ANTIGENIC ROLE OF SINGLE RESIDUES WITHIN THE MAIN IMMUNOGENIC REGION OF THE NICOTINIC ACETYLCHOLINE-RECEPTOR

The target of most of the autoantibodies against the acetylcholine receptor (AChR) in myasthenic sera is the main immunogenic region (MIR) on the extracellular side of the AChR α-subunit. Binding of anti-MIR monoclonal antibodies (mAbs) has been recently localized between residues α67 and α76 of Torpedo californica electric organ (WNPADYGGIK) and human muscle (WNPDDYGGVK) AChR. In order to evaluate the contribution of each residue to the antigenicity of the MIR, we synthesized peptides corresponding to resiodues α67-76, from Torpedo and human AChRs, together with 13 peptide analogues. Nine of these analogues had one residue of the Torpedo decapeptide replaced by L-alanine, three had a structure which was intermediate between those of the Torpedo and human α67-76 decapeptides, and one had D-alanine in position 73. Binding studies employing six anti-MIR mAbs and all 15 peptides revealed that some residues (Asn68 and Asp71) are indispensable for binding by all mAbs tested, whereas others are important only for binding by some mAbs. Antibody binding was mainly restricted to residues α68-74, the most critical sequence being α68-71. Fish electric organ and human MIR form two distinct groups of strongly overlapping epitopes. Some peptides analogues enhanced mAb binding compared with Torpedo and human peptides, suggesting that the construction of a very antigenic MIR is feasible.