[Cellular information and communication: receptors coupled to g-proteins]

A general model for membrane receptor action mechanisms indicates that information detected by receptors in the cell plasma membrane is transmitted to biological effectors, normally mediated by coupling elements. Seven-site transmembrane receptors coupled to G-proteins likely represent the most conserved and diversified category of membrane receptors. Information transfer from the extracellular medium to the cell involves three membrane proteins: a receptor, a G-protein (which binds and hydrolyzes GTP during its cycle), and an effector that regulates intracellular ion levels or second messengers. They detect various messages: light emissions, odorant molecules, peptide hormones, neurotransmitters and proteins. Structures of these membrane proteins include an extracellular N-terminal part, seven transmembrane alpha-helices and an intracellular C-terminal part. They are coupled to a G-protein which is stimulated upon ligand-induced receptor activation. This activated G-protein exchanges a GDP molecule for a GTP molecule which in turn acts on the effector. The effector is an enzyme (adenylate cyclase, phospholipase C, etc.) or an ionic channel (K+, Ca++ channels, etc.). Recognition of the message (which is specified by the membrane receptor), following amplification through activation of several catalytic cycles of G-proteins and the effector, thus leads to cellular events: modification of genetic transcription, cell growth, intercellular communication and modulation of the membrane potential. The present article briefly summarizes results on this topic with special stress on the angiotensin II receptor which is currently being investigated in our laboratory.