ENGINEERING A UNIQUELY REACTIVE THIOL INTO A CYSTEINE-RICH PEPTIDE

Cysteine mutagenesis for the purpose of chemical labelling was applied to the K+ channel neurotoxin charybdotoxin, a 37-residue peptide with six functionally essential cysteines. An additional 'spinster cystein e' was introduced at a position far away in space from the toxin's kno wn interaction surface where it contacts its K+ channel receptor. Desp ite the presence of the extra unpaired cysteine residue, the toxin sti ll folds efficiently and may be labelled by fluorescent and radioactiv e reagents to give a functionally competent toxin.