Cyclic GMP-binding sites and light control of free cGMP concentration in vertebrate rod photoreceptors

In this paper we describe the properties of cGMP-specific binding sites in rod outer segments (ROS) of vertebrate photoreceptors and their possible participation in the regulation of the concentration of cGMP. In particular we present data on the action of light, ATP and GTP on cGMP binding to specific sites in frog and bovine ROS. ATP and one of its non-hydrolyzable analogues induce a tenfold increase in cGMP binding to bleached ROS. Binding is half-maximal at ATP concentrations of 0.5-1.5μM. K D for cGMP binding to ATP-sensitive sites ranges from 0.7 to 7 μM, depending on the animal species and ATP concentration. There exists one binding site per 25-40 molecules of rhodopsin. Micromolar concentrations of GTP induce dissociation of cGMP from ATP-stimulated sites. In dark preparations, ATP and GTP do not affect cGMP binding. In the presence of ATP, light induces a fast (< 5 s at -12.5°C) binding of cGMP or τ < 10 ms when extrapolated to physiological temperature and protein concentrations. The presence of ATP and GTP results in cGMP desorption. Bleaching of 5 • 10 ~ 4 rhodopsin induces maximal binding. Direct photoaffinity labelling of ROS cGMP-binding proteins with [ 32 P]cGMP in the presence of ATP revealed a 58 kDa protein. A model of photoreceptor excitation is suggested based on light-stimulated cGMP adsorption-desorption, thereby controlling the free concentration of cGMP in a rod.