ABSTRACT
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 alpha-subunit. Binding of anti-MIR monoclonal antibodies (mAbs) has been recently localized between residues alpha 67 and alpha 76 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 residues alpha 67-76 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 alpha 67-76 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 alpha 68-74, the most critical sequence being alpha 68-71. Fish electric organ and human MIR form two distinct groups of strongly overlapping epitopes. Some peptide analogues enhanced mAb binding compared with Torpedo and human peptides, suggesting that the construction of a very antigenic MIR is feasible.
Subject(s)
Antigens/immunology , Oligopeptides/immunology , Receptors, Nicotinic/immunology , Amino Acid Sequence , Animals , Antibodies, Monoclonal/immunology , Binding, Competitive , Electric Organ/analysis , Epitopes/immunology , Humans , Molecular Sequence Data , Muscles/analysis , Structure-Activity Relationship , TorpedoABSTRACT
A comparative 1H-NMR spectral study of a synthetic decapeptide containing the main immunogenic region of the Torpedo acetylcholine receptor (AChR; WNPADYGGIK, representing the alpha 67-76 fragment of Torpedo AChR) with four analogous peptides (WNP3-D5YGGIK, WNPAA5YGGIK, WNPADYGGA9K, and WNPD4DYGGV9K) has been carried out in dimethyl sulfoxide. One- and two-dimensional nmr experiments [correlated spectroscopy (COSY), relayed COSY, and phase-sensitive nuclear Overhauser enhancement spectroscopy (NOESY)] were performed to obtain complete assignments of the proton resonances. The presence of strong and multiple short- and long-range NOEs, and especially a strong long-range NOE between the two Asn2-C alpha H and Gly7-C alpha H protons, argues in favor of a rigid folded structure in all five cases. Temperature dependence measurements indicate the existence of three intramolecular interactions involving the Asp3, Gly8, and Lys10 amide protons.
