Crystals of antibodies complexed with influenza virus neuraminidase show isosteric binding of antibody to wild-type and variant antigens.

We describe here, for the first time, crystals of antibodies bound to a viral antigen which diffract X-rays to beyond 3 A. Crystals have been grown of Fab fragments of monoclonal antibody NC41, complexed with influenza virus neuraminidase (NA) of the N9 subtype and with a variant of N9 NA having a sequence change of Asn to Asp at position 331. This reduces, but does not abolish, the binding of NC41 antibody (in the case of another variant, Ser 371 to Leu, binding of NC41 antibody appears to be abolished). We are presenting data on the three-dimensional structure of these two complexes which indicates that NC41 antibody binds isosterically to the wild type and variant neuraminidase molecules.