Characterization of an adult muscle acetylcholine receptor subunit by expression in fibroblasts

To study the functional and structural roles of the ε subunit in adult muscle acetylcholine receptor (AChR), we have co-expressed the α and ε subunits of the mouse receptor in transfected fibroblasts. Ligand binding studies suggest that association of ε with α subunit results in a lower association rate constant for ¹²⁵I-labeled α-bungarotoxin binding than that of the unassembled α subunit, approaching that for toxin binding to the AChR. Furthermore, αε complexes contain high affinity binding sites for competitive antagonists and agonists not present in the unassembled α subunit, but similar to one of the two nonequivalent binding sites in the adult AChR. Structural analysis of αε complexes by sucrose gradient velocity centrifugation suggests that some of the complexes formed are trimers or tetramers of α and ε subunits. Comparison of these data with those previously obtained for αγ complexes suggests that γ and ε have homologous functional roles and identical structural positions in the fetal and adult AChRs, respectively.