SNAKE-VENOM TOXINS, UNLIKE SMALLER ANTAGONISTS, APPEAR TO STABILIZE ARESTING STATE CONFORMATION OF THE NICOTINIC ACETYLCHOLINE-RECEPTOR

Previous studies have shown that the pattern and degree of 3-(trifluor omethyl)-3-(m-[I-125]iodophenyl ([I-125]TID) photoincorporation into t he nicotinic acetylcholine receptor (nAChR) can be used as a sensitive measure of nAChR conformation. Upon desensitization by prolonged expo sure to agonists, certain drugs and detergents, or reconstitution into desensitizing lipids, the levels of [I-125]TID incorporation into the subunits of the nAChR are dramatically reduced. In this study, we cha racterized the effects of the snake venom proteins alpha-bungarotoxin and alpha-cobrotoxin, as well as the smaller antagonists tubocurarine and gallamine, on [I-125]TID incorporation into the subunits of both p artially-purified nAChR in native lipids, or affinity-purified nAChR r econstituted into different combinations of lipids. Unlike all other c ompounds previously tested, alpha-bungarotoxin and alpha-cobrotoxin re producibly increased the level of [I-125]TID incorporation into all fo ur subunits of nAChR reconstituted into dioleoylphosphatidylcholine, d ioleoylphosphatidic acid and cholesterol. Gallamine had little or no e ffect on [I-125]TID incorporation at any concentration tested (0.1 mu M-5 mM). Tubocurarine had no effect on [I-125]TID incorporation at low concentrations, but at higher concentrations reduced the level of [I- 125]TID labeling. The snake venom proteins may shift the population of nAChR, which exists as a mixture of resting state and desensitized co nformations, entirely to the resting state. However, the binding of th e snake venom toxins does not appear sufficient to induce the resting state conformation in nAChR which have been desensitized by other mean s, such as solubilization in desensitizing detergents or reconstitutio n in desensitizing lipids.