Neuron 1993 Sep 01;113:459-66. doi: 10.1016/0896-6273(93)90150-p.

Identification of an external divalent cation-binding site in the pore of a cGMP-activated channel.

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Divalent cation blockade of cGMP-gated channels in photoreceptor cells ensures the low open channel noise required for a highly sensitive visual transduction process. This study identifies a divalent cation-binding site in the pore of a retinal cGMP-gated channel expressed in Xenopus oocytes. Substitution of a specific glutamate residue by a neutral amino acid renders the channel insensitive to external Mg2+ and Ca2+ and affects the conduction of Na+. The mutated channels remain sensitive to internal divalent cations. These results place the glutamate residue in the ion conduction pathway close to the extracellular surface.

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