Site of covalent attachment of alpha-scorpion toxin derivatives in domain I of the sodium channel alpha subunit.

Purified and reconstituted sodium channels from rat brain have been photoaffinity labeled with a photoactivable derivative of the alpha-scorpion toxin V from Leiurus quinquestriatus (LqTx). A battery of sequence-specific antibodies has been used to determine which of the peptides produced by chemical and enzymatic cleavage of the photolabeled sodium-channel alpha subunit contain covalently attached LqTx. Nearly all of the covalently attached LqTx is found within homologous domain I. Two site-directed antisera, which recognize residues 317 to 335 and residues 382 to 400, respectively, specifically immunoprecipitate a 14-kDa peptide produced by CNBr digestion to which LqTx is covalently attached. It is proposed that a portion of the receptor site for alpha-scorpion toxins is formed by peptide segment(s) between amino acid residues 335 and 378 which is located in an extracellular loop between transmembrane helices S5 and S6 of homologous domain I of the sodium channel alpha subunit.