G Protein-Coupled Receptor Systems Involved in Cell Growth and Oncogenesis

THE family of heterotrimeric G proteins, referred to as G proteins, functions by coupling specific cell surface receptors to the control of intracellular signal transduction pathways. G protein-coupled receptors have a characteristic seven transmembrane (STM) structure, often leading to their description as serpentine. The extracellular and membrane domains of serpentine receptors vary significantly in sequence to allow selective binding of a variety of ligands including photons, ions, odorants, small molecules such as acetylcholine and catecholamines, peptides, and proteases (thrombin). The array of serpentine receptors couples to members of the G protein family. The known G proteins can be broadly categorized into four families based on sequence and/or functional homologies of the a-subunits (Table 1). Known effectors for G proteins include adenylyl cyclases, phosphatidylinositol phospholipase C (PLC) isof orms, novel phosphatidylinositol-3 kinases, cGMP phosphodiesterase, and selected ion channels. The list of G protein effectors is certain to grow and may include proteins such as the Na+/H+ antiporter and specific tyrosine kinases and phosphatases.