Molecular design of an amplification cascade in vision

Abstract The photoexcitation of rhodopsin triggers a cascade that results in the hydrolysis of a large number of molecules of cyclic GMP. The molecular mechanism of this amplification cascade has been delineated. Transducin, a multisubunit perpheral membrane protein, is the information‐carrying intermediate in the activation of the cyclic GMP phosphodiesterase. Photoexcited rhodopsin (R*) castalyzes the exchange of GRP for GDP bound to the α‐subunit of transducin (T). About 500 molecules of T α ‐GTP are formed per absorbed photon at low light levels. T α ‐GTP, rekeased from the β‐ and γ‐subunits of transducin, then activates the phosphodiesterase by relieving an inhibitory constraint imposed by its small sununit. Each actived phosphodiesterase molecule hydrolyzes more than 100 cyclic GMP/s, giving an overall gain of more than 500,000. Photoexcited rhodopsin triggers the activation of a molecule of transducin in a millisecond, which is sufficiently rapid to enable this cascade to participate in visual excitation. Hydrolysis of GTP bound to T α seves to restore the system to the dark state. Transducin, like the G proteins of the adenylate cyclase casecade, can be specifically ADP‐ribosylated by cholera toxin and pertussis toxin. In both cascades, labling by pertussis toxin blocks the capacity of transducin to interact with the excited receptor, whereas labeling by cholera toxin inhibits the hydrolysis of bound GTP, leading to persistent activation. Moreover, the moleculaar design of the hormone‐triggered cyclic AMP cascade is similar to that of the light‐triggered cyclic GMP cascade. It seems likely that transducin, the stimulatory G protein, the inhibitor G protein, and the ras protein are members of the same family of signal amplifiers. The study of the cyclic nucleotide cascade of vision is providing rewarding views of recurring motifs of signal amplification in nature.