Three-dimensional structure of neuraminidase of subtype N9 from an avian influenza virus

Neuraminidases from different subtypes of influenza virus are characterized by the absence of serlogical cross‐reactivity and an amino acid sequence homology of approximately 50%. The three‐dimensional structure of the neuraminidase antigen of subtype N9 from an avian influenza virus (A/tern/Australia/G70c/75) has been determined by X‐ray crystallography and shown to be folded similarly to neuraminidase of subtype N2 isolated from a human influenza virus. This result demontrates that absence of immunological cross‐reactivity is no measure of dissimilarity of polypeptide chain folding. Small differences in the way in which the subunits are organized around the molecular fourfold axis are observed. Insertions and deletions with respect to subtype N2 neuraminidase occur in four regions, only one of which is located within the major antigenic determinants around the enzyme active site.