A NEW CYSTEINE FRAMEWORK IN SODIUM-CHANNEL BLOCKING CONOTOXINS

TWO novel sodium channel blocking peptides from the venom of the mollu scivorous snail Conus pennaceus, mu PnIVA and mu PnIVB, are described. Elucidation of their amino acid sequences was complicated by a previo usly undescribed anomalous product of reduction and pyridylethylation, which occurs on N-terminal cysteine residues and gives a PTH derivati ve eluting at the same position as PTH-Trp in reverse-phase chromatogr aphy. The amino acid sequences of the toxins were determined by a comb ination of Edman degradation and mass-spectrometric techniques as CCKY GWTCLLGCSPCGC (PnIVA) and CCKYGWTCWLGCSPCGC (PnIVB). These toxins bloc k sodium channels in molluscan neurons, but have no effect on sodium c urrents in bovine chromaffin cells or in rat brain synaptosomes. Altho ugh there is only one amino acid difference in the two sequences, PnIV B is approximately 6 times more potent than PnIVA in blockade of the s odium current in Lymnaea neurons. The PnIV sequences reveal a new cyst eine residue framework for conotoxins (CC-----C---C--C-C). Strikingly, the only charged residue in PnIVA/B is Lys3, Iodination reaction expe riments on the adjacent Tyr4 suggest that this region of the peptide m ust be solvent exposed and essential for activity, These structurally novel mu-conotoxins target a sodium channel subtype with low affinity for tetrodotoxin and therefore provide new probes for functional studi es on sodium channels.