Truncated Forms of a Novel Yeast Protein Suppress the Lethality of a G Protein α Subunit Deficiency by Interacting with the β Subunit (∗)

In Saccharomyces cerevisiae, the mating pheromone-initiated signal is transduced by a heterotrimeric G protein and normally results in transient cell cycle arrest and differentiation. A null allele of the Gα (GPA1/SCG1) subunit results in cell death due to unchecked signaling from the Gβγ (STE4, STE18, respectively) heterodimer. We have identified three high copy suppressors of gpa1 lethality. Two of these genes encode known transcription factors, Matα2p and Mcm1p. The third is a truncated form of a novel gene, SYG1. Overexpressed wild type SYG1 is a weak suppressor of gpa1. In contrast, the isolated mutant allele SYG1-1 is a strong suppressor that completely blocks the cell cycle arrest and differentiation phenotypes of gpa1 cells of both mating types. One deletion mutant (SYG1Δ340) can suppress the cell cycle arrest associated with gpa1, but the cells retain a differentiated morphology. SYG1-1 can suppress the effects of overexpressed wild type Gβ but is not able to suppress the lethality of an activated Gβ mutant (STE4Hpl). Consistent with these genetic observations, the suppressing form of Syg1p can interact with the STE4 gene product, as determined by a two-hybrid assay. SYG1-1 is also capable of promoting pheromone recovery in wild type cells, as judged by halo assay. The sequence of SYG1 predicts eight membrane-spanning domains. Deletion mutants of SYG1 indicate that complete gpa1 suppression requires removal of all of these hydrophobic regions. Interestingly, this truncated protein localizes to the same plasma membrane-enriched subcellular fraction as does full-length Syg1p. Three hypothetical yeast proteins, identified by their similarity to the SYG1 primary sequence within the gpa1 suppression domain, also appear to have related structures. The properties of Syg1p are consistent with those of a transmembrane signaling component that can respond to, or transduce signals through, Gβ or Gβγ.