G-proteins share features that distinguish them from other GTP-binding proteins. These features include (1) association with the cytoplasmic surface of the plasma membrane (ras p21 and some other low molecular weight GTP-binding proteins are also associated with the cytoplasmic membrane surface); (2) function as receptor–effector couplers; and (3) heterotrimeric structure. Receptors coupled to G-proteins share a common overall topographic structure. Hydrophobicity plots of the amino acid sequences predicted by cDNAs encoding G-protein-coupled receptors suggest that a “generic” receptor consists of a single polypeptide chain that spans the plasma membrane seven times. Effectors regulated by G-proteins include enzymes of second messenger metabolism and ion channels. Some of these are clearly transmembrane proteins (e.g., adenylyl cyclase), but others (e.g., cGMP phosphodiesterase) are peripheral membrane proteins, and for others the structure has yet to be elucidated.
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