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