Affinity labelling of neuronal acetylcholine receptors localizes acetylcholine-binding sites to their beta-subunits

Neuronal nicotinic acetylcholine receptors (AChRs) from brains of chickens and rats consist of two types of subunits, alpha and beta, of which alpha shares some antigenic determinants with alpha-subunits from AChRs of electric organ and muscle [(1986) Biochemistry 25, 2082-2093; (1986) J. Neurosci. G, 3061-3069; (1986) Proc. Natl. Acad. Sci. USA, in press]. Here we demonstrate that after reduction with dithiothreitol (DTT) the AChRs can be specifically labelled with the acetylcholine-binding site directed reagent 4-(N-maleimido)benzyltri [3H]methylammonium iodide. Labelling of the beta-subunits of neuronal nicotinic AChRs indicates that the acetylcholine-binding site, and amino acids which may be homologous to Cys 192-193 of the alpha-subunits of AChRs from electric organ and muscle, are located on the beta-subunit of neuronal AChRs. These results suggest that although neuronal nicotinic AChRs have some structural homologies to AChRs from muscle and electric organs, the AChRs from these sources are quite distant relatives in an extended gene family.