| Influenza A viruses expresses both hemagglutinin (HA), which is responsible for binding to the terminal sialic acid of sialyloligosaccharides on the cell surface, and neuraminidase (NA), which contains a sialidase activity that removes sialic acid from sialyloligosaccharides. Interplay between HA receptor-binding and NA receptor-destroying sialidase activity appears important for replication of the virus. To investigate the sialidase requirement of influenza viruses further, we generated a series of sialidase-deficient mutants. We discovered these adapted viruses were capable of undergoing multiple cycles of replication in cell culture, eggs, and mice.; To understand this adaptation, we investigated changes in the HA receptor-binding affinity of the sialidase-deficient mutants. The results show that mutations around the HA receptor binding pocket reduce the virus' affinity for cellular receptors, thereby compensating for the loss of sialidase. Thus, sialidase activity is not absolutely required in the influenza A virus life cycle, but appears necessary for efficient virus replication. In addition, we investigated the adaptation of influenza virus to growth on a cell line, MaKS, which expresses low levels of virus receptor. Our results demonstrate that adaptation to such an environment results from loss of functional NA sialidase activity without alteration of HA binding affinity. Thus, in the presence of low levels of virus receptor, HA receptor-binding activity is insufficient to prevent progeny virus release. Thus these studies have demonstrated influenza A virus must maintain a balance between cellular binding affinity (HA activity) and progeny virion release (NA activity or viral receptor levels). |
