Millisecond activation of transducin in the cyclic nucleotide cascade of vision

Cyclic GMP has been implicated as a messenger molecule involved in visual transduction 1 . Photoexcited rhodopsin (R * ) binds to a multisubunit membrane protein called transducin (T) and stimulates the exchange of a bound GDP molecule for GTP. This leads to the release of the α -subunit of T with bound GTP (T α –GTP), which activates a cyclic GMP phosphodiesterase 2–6 . The question arises as to whether the hydrolysis of cyclic GMP that results from activation of the phosphodiesterase is sufficiently rapid to be involved in visual excitation, which occurs on a time scale of ∼2 s in the single-photon limit 7 . Previous studies have suggested that the cyclic GMP phosphodiesterase is activated in less than 100 ms at moderate light levels 3,8,9 . We report here light scattering studies of magnetically orientated frog rod outer segments which show that a molecule of R * catalyses the activation of a molecule of T in about 1 ms. Thus, hundreds of molecules can be activated within the response time of vision in the single-photon limit, and the formation of T α –GTP is fast enough for it to be a key step in visual transduction.