Formation of Oligomers Containing the β3 and β4 Subunits of the Rat Nicotinic Receptor

Abstract

The role of the β3 and β4 subunits of the nicotinic acetylcholine receptor in brain is still unclear. We investigated nicotinic receptor structure with antibodies directed against unique regions of the β3 and β4 subunits of the rat nicotinic acetylcholine receptor. Anti-β4 detected a single band of 66 kDa in most regions of the brain that was strongest in striatum and cerebellum. The 60 kDa β3 subunit was detected primarily in striatum and cerebellum, and faintly in hippocampus. Immunoprecipitation experiments established that the two subunits were coassembled in the cerebellum along with the β2 subunit. Antibodies against the α4, β2, β3, and β4 subunits immunoprecipitated ∼75% of the bungarotoxin-insensitive nicotinic receptor from cerebellar extracts as determined by nicotine-dependent acetylcholine binding. Transfection of COS cells with cDNAs for these four subunits induced expression of a high affinity nicotinic receptor. Omission of only a single subunit from the transfection affected either theB_max or the apparentK_D of the receptor. Our data suggest that the β3 subunit functions as a structural entity that links a relatively unstable α4β2 heterodimer to a more stable α4β4 heterodimer. The agonist-binding site formed by α4β2 has a much greater affinity than does that formed by α4β4. In this respect, nicotinic receptors that contain the β3 subunit are structurally homologous to the muscle nicotinic receptor.