In mammals, G protein (alpha), (beta) and (gamma) polypeptides are encoded by at least 16, 4 and 7 genes, respectively. Most interest in G proteins has been focused on their (alpha) subunits, since these proteins bind and hydrolyze GTP and most obviously regulate the activity of the best studied effectors. Four distinct classes of G(alpha) subunits have been identified; these include Gs, Gi, Gq and G(alpha) 12/13. The Gi class comprises all the known (alpha) subunits that are susceptible to pertussis toxin modifications, including G(alpha) i-1, G(alpha) i-2, G(alpha) i-3, G(alpha) o, G(alpha) t1, G(alpha) t2, G(alpha) z and G(alpha) gust. In the well characterized visual system, photorhodopsin catalyzes the exchange of guanine nucleotides bound to the visual transducin G(alpha) subunits (G(alpha) t1 in rod cells and G(alpha) t2 in cone cells).