This heterotrimeric G protein is illustrated with its theoretical lipid anchors. GDP is black. Alpha chain is yellow. Beta and gamma chains are blue.
3D structure of a heterotrimeric G protein
"G protein" usually refers to the membrane-associated heterotrimeric G proteins, sometimes referred to as the "large" G proteins (as opposed to the subclass of smaller, monomeric small GTPases) . These proteins are activated by G protein-coupled receptors and are made up of alpha (α), beta (β) and gamma (γ) subunits, the latter two referred to as the beta-gamma complex.
There are four main families of G proteins: Gi/Go, Gq, Gs, and G12.
Reconstitution experiments carried out in the early 1980s showed that purified Gα subunits can directly activate effector enzymes. The GTP form of the α subunit of transducin (Gt) activates the cyclic GMP phosphodiesterase from retinal rod outer segments, and the GTP form of the α subunit of the stimulatory G protein (Gs) activates hormone-sensitive adenylate cyclase.
Gα subunits consist of two domains, the GTPase domain, and the alpha-helical domain.
There exist at least 20 different Gα subunits, which are separated into four main families. This nomenclature is based on their sequence homologies:
The β and γ subunits are closely bound to one another and are referred to as the beta-gamma complex. Upon activation of the GPCR, the Gβγ complex is released from the Gα subunit after its GDP-GTP exchange.
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^May, DC; Ross, EM; Gilman, AG; Smigel, MD (1985). "Reconstitution of catecholamine-stimulated adenylate cyclase activity using three purified proteins". J. Biol. Chem.260 (29): 15829–15833. PMID2999139.