G-proteins

Guanine nucleotide binding protein that binds GTP when activated, which it hydrolyses to GDP.  The active form of the protein can interact with a number of down stream effectors such as adenylyl cyclase (producing cAMP from ATP), and phospholipase C (PLC).

The G-proteins can be divided in to two families - the heterotrimeric G-proteins and the small or mono-meric G-proteins.

G-protein coupled receptors (GPCR) upon binding of stimulus activates G-protein.