Synthetic peptides corresponding to sequences of snake venom neurotoxins and rabies virus glycoprotein bind to the nicotinic acetylcholine receptor

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Abstract
Peptides corresponding to portions of loop 2 of snake venom curare‐mimetic neurotoxins and to a structually similar region of rabies virus glycoprotein were synthesized. Interaction of these peptides with purified Torpedo electric organ acetylcholine receptor was tested by measuring their ability to block the binding of 125I‐labeled α‐bungarotoxin to the receptor. In addition, inhibition of α‐bungarotoxin binding to a 32‐residue synthetic peptide corresponding to positions 173–204 of the α‐subunit was determined. Neurotoxin and glycoprotein peptides corresponding to toxin loop 2 inhibited labeled toxin binding to the receptor with IC50 values comparable to those of nicotine and the competitive antagonist d‐tubocurarine and to the α‐subunit peptides with apparent affinities between those of d‐tubocurarine and α‐cobratoxin. Substitution of neurotoxin residue Arg37, the proposed counterpart of the quatenary ammonium of acetylcholine, with a negatively charged Glu residue reduced the apparent affinity about 10‐fold. Peptides containg the neutotoxin invarint residue Trp29 had 10‐ to 100‐fold higher affinites than peptides lacking this residue. These results demonstrate that relatively short synthetic peptides retain some of binding ability of the native protein from which they are derived, indicating that such peptides are useful in the study of proetin interactions. The ability of the peptides to compete α‐bungarotoxin binding to the receptor with apparent affinites comparable to those of other cholinergic ligands indicates that loop 2 of the neurotoxins and the strucually similar segment of the rabies virus glycoprotein act as recognition sites for the acetylcholine receptor. Invarient toxin residues Arg37 and Trp29 and their viral homologs play important, although not essential, roles in binding, possibly by interaction with complementary anionic and hydrophobic subsites on the acetylcholine receptor. The α‐subunit peptide most likely contains all of the determinants for binding of the toxin and glycoprotein peptides present on the α‐subunit, because these peptides bind to the 32‐residue α‐subunit peptides with the same or greater affinity as to the intact subunit.

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