Signal transduction by a nondissociable heterotrimeric yeast G protein

Abstract

Many signal transduction pathways involve heterotrimeric G proteins. The accepted model for activation of heterotrimeric G proteins states that the protein dissociates to the free G␣ (GTP)-bound subunit and free G␤␥ dimer. On GTP hydrolysis, G␣ (GDP) then reassociates with G␤␥ [Gilman, A. G. (1987) Annu. Rev. Biochem. 56, 615-649]. We reexamined this hypothesis, by using the mating G protein of the yeast Saccharomyces cerevisiae encoded by the genes GPA1, STE4, and STE18. In the absence of mating pheromone, the G␣ (Gpa1) subunit represses the mating pathway. On activation by binding of pheromone to a serpentine receptor, the G␤␥ (Ste4, Ste18) dimer transmits the signal to a mitogen-activated protein kinase cascade, leading to gene activation, arrest in the G1 stage of the cell cycle, production of shmoos (mating projections), and cell fusion. We found that a Ste4-Gpa1 fusion protein transmitted the pheromone signal and activated the mating pathway as effectively as when Ste4 (G␤) and Gpa1 (G␣) were coexpressed as separate proteins. Hence, dissociation of this G protein is not required for its activation. Rather, a conformational change in the heterotrimeric complex is likely to be involved in signal transduction.

The Role of Gβγ Subunits in the Organization, Assembly, and Function of GPCR Signaling Complexes

“…Some studies have indicated that conformational changes within the heterotrimeric G protein (13–16) rather than subunit dissociation per se are sufficient to reveal distinct effector interacting surfaces (reviewed in 12). This model is also supported by chemical (17) and molecular (18) cross-linking studies that showed physically tethered heterotrimers are still functional. Two recent studies appear to support the original hypothesis (19, 20), although matters are made more complicated by evidence that both scenarios are operative (21, 22).…”

The GPCR heterotetramer: challenging classical pharmacology

“…The readout of experiments with RET donor and acceptor molecules fused to Gα and Gγ subunits indicates that G protein activation upon ligand binding to the receptors does not lead to dissociation, but implies a conformational change with rearrangement, reorientation, of its subunits [21,22]. Levitzki and collaborators already advanced this interpretation after finding successful signal transduction of a yeast pheromone receptor with fused G protein subunits [23]. Levitzki was also first to suggest that G proteins form stable complexes with AC being independent of the activation state of G proteins, mostly based upon co-purification of AC from turkey erythrocyte membranes (reviewed in ref.…”

Structural mechanism of G protein activation by G protein-coupled receptor

“…However, a lot of studies suggest that GPCR-G protein interaction occurs before the activation process ( [Frank et al, 2005,Galés et al, 2005,Klein et al, 2000). Indeed, an alternative hypothesis proposed that G protein could be preassembled with inactive GPCRs (Nanof et al, 1991;Nanof and Stiles, 1993).…”