Heterotrimeric G protein complexes are composed of three distinct protein subunits named alpha (α), beta (β) and gamma (γ) subunits. The alpha subunits contain the GTP binding/GTPase domain flanked by long regulatory regions, while the beta and gamma subunits form a stable dimeric complex referred to as the beta-gamma complex. When activated, a heterotrimeric G protein dissociates into activated, GTP-bound alpha subunit and separate beta-gamma subunit, each of which can perform distinct signaling roles. The α and γ subunit are modified by lipid anchors to increase their association with the inner leaflet of the plasma membrane.
Heterotrimeric G proteins act as the transducers of G protein-coupled receptors, coupling receptor activation to downstream signaling effectors and second messengers. In unstimulated cells, heterotrimeric G proteins are assembled as the GDP bound, inactive trimer (G-GDP-G complex). Upon receptor activation, the activated receptor intracellular domain acts as GEF to release GDP from the G protein complex and to promote binding of GTP in its place. The GTP-bound complex undergoes an activating conformation shift that dissociates it from the receptor and also breaks the complex into its component G protein alpha and beta-gamma subunit components. While these activated G protein subunits are now free to activate their effectors, the active receptor is likewise free to activate additional G proteins – this allows catalytic activation and amplification where one receptor may activate many G proteins.
G protein signaling is terminated by hydrolysis of bound GTP to bound GDP. This can occur through the intrinsic GTPase activity of the α subunit, or be accelerated by separate regulatory proteins that act as GTPase-activating proteins (GAPs), such as members of the Regulator of G protein signaling (RGS) family). The speed of the hydrolysis reaction works as an internal clock limiting the length of the signal. Once G is returned to being GDP bound, the two parts of the heterotrimer re-associate to the original, inactive state.
The heterotrimeric G proteins can be classified by sequence homology of the α unit and by their functional targets into four families: G family, G family, G family and G family. Each of these G protein families contains multiple members, such that the mammals have 16 distinct -subunit genes. The G and G are likewise composed of many members, increasing heterotrimer structural and functional diversity. Among the target molecules of the specific G proteins are the second messenger-generating enzymes adenylyl cyclase and phospholipase C, as well as various ion channels.