Cell Biology
Challenging the Gβ
Paradigm
Nancy R. Gough
Science Signaling, AAAS, Washington, DC 20005, USA
The dogma states that heterotrimeric G proteins have two active parts: the guanine nucleotide-binding G
subunit and a dimeric subunit Gβ
composed of two proteins, the Gβ and G subunits. Additionally, most evidence suggests that the Gβ complex with G bound to GDP is inactive and that GTP binding to G triggers the dissociation of the G subunit from Gβ, 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 G and separation of Gβ from the activated complex, and Gβ 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 G (KlG) 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 KlG in K. lactis failed to compromise mating as long as the KlG subunit was present, but if both KlG and KlG were knocked out, then mating was compromised. Thus, mating appears to require Gβ and either G or G, which suggests that either a Gβ or a Gβ complex may function in this pathway. The ability of KlG and KlGβ and for KlG and KlGβ to interact was confirmed by yeast two-hybrid assay. One role of the subunit in the function of Gβ complexes is recruitment of the β subunit to membranes, and in S. cerevisiae mutation of the cysteine residue that is farnesylated in G (GS) behaves as a dominant-negative G. However, the equivalent mutation in K. lactis KlG failed to inhibit mating. KlGβ was associated with the membrane fraction in cells lacking KlG but was completely cytosolic in cells lacking both KlG and KlG. Adding a farnesylation motif to KlGβ partially targeted the protein to the membrane and partially rescued the mating defect in cells deficient in both KlG and KlG. Thus, it appears that in K. lactis, Gβ can signal without G 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 subunit. Mol. Biol. Cell 21, 489–498 (2010). [Abstract] [Full Text]
