Snake venom toxins, unlike smaller antagonists, appear to stabilize a resting state conformation of the nicotinic acetylcholine receptor

Abstract

Previous studies have shown that the pattern and degree of 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine ([125I]TID) photoincorporation into the nicotinic acetylcholine receptor (nAChR) can be used as a sensitive measure of nAChR conformation. Upon desensitization by prolonged exposure to agonists, certain drugs and detergents, or reconstitution into desensitizing lipids, the levels of [125I]TID incorporation into the subunits of the nAChR are dramatically reduced. In this study, we characterized the effects of the snake venom proteins alpha-bungarotoxin and alpha-cobrotoxin, as well as the smaller antagonists tubocurarine and gallamine, on [125I]TID incorporation into the subunits of both partially-purified nAChR in native lipids, or affinity-purified nAChR reconstituted into different combinations of lipids. Unlike all other compounds previously tested, alpha-bungarotoxin and alpha-cobrotoxin reproducibly increased the level of [125I]TID incorporation into all four subunits of nAChR reconstituted into dioleoylphosphatidylcholine, dioleoylphosphatidic acid and cholesterol. Gallamine had little or no effect on [125I]TID incorporation at any concentration tested (0.1 microM-5 mM). Tubocurarine had no effect on [125I]TID incorporation at low concentrations, but at higher concentrations reduced the level of [125I]TID labeling. The snake venom proteins may shift the population of nAChR, which exists as a mixture of resting state and desensitized conformations, entirely to the resting state. However, the binding of the snake venom toxins does not appear sufficient to induce the resting state conformation in nAChR which have been desensitized by other means, such as solubilization in desensitizing detergents or reconstitution in densitizing lipids.

Rational Understanding of Nicotinic Receptors Drug Binding

“…The modeled structure is supported by three independent observations. First, the structure shows an "opening" of the binding site, thus allowing an easier access from the external surface of the pentamer for large snake venom toxins [8] that stabilize the basal "resting" state of nAChR [19,20]. Second, the docking of small ligands (ACh, nicotine and epibatidine) in the modeled sites results in lower affinities for the basal state in agreement with what is experimentally observed [6].…”

“Weak Toxin” from Naja kaouthia Is a Nontoxic Antagonist of α7 and Muscle-type Nicotinic Acetylcholine Receptors

“…In particular, long times required to restore the sensitivity of the ␣7 AChR to Ach after applying and washing out WTX might be associated with the conformational heterogeneity of WTX (44) and conformational changes that WTX might undergo itself on binding to the receptor or induce in the latter. Noteworthy allosteric effects of antagonists and agonists, some of them influencing the channel moiety, were described for the Torpedo AChR (45,46). In summary, the results obtained suggest that the biological target of WTX, a member of the family of weak toxins, may be nicotinic AChRs.…”

Models of the extracellular domain of the nicotinic receptors and of agonist- and Ca 2+ -binding sites

“…A model of interaction between ␣-bungarotoxin and AChBP has been presented by Harel et al (46), on the basis of the crystallized-compact-form of AChBP. However, it has been shown by independent approaches that snake toxins block the receptor by stabilizing the basal state (7)(8)(9). In agreement with this scheme, Harel's model exhibits numerous clashes between the toxin and the subunit carrying the complementary component of the binding site.…”