Crystallization and preliminary X-ray diffraction analysis of two homologous antigen-binding fragments in complex with different carbohydrate antigens

The antigen-binding fragments (Fab) of two murine monoclonal antibodies (mA b) S25-2 and S45-18, specific for carbohydrate epitopes in the lipopolysacc haide of the bacterial family Chlamydiaceae, have been crystallized in the presence and absence of synthetic oligosaccharides corresponding to their r espective haptens. Crystals of both Fabs show different morphology dependin g on the presence of antigens. The sequence of mAb S45-18 was determined an d shows a remarkable homology to that reported for mAb S25-2. These crystal s offer an unparalleled opportunity to compare the structure and modes of b inding of two homologous antibodies to similar but distinct carbohydrate ep itopes.