ONLY SNAKE CURAREMIMETIC TOXINS WITH A 5TH DISULFIDE BOND HAVE HIGH-AFFINITY FOR THE NEURONAL ALPHA-7 NICOTINIC RECEPTOR

Long chain and short chain curaremimetic toxins from snakes possess 66 -74 residues with five disulfide bonds and 60-62 residues with four di sulfide bonds, respectively, Despite their structural differences all of these toxins bind with high affinity to the peripheral nicotinic ac etylcholine receptors (AChR), Binding experiments have now revealed th at long chain toxins only, like the neuronal K-bungarotoxin, have a hi gh affinity for a chimeric form of the neuronal alpha 7 receptor, with K-d values ranging from about 1 to 12 nM, In contrast, all other toxi ns bind to the chimeric alpha 7 receptor with a low affinity, with K-d values ranging between 3 and 22 mu M. These results are supported by electrophysiological recordings on both the wild-type and chimeric alp ha 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 agreeme nt with this conclusion, we show that a long chain toxin whose fifth d isulfide is reduced and then dithiopyridylated has a low affinity (K-d = 12 mu M) for the neuronal alpha 7 receptor, whereas it retains a hi gh affinity (K-d = 0.35 nM) for the peripheral AChR, Thus, a long chai n curaremimetic toxin having a reduced fifth disulfide bond behaves li ke a short chain toxin toward both the peripheral and neuronal AChR, T herefore, functional classification of toxins that bind to AChRs shoul d probably be done by considering their activities on both peripheral and neuronal receptors.