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. [NHppG 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 pNHppG. These findings strongly suggest that transducin is the first amplified information-carrying intermediate in the cyclic nucleotide cascade of vision.