THE N-TERMINAL DOMAINS OF ACETYLCHOLINE-RECEPTOR SUBUNITS CONTAIN RECOGNITION SIGNALS FOR THE INITIAL STEPS OF RECEPTOR ASSEMBLY

Ligand-gated ion channels are oligomeric membrane proteins in which homologous subunits specifically recognize one another and assemble around an aqueous pore. To identity domains responsible for the specificity of subunit association, we used a dominant-negative assay in which truncated subunits of the mouse muscle acetylcholine receptor (AChR) were coexpressed with the four wild-type subunits in transfected COS cells. Fragments of the alpha, delta, and gamma-subunits consisting solely of the extracellular N-terminal domain blocked surface expression of the AChR and the formation of alpha-delta-heterodimers, an early step in the assembly pathway of the AChR. Immunoprecipitation and sucrose gradient sedimentation experiments showed that an N-terminal fragment of the alpha-subunit forms a specific complex with the intact delta-subunit. Thus the extracellular N-terminal domain of the alpha, delta, and gamma-subunits contains the information necessary for specific subunit association.