The fifth subunit in a3b4 nicotinic receptor is more than an accessory subunit

The a3b4 subtype is the predominant neuronal nicotinic acetylcholine receptor present in the sensory and autonomic ganglia and in a subpopulation of brain neurons. This subtype can form pentameric receptors with either 2 or 3 b4 subunits that have different pharmacologic and functional properties. To further investigate the role of the fifth subunit, we coexpressed a dimeric construct coding for a single polypeptide containing the b4 and a3 subunit sequences, with different monomeric subunits. With this strategy, which allowed the formation of single populations of receptors with unique stoichiometry, we demonstrated with immunofluorescence and biochemical and functional assays that only the receptors with 3 b4 subunits are efficiently expressed at the plasma membrane. Moreover, the LFM export motif of b4 subunit in the fifth position exerts a unique function in the regulation of the intracellular trafficking of the receptors, their exposure at the cell surface, and consequently, their function, whereas the same export motif present in the b4 subunits forming the acetylcholine binding site is dispensable.