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.

The structure of a GPCR shows 7 transmembrane spans, with 3 exoloops and 4 cytoloops. The N-terminal is found on the outside of the membrane and the C-terminal found on the cytosolic side of the membrane.