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

Abstract

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.