Molecular cloning of a putative cyclic nucleotide-gated ion channel cDNA from Limulus polyphemus

Cyclic nucleotide-gated channels have been proposed to mediate the electric al response to light in the Ventral photoreceptor cells of the horseshoe cr ab, 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 clo nes. The open reading frame encodes 905 amino acids; the sequence shows 44% identity to that of the Lu subunit of the bovine rod cyclic GMP-gated chan nel over the region containing the transmembrane domains and the cyclic nuc leotide binding domain, This Limulus channel has a novel C-terminal region of similar to 200 amino acids, containing three putative Src homology domai n 3 binding motifs and a putative coiled-coil domain. The possibility that this cloned channel is the same as that detected previously in excised patc hes from the photoreceptive membrane of Limulus Ventral photoreceptors is d iscussed in terms of its sequence and its expression in the ventral eye ner ves.