Mass spectrometric-based revision of the structure of a cysteine-rich peptide toxin with gamma-carboxyglutamic acid, TxVIIA, from the sea snail, Conus textile.

A mollusk-specific toxin, TxVIIA, having potent paralytic activity was isolated from the venom of sea snail Conus textile (Fainzilber M et al., 1991, Eur J Biochem 202:589-595). The structure reported above was based upon amino acid analysis and the Edman degradation. We have recently reinvestigated this toxin employing some of the most novel techniques in mass spectrometry. We now report a revised structure based primarily on high-energy collision-induced dissociation analysis of the two Asp17-N peptides of the reduced, pyridinylethyl derivative representing the entire sequence using matrix-assisted laser desorption ionization (MALDI) as CGGYSTYC gamma VDS gamma CCSDNCVRSYCTLF-NH2 (gamma, gamma-carboxyglutamic acid or Gla). The N-terminus of the previous sequence was incorrect, apparently due to a side reaction of reduction and alkylation, which led to the erroneous assignment of Trp for the N-terminal residue. In addition, the last two C-terminal amino acids and the C-terminal amidation had not been detected. Also, a combination of electrospray ionization mass spectrometry and positive and negative ion MALDI mass spectrometry provided information on the molecular weights of the native and derivatized toxin and presence of two Gla residues. Thus, TxVIIA does not have an "unusual" sequence as previously reported, but in fact belongs to the conserved Cys framework for omega- and delta-conotoxins. However, the four net negative charges with the cysteine-rich structure of this revised sequence is highly unusual for conopeptides.