DEDUCED AMINO-ACID-SEQUENCE OF A PUTATIVE SODIUM-CHANNEL FROM THE SCYPHOZOAN JELLYFISH CYANEA-CAPILLATA

Members of the phylum Cnidaria are the lowest extant organisms to poss ess a nervous system and are the first that are known to contain cells that produce action potentials carried exclusively by Na+ ions. They thus occupy an important position in the evolution of Na+ channels. A cDNA encoding a 198-kDa protein with high sequence identity to known N a+ channels was isolated from the scyphozoan jellyfish Cyanea capillat a. The similarity between this and other Na+ channels is greatest in t he transmembrane segments and the putative pore region and less so in the cytoplasmic loops that link the four domains of the protein. Phylo genetic analysis of the deduced protein reveals that it is closely rel ated to known Na+ channels, particularly those of squid and Drosophila , and more distantly separated from Ca2+ channels. Scrutiny of the Cya nea channel in regions corresponding to those purported to form the te trodotoxin receptor and selectivity filter of Na+ channels in higher a nimals reveals several anomalies that suggest that current models of t he location of the tetrodotoxin binding site and Na+ channel selectivi ty filter are incomplete.