Millisecond activation of transducin in the cyclic nucleotide cascade of vision

Abstract

Cyclic GMP has been implicated as a messenger molecule involved in visual transduction¹. 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 limit⁷. 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.