Trimeric G-proteins are transducers which convert a signal from the G-protein linked receptor onto other proteins on the plasma membrane. The trimeric G-protein consists of 3 heterologous subunits named alpha, beta and gamma. The alpha subunit is a GTP-ase which binds GDP in its resting state. When the G-protein is activated by an external signal it splits into the activated alpha subunit (which is now bound to GTP) and the activated beta-gamma complex. Depending on the signalling pathway, either one of these molecules can be the one to interact with the enzyme in the next step of the cell signalling process. 
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.
- ↑ B Alberts, A Johnson, J Lewis, D Morgan, M Raff, K Roberts, P Walter. Molecular Biology of the Cell. 6th Ed. New York: Garland Science. 2014. Chapter 15, Page 817-819.