How do snake curaremimetic toxins discriminate between nicotinic acetylcholine receptor subtypes

Curaremimetic toxins from snake venoms form a large family of small protein s that adopt a similar fold and which bind to Torpedo nicotinic acetylcholi ne receptors with high affinity. Notwithstanding its apparent homogeneity, the toxin family is subdivided into short-chain (60-62 residues and four di sulfide bonds) and long-chain toxins (66-74 residues and five disulfide bon ds). In agreement with this structurally-based distinction we recently show ed that only long-chain toxins bind with high affinity to the neuronal nico tinic acetylcholine alpha 7 receptor. We suggested that a small loop cycliz ed by a disulfide bond and uniquely present in long-chain toxins may act as a major discriminative element. To assess the validity of this proposal we prepared various derivatives of a long-chain toxin, using stepwise solid-p hase synthesis. We found that replacement of both half cystines of the smal l loop by a serine caused a 35-fold affinity decrease for the neuronal rece ptor and only a 6-fold affinity decrease for Torpedo receptor. In addition, insertion of this loop at a homologous position of a short-chain toxin cau sed a 20-fold affinity increase for the neuronal receptor whereas it did no t modify its affinity for the Torpedo receptor. Our findings, therefore, re veal that a small structural deviation from a toxin fold can generate exqui site discriminative recognition for some receptor subtypes. (C) 1998 Elsevi er Science Ireland Ltd. All rights reserved.