In mammals, G-protein α, β, γ polypeptides are encoded by at least 16, 4 and 7 genes, respectively. G
α-subunits bind and hydrolyze GTP and have the sites for bacterial toxin-catalyzed ADP-ribosylation. A structural model of G
α-subunits can be defined on the basis of similarities between G
α and other members of the GTP-binding proteins. The resulting G
α model specifies the spatial relationship among the guanine nucleotide binding site, the binding site of the G
βγ-subunit complex, likely regions of effector and receptor interaction, and sites of cholera or pertussis toxin-induced modification. The architecture of the G
α core is the same as that of p21
ras. Experimental evidence from immunological, molecular genetic and biochemical studies support the G
α model. The G
α-subunits alone were previously thought to act on the effector enzymes; However, recent evidence indicates that the G
βγ-dimer also plays an important part in effector activation.
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