G protein

- located in cytoplasm
- coupled to transmembrane receptor (G protein coupled receptor) - seven helical membrane receptors
- ligand binding site is exposed outside surface of cell. Many ligands binds to GPCR, such as TSH, ACTH, serotonin and GABA
- Ligand binding initiates secondary messenger - cAMP or IP3
- best described human disease caused by mutation of G protein is pseudohypoparathyroidism (Albright's hereditary osteodystrophy)