Cell Biology

Challenging the Gβγ Paradigm

Science Signaling  09 Feb 2010:
Vol. 3, Issue 108, pp. ec46
DOI: 10.1126/scisignal.3108ec46

The dogma states that heterotrimeric G proteins have two active parts: the guanine nucleotide-binding Gα subunit and a dimeric subunit Gβγ composed of two proteins, the Gβ and Gγ subunits. Additionally, most evidence suggests that the Gαβγ complex with Gα bound to GDP is inactive and that GTP binding to Gα triggers the dissociation of the Gα subunit from Gβγ, allowing each subunit to then regulate the activity of downstream effectors. The study of the mating response of the budding yeast Kluyveromyces lactis by Navarro-Olmos et al. challenges two of these established paradigms in G protein signaling. In the budding yeast Saccharomyces cerevisiae, mating pheromone triggers the conventional pathway of GTP binding to Gα and separation of Gβγ from the activated complex, and Gβγ then activates the mitogen-activated protein kinase cascade to mediate the mating response. Despite structural similarities and sequence conservation and the ability to interact in a yeast two-hybrid system, the K. lactis Gβ (KlGβ) and Gγ (KlGγ) failed to suppress the sterile phenotype of S. cerevisiae, which suggests that the pathway may be different. Knocking out KlGβ alone abolished mating. Unexpectedly, knocking out KlGγ in K. lactis failed to compromise mating as long as the KlGα subunit was present, but if both KlGα and KlGγ were knocked out, then mating was compromised. Thus, mating appears to require Gβ and either Gα or Gγ, which suggests that either a Gαβ or a Gβγ complex may function in this pathway. The ability of KlGγ and KlGβ and for KlGα and KlGβ to interact was confirmed by yeast two-hybrid assay. One role of the γ subunit in the function of Gβγ complexes is recruitment of the β subunit to membranes, and in S. cerevisiae mutation of the cysteine residue that is farnesylated in Gγ (GγS) behaves as a dominant-negative Gγ. However, the equivalent mutation in K. lactis KlGγ failed to inhibit mating. KlGβ was associated with the membrane fraction in cells lacking KlGγ but was completely cytosolic in cells lacking both KlGγ and KlGα. Adding a farnesylation motif to KlGβ partially targeted the protein to the membrane and partially rescued the mating defect in cells deficient in both KlGγ and KlGα. Thus, it appears that in K. lactis, Gβ can signal without Gγ as long as it has a mechanism to reach the membrane.

R. Navarro-Olmos, L. Kawasaki, L. Domínguez-Ramírez, L. Ongay-Larios, R. Pérez-Molina, R. Coria, The β subunit of the heterotrimeric G protein triggers the Kluyveromyces lactis pheromone response pathway in the absence of the γ subunit. Mol. Biol. Cell 21, 489–498 (2010). [Abstract] [Full Text]