Flow of information in the light-triggered cyclic nucleotide cascade of vision

Photolyzed rhodopsin catalyzes the exchange of GTP for FDP bound to a protein in retinal rod outer segments. We previously proposed that the GTP complex of this protein regulates the cyclic GMP phosphodiesterase and that it may be the first amplified intermediate in visual excitation [Fung, B. K.-K. & Stryer, L. (1980) Proc. Natl. Acad. Sci. USA 77, 2500-2504]. We report here the identification and characterization of transducin, a regulatory protein consisting of three kinds of polypeptide chains: T alpha (39 kilodaltons), T beta (36 kilodaltons), and T gamma (approximately 10 kilodaltons). Reconstituted membranes containing transducin and rhodopsin but no phosphodiesterase exhibit GTPase activity and amplified binding of guanosine 5'[beta, gamma-imido]triphosphate (p[NH]ppG), a nonhydrolyzable analog of GTP, on illumination. A single photolyzed rhodopsin molecule led to the uptake of p[NH]ppG by 71 molecules of transducin. High-pressure liquid chromatography showed that the binding site for GTP is on the alpha subunit of transducin. The isolation of the complex of ;[NH]ppG with T alpha enabled us to determine whether this species is the activator of the phosphodiesterase. We found that phosphodiesterase on unilluminated disc membranes can indeed be fully activated by addition of T alpha containing bound p[NH]ppG. These findings strongly suggest that transducin is the first amplified information-carrying intermediate in the cyclic nucleotide cascade of vision.