Visual Signal Transduction; The Cycle of Transducin Shuttling between Rhodopsin and cGMP Phosphodiesterase

Among the numerous G proteins that couple membrane receptors to effectors of intracellular messengers, the system of transducin (T) shuttling between photo-excited rhodopsin (R*) and cGMP phosphodiesterase (PDE) constitutes a most interesting model. Three features of this system have made a large variety of in situ biophysical measurements possible and have greatly facilitated biochemical purification and reconstitution experiments: (1) the exceptionally high concentration of the components (rhodopsin, transducin, and PDE) in retinal rod outer segments (ROS); (2) the convenient excitability of the system by light flashes; and (3) the peculiarity of the solubility of transducin and PDE in the total absence of detergent.