Beta-gamma complex

The G beta-gamma complex (G_βγ) is a tightly bound dimeric protein complex, composed of one G_β and one G_γ subunit, and is a component of heterotrimeric G proteins. Heterotrimeric G proteins, also called guanosine nucleotide-binding proteins, consist of three subunits, called alpha, beta, and gamma subunits, or G_α, G_β, and G_γ. When a G protein-coupled receptor (GPCR) is activated, G_α dissociates from G_βγ, allowing both subunits to perform their respective downstream signaling effects. One of the major functions of G_βγ is the inhibition of the G_α subunit.

The individual subunits of the G protein complex were first identified in 1980 when the regulatory component of adenylate cyclase was successfully purified, yielding three polypeptides of different molecular weights. Initially, it was thought that G_α, the largest subunit, was the major effector regulatory subunit, and that G_βγ was largely responsible for inactivating the G_α subunit and enhancing membrane binding. However, downstream signalling effects of G_βγ were later discovered when the purified G_βγ complex was found to activate a cardiac muscarinic K+ channel. Shortly after, the G_βγ complex associated with a mating factor receptor-coupled G protein in yeast was found to initiate a pheromone response. Although these hypotheses were initially controversial, G_βγ has since been shown to directly regulate as many different protein targets as the G_α subunit.

Recently, possible roles of the G_βγ complex in retinal rod photoreceptors have been investigated, with some evidence for the maintenance of G_α inactivation. However, these conclusions were drawn from in vitro experiments under unphysiological conditions, and the physiological role of the G_βγ complex in vision is still unclear. Nevertheless, recent in vivo findings demonstrate the necessity of the transducin G_βγ complex in the functioning of rod photoreceptors under low light conditions.

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