kappa-Bungarotoxin (kappa-bgt) is a 66-residue peptide originally purified from snake venom that acts as an antagonist at certain acetylcholine receptors. It is one of four homologous kappa-neurotoxins that are distinguished from the structurally related alpha-neurotoxins by their ability to block the alpha 3-subunit-containing neuronal nicotinic acetylcholine receptor (nAChR). It has been reported that venom-purified kappa-bgt also displays some affinity for the alpha 1-subunit-containing muscle nAChR to which the alpha-neurotoxins bind with high affinity. Here we report the effects of particular mutations on the ability of recombinant kappa-bgt to block the binding of 125I-alpha-bgt to nAChRs found in fetal mouse muscle and chick skeletal muscle. While the replacement of a proline residue found in all kappa-neurotoxins with an alanine (P-42-A) has relatively little effect, the introduction of a lysine, which is found in 90% of active alpha-neurotoxins at the same position (P-42-K), eliminates muscle receptor affinity at the concentrations tested. In contrast, the replacement of a glutamine in kappa-bgt with a tryptophan found in all active alpha-neurotoxins (Q-32-W) increases the affinity of kappa-bgt for the muscle receptor. When the arginine residue found in all active alpha- and kappa-neurotoxins is replaced by an alanine (R-40-A), the ability of kappa-bgt to block the muscle receptor is reduced to undetectable levels.(ABSTRACT TRUNCATED AT 250 WORDS)