CLONING OF SACCHAROMYCES-CEREVISIAE STE5 AS A SUPPRESSOR OF A STE20 PROTEIN-KINASE MUTANT - STRUCTURAL AND FUNCTIONAL SIMILARITY OF STE5 TOFAR1

The beta and gamma subunits of the mating response G-protein in the ye ast Saccharomyces cerevisiae have been shown to transmit the mating ph eromone signal to downstream components of the pheromone response path way. A protein kinase homologue encoded by the STE20 gene has recently been identified as a potential G(betagamma) target. We have searched multicopy plasmid genomic DNA libraries for high gene dosage suppresso rs of the signal transduction defect of ste20 mutant cells. This scree n identified the STE5 gene encoding an essential component of the pher omone signal transduction pathway. We provide genetic evidence for a f unctional interrelationship between the STE5 gene product and the Ste2 0 protein kinase. We have sequenced the STE5 gene, which encodes a pre dicted protein of 917 amino acids and is specifically transcribed in h aploid cells. Transcription is slightly induced by treatment of cells with pheromone. Ste5 has homology with Far1, a yeast protein required for efficient mating and the pheromone-inducible inhibition of a G1 cy clin, Cln2. A STE5 multicopy plasmid is able to suppress the signal tr ansduction defect of far1 null mutant cells suggesting that Ste5, at e levated levels, is able functionally to replace Far1. The genetically predicted point of function of Ste5 within the pheromone signalling pa thway suggests that Ste5 is involved in the regulation of a G(betagamm a)-activated protein kinase cascade which links a G-protein coupled re ceptor to yeast homologues of mitogen-activated protein kinases.