The KlGpa1 Gene Encodes a G-Protein a Subunit That Is a Positive Control Element in the Mating Pathway of the Budding Yeast Kluyveromyces lactis

The cloning of the gene encoding the KlGpa1p subunit was achieved by standard PCR techniques and by screening a Kluyveromyces lactis genomic library using the PCR product as a probe. The full-length open reading frame spans 1,344 nucleotides including the stop codon. The deduced primary structure of the protein (447 amino acid residues) strongly resembles that of Gpa1p, the G-protein a subunit from Saccharomyces cerevisiae involved in the mating pheromone response pathway. Nevertheless, unlike disruption of Gpa1 from S. cerevisiae, disruption of KlGpa1 rendered viable cells with a reduced capacity to mate. Expression of a plasmidic KlGpa1 copy in a DKlgpa1 mutant restores full mating competence; hence we conclude that KlGpa1p plays a positive role in the mating pathway. Overexpression of the constitutive subunit KlGpa1p(K) (GTP bound) does not induce constitutive mating; instead it partially blocks wild-type mating and is unable to reverse the sterile phenotype of DKlgpa1 mutant cells. K. lactis expresses a second Ga subunit, KlGpa2p, which is involved in regulating cyclic AMP levels upon glucose stimulation. This subunit does not rescue DKlgpa1 cells from sterility; instead, overproduction of KlGpa2p slightly reduces the mating of wild-type cells, suggesting cross talk within the pheromone response pathway mediated by KlGpa1p and glucose metabolism mediated by KlGpa2p. The DKlgpa1 DKlgpa2 double mutant, although viable, showed the mating deficiency observed in the single DKlgpa1 mutant.