Molecular Cloning of a Putative Cyclic Nucleotide‐Gated Ion Channel cDNA from Limulus polyphemus

: Cyclic nucleotide‐gated channels have been proposed to mediate the electrical response to light in the ventral photoreceptor cells of the horseshoe crab, Limulus polyphemus. However, a cyclic nucleotide‐gated channel has not been identified from Limulus. We have cloned a putative full‐length cyclic nucleotide‐gated channel cDNA by screening cDNA libraries constructed from Limulus brain using a probe developed from Limulus ventral eye nerves. The putative full‐length cDNA was derived from two overlapping partial cDNA clones. The open reading frame encodes 905 amino acids ; the sequence shows 44% identity to that of the α subunit of the bovine rod cyclic GMP‐gated channel over the region containing the transmembrane domains and the cyclic nucleotide binding domain. This Limulus channel has a novel C‐terminal region of ~200 amino acids, containing three putative Src homology domain 3 binding motifs and a putative coiled‐coil domain. The possibility that this cloned channel is the same as that detected previously in excised patches from the photoreceptive membrane of Limulus ventral photoreceptors is discussed in terms of its sequence and its expression in the ventral eye nerves.