Binding stoichiometry of a fluorescent cGMP analogue to membranes of retinal rod outer segments

The high-affinity binding of the cGMP analogue 8-(5-thioacetamidofluorescein)-cGMP to rod outer segment membranes depleted of peripherally bound proteins has been defined by equilibrium dialysis (mean ± SD): (a) membranes contain about one cGMP binding site per 130 rhodopsin molecules; (b) the concentration of free ligand for half saturation is 2.0 ± 0.6 μM; (c) the apparent Hill coefficient of the bound versus free ligand relationship is 1.7 ± 0.5; (d) half saturation of the binding sites is sufficient for 85% activation of calcium permeability. A gating mechanism is proposed.