Postsynaptic polypeptide neurotoxins isolated from the venoms of elapid and hydrophid snakes exhibit the ability to bind selectively to and inhibit different types of receptors that function in nerve signal transmission. On the basis of their amino acid sequences and three-dimensional structures, these neurotoxins are clearly related, but nothing is yet known about the basis for their physiological receptor specificity. In this report, site-directed mutants of kappa-bungarotoxin, produced by an Escherichia coli expression system, are tested to determine the function of selected amino acid side chains in the interaction between toxin and receptor. Highly conserved residues at the bottom of the second loop (a region that has been shown to be a major point of contact with the receptor), particularly those residues at the junction between the beta-sheet and the end of the loop, were selected. The results demonstrate that a single amino acid substitution of the invariant arginine residue (Arg-40 to Ala-40) renders the toxin unable to inhibit nerve transmission in the chick ciliary ganglion up to a concentration of 10 microM. Significantly, the results also show that conversion to alanine of the nearby proline residue (Pro-42) found to be invariant in all kappa-neurotoxins, but not found in any potent alpha-neurotoxin, produces a toxin with full inhibitory capacity. However, the introduction of a lysine residue at this position (P-42-K), like that found in alpha-bungarotoxin, reduces activity significantly.(ABSTRACT TRUNCATED AT 250 WORDS)