FORMATION OF OLIGOMERS CONTAINING THE BETA-3 AND BETA-4 SUBUNITS OF THE RAT NICOTINIC RECEPTOR

The role of the beta 3 and beta 4 subunits of the nicotinic acetylchol ine receptor in brain is still unclear. We investigated nicotinic rece ptor structure with antibodies directed against unique regions of the beta 3 and beta 4 subunits of the rat nicotinic acetylcholine receptor . Anti-beta 4 detected a single band of 66 kDa in most regions of the brain that was strongest in striatum and cerebellum. The 60 kDa beta 3 subunit was detected primarily in striatum and cerebellum, and faintl y in hippocampus. Immunoprecipitation experiments established that the two subunits were coassembled in the cerebellum along with the beta 2 subunit. Antibodies against the alpha 4, beta 2, beta 3, and beta 4 s ubunits immunoprecipitated similar to 75% of the bungarotoxin-insensit ive nicotinic receptor from cerebellar extracts as determined by nicot ine-dependent acetylcholine binding. Transfection of COS cells with cD NAs for these four subunits induced expression of a high affinity nico tinic receptor. Omission of only a single subunit from the transfectio n affected either the B-max or the apparent K-D of the receptor. Our d ata suggest that the beta 3 subunit functions as a structural entity t hat links a relatively unstable alpha 4 beta 2 heterodimer to a more s table alpha 4 beta 4 heterodimer. The agonist-binding site formed by a lpha 4 beta 2 has a much greater affinity than does that formed by alp ha 4 beta 4. In this respect, nicotinic receptors that contain the bet a 3 subunit are structurally homologous to the muscle nicotinic recept or.