A putative voltage-gated sodium channel alpha subunit (PpSCN1) from the hydrozoan jellyfish, Polyorchis penicillatus: structural comparisons and evolutionary considerations.

Extant cnidarians are probably the simplest metazoans with discrete nervous systems and rapid, transient voltage-gated currents carried exclusively by Na+ ions. Thus cnidarians are pivotal organisms for studying the evolution of voltage-gated Na+ channels. We have isolated a full-length Na+ channel alpha subunit cDNA (PpSCN1) from the hydrozoan jellyfish, Polyorchis penicillatus, that has one of the smallest known coding regions of a four domain Na+ channel (1695 amino acids). Homologous residues that have a critical bearing on the selectivity filter, voltage-sensor and binding sites for tetrodotoxin and lidocaine in vertebrates and most invertebrates differ in cnidarians. PpSCN1 is not alternatively-spliced and may be the only pore-forming alpha subunit available to account for at least three electrophysiologically distinct Na+ currents that have been studied in P. penicillatus.