cGMP-dependent cation channel of retinal rod outer segments

Light-modulated cytoplasmic cGMP simultaneously controls plasma membrane Na + conductance in visual excitation and Ca 2+ entry into rods by direct interaction with the cation channel 1–3 . Cytoplasmic Ca 2+ in turn may set operating points and contribute to the dynamics of several enzymes that regulate cGMP levels in the dark 4–6 , recovery from excitation and receptor adaptation or down regulation 7 . Similar channels may couple electrical activity to internal nucleotide metabolism in other tissues. We here report the identification, partial purification and behaviour after reconstitution of a protein of relative molecular mass 39,000 ( M r 39K) present in both disk and plasma membranes from bovine rod outer segments that mediates these cGMP-dependent cation fluxes. Its cGMP agonist specificity, kinetic cooperativity, ionic selectivity, membrane density and other features closely match the properties of the visual cGMP-dependent conductance inferred from elec-trophysiological measurements 1,2,8,9 .