Distribution of sequence differences in influenza N9 neuraminidase of tern and whale viruses and crystallization of the whale neuraminidase complexed with antibodies.

Neuraminidase genes from A/tern/Australia/G70C/75 (H11N9) and A/whale/Maine/1/84 (H13N9) influenza viruses have been sequenced. Seventy-two nucleotide changes were found, 17 of which result in changes in the amino acid sequence of the neuraminidase; 3 in the stalk region and 14 in the heads. To our surprise, all of the sequence changes in the head region are located on the base of the neuraminidase tetramer, resulting in conservation of antigenic sites on top of the neuraminidase which vary extensively in human influenza virus neuraminidase. Whale N9 neuraminidase, like tern N9 neuraminidase, possesses high levels of hemagglutinating activity but, unlike the tern neuraminidase, failed to form large well-ordered crystals. However, when the neuraminidase was complexed with Fab fragments of monoclonal antibodies, which were made against the tern N9 neuraminidase, large crystals of the complexes were obtained which diffract X-rays to beyond 3 A.