Small angle X-ray diffraction (SAXD) and electron microscopy (EM) of membranes enriched in acetylcholine receptor (AcChR) have produced 20-30 angstrom-resolution three-dimensional models of the AcChR. Our unique library of anti-AcChR monoclonal antibodies (mAbs) define a set of epitopes on the AcChR each of which mark a functionally sensitive site. We propose to map these epitopes onto the three-dimensional model of the AcChR using SAXD and EM analysis of AcChR complexed to the mAbs or their Fab fragments. As we identify the amino acid residues comprising the epitopes for the mAbs in Project 5, our Fab locations will serve to map the residues onto AcChR subregions of dimensions (13 x 6 x 15 angstrom). In addition, we propose to localize the bromoacetylcholine affinity site of the AcChR, along with a second sulfhydryl site, by analyzing fluorescent dyes covalently attached to these two sites. The distance between these sites, and others, will be determined by resonance energy transfer. In addition, mAbs directed against the dyes will be used to identify their sites of attachment on the three-dimensional model of the AcChR. Hence, these studies will have mapped portions of the primary structure of the AcChR onto the model of its tertiary structure.
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