Characterisation, crystallisation and preliminary X-ray diffraction analysis of a Fab fragment of a rat monoclonal antibody with very high affinity for the human muscle acetylcholine receptor

The Fab fragment of a rat monoclonal antibody (no. 192) with very high affinity for the main immunogenic region of the human muscle nicotinic acetylcholine receptor (AChR) has been purified, characterised and crystallised using vapour diffusion techniques. Its K(d) for human AChR was determined to be 5x10^-11 M. Its cross-reactivity pattern suggests that residue α23 of the AChR strongly affects its epitope. Crystals suitable for X-ray analysis, obtained by micro- and macroseeding techniques, belong to the orthorhombic space group C222₁ and they diffract to 2.8 Å resolution using synchrotron radiation. The unit cell dimensions are α = 83.4 Å, b = 110.0 Å and c = 212.2 Å and there are two Fab molecules per asymmetric unit.