Heterotrimeric G-proteins

Heterotrimeric G-proteins are made up of three subunits: alpha (α), beta (β) and gamma (γ). In its resting state, the three subunits are associated together, and GDP is bound to the α subunit. The α subunit has GTP-hydrolysis activity. 

The G-protein acts as the transducer in the G-protein coupled receptor signalling system. It is a guanine nucleotide binding protein.

On activation of the receptor, the alpha unit releases GDP and binds GTP, and dissociates from the other subunits. The alpha unit has the ability to remian in an empty form once the GDP has been released. The alpha subunit consists of four different families: i, s, q and 12 which interact with effectors, leading to various different outcomes. The beta and gamma subunits form a dimer.