Human α4β2 Acetylcholine Receptors Formed from Linked Subunits

Abstract

We prepared concatamers of α4 and β2 subunits for human nicotinic acetylcholine receptors (AChRs), in which the C terminus of α4 was linked to the N terminus of β2, or vice versa, via a tripeptide sequence repeated 6 or 12 times, and expressed them in Xenopus oocytes. Linkage did not substantially alter channel amplitude or channel open-duration. Linkage at the C terminus of α4 prevented AChR potentiation by 17-β-estradiol by disruption of its binding site. Assembly of AChRs from concatamers was less efficient, but function was much more efficient than that of unlinked subunits. With both linked and free subunits, greater ACh-induced currents per surface AChR were observed with the (α4)3(β2)2 stoichiometry than with the (α4)2(β2)3 stoichiometry. The (α4)3(β2)2 stoichiometry exhibited much lower ACh sensitivity. When concatamers were expressed alone, dipentameric AChRs were formed in which the (α4)2(β2)3 pentamer was linked to the (α4)3(β2)2 pentamer. Dipentamers were selectively expressed on the cell surface, whereas most monopentamers with dangling subunits were retained intracellularly. Coexpression of concatamers with monomeric β2, β4, or α4 subunits resulted in monopentamers, the stoichiometry of which was determined by the free subunit added. Linkage between the C terminus of β2 and the N terminus of α4 favored formation of ACh-binding sites within the concatamer, whereas linkage between the C terminus of α4 and the N terminus of β2 favored formation of ACh-binding sites between concatamers. These protein-engineering studies provide insight into the structure and function of α4β2 AChRs, emphasizing the functional differences between α4β2 AChRs of different stoichiometries.