontaneously exerts an intracellular signal that mimics the action of mating factor. We have cloned the SGP2 gene, which was first identified as a secondary mutation that allowed a gpa1::HIS3 mutant to grow and to show a non-cell-type-specific sterile phenotype. Disruption of SGP2 confers temperature-sensitive growth and a-specific sterile phenotypes, characteristics similar to those conferred by the dpr1 (ram) mutation, a suppressor of RAS2Val-19. The following observations indicate that SGP2 and DPR1 are in fact identical. (i) The cloned SGP2 complements both the temperature-sensitive growth and the a-specific sterility of the dpr1 mutant and can be integrated into the chromosomal DPR1 locus. (ii) The cloned DPR1, in turn, complements the ability of sgp2 to suppress the lethality of gpa1::HIS3. (iii) The dpr1 mutation suppresses the growth defect of gpa1::HIS3, and the dpr1 gpa1::HIS3 strain shows a non-cell-type-specific sterile phenotype. (iv) sgp2 is closely linked to the dpr1 locus. The DPR1 product has been shown to be responsible for processing and fatty acid acylation of a-factor and RAS proteins at their carboxyl termini. Therefore, the SGP2 (DPR1) product may be involved in membrane localization of an essential component in the mating-factor signaling pathway.