[Structure and function of heterotrimeric G-proteins].

In mammals, G-protein alpha, beta, gamma polypeptides are encoded by at least 16, 4 and 7 genes, respectively. G alpha-subunits bind and hydrolyze GTP and have the sites for bacterial toxin-catalyzed ADP-ribosylation. A structural model of G alpha-subunits can be defined on the basis of similarities between G alpha and other members of the GTP-binding proteins. The resulting G alpha model specifies the spatial relationship among the guanine nucleotide binding site, the binding site of the G beta gamma-subunit complex, likely regions of effector and receptor interaction, and sites of cholera or pertussis toxin-induced modification. The architecture of the G alpha core is the same as that of p21ras. Experimental evidence from immunological, molecular genetic and biochemical studies support the G alpha model. The G alpha-subunits alone were previously thought to act on the effector enzymes; However, recent evidence indicates that the G beta gamma-dimer also plays an important part in effector activation.