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Flow of Information in the Light-Triggered Cyclic Nucleotide Cascade of Vision

Bernard K.-K. Fung, James B. Hurley and Lubert Stryer
Proceedings of the National Academy of Sciences of the United States of America
Vol. 78, No. 1, [Part 2: Biological Sciences] (Jan., 1981), pp. 152-156
Stable URL: http://www.jstor.org/stable/10001
Page Count: 5
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Flow of Information in the Light-Triggered Cyclic Nucleotide Cascade of Vision
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Abstract

Photolyzed rhodopsin catalyzes the exchange of GTP for GDP 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: Tα (39 kilodaltons), Tβ (36 kilodaltons), and Tγ (≈ 10 kilodaltons). Reconstituted membranes containing transducin and rhodopsin but no phosphodiesterase exhibit GTPase activity and amplified binding of guanosine 5′-[β , γ -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 α subunit of transducin. The isolation of the complex of p[NH]ppG with Tα 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 Tα 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.

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