SPECIFIC BINDING OF ATP TO EXTRACELLULAR SITES ON TORPEDO ACETYLCHOLINE-RECEPTOR

The beta- and delta-subunits of the nicotinic acetylcholine receptor from Torpedo californica were covalently photolabeled at the synaptic surface with the ATP photoaffinity analogue [alpha-P-32]-8-azido-ATP. The specificity of labeling for nucleotide binding sites was demonstrated by the saturation of labeling with increasing concentration of 8-azido-ATP and the inhibition of photolabeling by ATP. Protection studies suggest that the binding sites for the photolabel are unique and are not associated with the cholinergic ligand binding sites.