Selective labeling of alpha-bungarotoxin with fluorescein isothiocyanate and its use for the study of toxin-acetylcholine receptor interactions.

The main product of the reaction of fluorescein isothiocyanate (FITC) and bungarotoxin (Bgt) under near stoichiometric conditions is a monofluorescein derivative preferentially labeled at Lys 26, a highly conserved residue known to be involved in the binding (McDaniel, C.S., Manshouri, T., and Atassi, M.Z. (1987) J. Prot. Chem. 6, 455-461; Garcia-Borron, J.C., Bieber, A.L., and Martinez-Carrion, M. (1987) Biochemistry 26, 4295-4303) of postsynaptic neurotoxins specific for the nicotinic acetylcholine receptor (AcChR). The fluorescently labeled toxin retains a high affinity for the AcChR, and an unaltered specificity. Binding of FITC-Bgt to AcChR results in a significant decrease in the fluorescence intensity of the probe. This AcChR-mediated quenching of FITC-Bgt fluorescence allows for a continuous monitoring of the binding process. The quenching of free and bound FITC-Bgt by charged and neutral quenchers shows few fluorophore accessibility changes as induced by the toxin-bound state. The results are consistent with a model in which the positively charged concave surface of the toxin interacts with a negatively charged complementary surface in the receptor molecule.