Long chain and short chain curaremimetic toxins from snakes possess 66 -74 residues with five disulfide bonds and 60 -62 residues with four disulfide bonds, respectively. Despite their structural differences all of these toxins bind with high affinity to the peripheral nicotinic acetylcholine receptors (AChR). Binding experiments have now revealed that long chain toxins only, like the neuronal -bungarotoxin, have a high affinity for a chimeric form of the neuronal ␣7 receptor, with K d values ranging from about 1 to 12 nM. In contrast, all other toxins bind to the chimeric ␣7 receptor with a low affinity, with K d values ranging between 3 and 22 M. These results are supported by electrophysiological recordings on both the wild-type and chimeric ␣7 receptors. Amino acid sequence analyses have suggested that high affinities for the neuronal receptor are associated with the presence of the fifth disulfide at the tip of the toxin second loop. In agreement with this conclusion, we show that a long chain toxin whose fifth disulfide is reduced and then dithiopyridylated has a low affinity (K d ‫؍‬ 12 M) for the neuronal ␣7 receptor, whereas it retains a high affinity (K d ‫؍‬ 0.35 nM) for the peripheral AChR. Thus, a long chain curaremimetic toxin having a reduced fifth disulfide bond behaves like a short chain toxin toward both the peripheral and neuronal AChR. Therefore, functional classification of toxins that bind to AChRs should probably be done by considering their activities on both peripheral and neuronal receptors.

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