Special features of the 2009 pandemic swine-origin influenza A H1N1 hemagglutinin and neuraminidase

Since the 2009 pandemic H1N1 swine-origin influenza A virus (09 S-OIV) has reminded the world about the global threat of the ever changing influenza virus, many questions regarding the detailed re-assortment of influenza viruses yet remain unanswered. Influenza A virus is the causative agent of the pandemic flu and contains 2 major antigenic glycoproteins on its surface: (i) hemagglutinin (HA); and (ii) neuraminidase (NA). The structures of the 09 S-OIV HA and NA proteins (09H1 and 09N1) have recently been resolved in our laboratory and provide some clues as to why the 09 S-OIV re-assortment virus is highly infectious with severe consequences in humans. For example, the 09H1 is highly similar to the HA of the 1918 influenza A pandemic virus in overall structure and especially in regards to its 5 defined antibody binding epitopes. For 09N1, its most distinctive feature is the lack of a 150-loop active site cavity, which was previously predicted to be present in all N1 NAs, and we hypothesize that the 150-loop may play a important role in the substrate specificity (alpha 2,3 or alpha 2,6 linked sialic acid receptors) and enzymatic mechanism of influenza NA. Combination of the HA and NA with special characteristics for the 09 S-OIV might contribute to its high increased transmissibility in humans.