ASSEMBLY OF THE MAMMALIAN MUSCLE ACETYLCHOLINE-RECEPTOR IN TRANSFECTED COS CELLS

We have investigated the mechanisms of assembly and transport to the cell surface of the mouse muscle nicotinic acetylcholine receptor (AChR) in transiently transfected COS cells. In cells transfected with all four subunit cDNAs, AChR was expressed on the surface with properties resembling those seen in mouse muscle cells (Gu, Y., A. F. Franco, Jr., P. D. Gardner, J. B. Lansman, J. R. Forsayeth, and Z. W. Hall. 1990. Neuron. 5:147-157). When incomplete combinations of AChR subunits were expressed, surface binding of I-125-alpha-bungarotoxin was not detected except in the case of alpha-beta-gamma which expressed < 15% of that seen with all four subunits. Immunoprecipitation and sucrose gradient sedimentation experiments showed that in cells expressing pairs of subunits, alpha-delta and alpha-gamma-heterodimers were formed, but alpha-beta was not. When three subunits were expressed, alpha-delta-beta and alpha-gamma-beta complexes were formed. Variation of the ratios of the four subunit cDNAs used in the transfection mixture showed that surface AChR expression was decreased by high concentrations of delta or gamma cDNAs in a mutually competitive manner. High expression of delta or gamma-subunits also each inhibited formation of a heterodimer with alpha and the other subunit. These results are consistent with a defined pathway for AChR assembly in which alpha-delta and alpha-gamma-heterodimers are formed first, followed by association with the beta-subunit and with each other to form the complete AChR.