Identification of two amino acid residues in the epsilon subunit that promote mammalian muscle acetylcholine receptor assembly in COS cells.

We have used a species difference in epsilon subunits of the acetylcholine receptor (AChR) to investigate regions of the subunit protein that are important in receptor assembly. Upon transient transfection of COS cells, mouse epsilon subunit cDNA is approximately 10 times more effective than that of the rat in supporting expression of surface AChRs when the other subunits are from either mouse or rat. In cells transfected with only alpha and epsilon subunit cDNAs, the formation of an alpha epsilon heterodimer, a presumed assembly intermediate, is also less efficient with rat than with mouse epsilon subunit. By site-directed mutagenesis, we have found that these differences can be accounted for by 2 amino acid differences in the N-terminal domain at positions 106 and 115 of the rat and mouse epsilon subunits, suggesting that the region near these 2 amino acid residues is important for AChR assembly.