The β Subunit of the Heterotrimeric G Protein Triggers the Kluyveromyces lactis Pheromone Response Pathway in the Absence of the γ Subunit

The Kluyveromyces lactis heterotrimeric G protein is a canonical Galphabetagamma complex; however, in contrast to Saccharomyces cerevisiae, where the Ggamma subunit is essential for mating, disruption of the KlGgamma gene yielded cells with almost intact mating capacity. Expression of a nonfarnesylated Ggamma, which behaves as a dominant-negative in S. cerevisiae, did not affect mating in wild-type and DeltaGgamma cells of K. lactis. In contrast to the moderate sterility shown by the single DeltaKlGalpha, the double DeltaKlGalpha DeltaKlGgamma mutant displayed full sterility. A partial sterile phenotype of the DeltaKlGgamma mutant was obtained in conditions where the KlGbeta subunit interacted defectively with the Galpha subunit. The addition of a CCAAX motif to the C-end of KlGbeta, partially suppressed the lack of both KlGalpha and KlGgamma subunits. In cells lacking KlGgamma, the KlGbeta subunit cofractionated with KlGalpha in the plasma membrane, but in the DeltaKlGalpha DeltaKlGgamma strain was located in the cytosol. When the KlGbeta-KlGalpha interaction was affected in the DeltaKlGgamma mutant, most KlGbeta fractionated to the cytosol. In contrast to the generic model of G-protein function, the Gbeta subunit of K. lactis has the capacity to attach to the membrane and to activate mating effectors in absence of the Ggamma subunit.