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.