Receptor-mediated activation of heterotrimeric G-proteins: current structural insights.

G-protein-coupled receptors (GPCRs) serve as catalytic activators of heterotrimeric G-proteins (Galphabetagamma) by exchanging GTP for the bound GDP on the Galpha subunit. This guanine nucleotide exchange factor activity of GPCRs is the initial step in the G-protein cycle and determines the onset of various intracellular signaling pathways that govern critical physiological responses to extracellular cues. Although the structural basis for many steps in the G-protein nucleotide cycle have been made clear over the past decade, the precise mechanism for receptor-mediated G-protein activation remains incompletely defined. Given that these receptors have historically represented a set of rich drug targets, a more complete understanding of their mechanism of action should provide further avenues for drug discovery. Several models have been proposed to explain the communication between activated GPCRs and Galphabetagamma leading to the structural changes required for guanine nucleotide exchange. This review is focused on the structural biology of G-protein signal transduction with an emphasis on the current hypotheses regarding Galphabetagamma activation. We highlight several recent results shedding new light on the structural changes in Galpha that may underlie GDP release.