Structure of Influenza Virus N7: the Last Piece of the Neuraminidase “Jigsaw” Puzzle

Abstract

There are nine subtypes of influenza A virus neuraminidase (NA), N1 to N9. In addition, influenza B virus also contains NA, and there are two influenza virus NA-like molecules, N10 and N11, which were recently identified from bats. Crystal structures for all of these proteins have been solved, with the exception of N7, and there is no published report of N6, although a structure has been deposited in the Protein Data Bank. Here, we present the N7 and N6 structures at 2.1 Å and 1.8 Å, respectively. Structural comparison of all NA subtypes shows that both N7 and N6 highly resemble typical group 2 NA structures with some special characteristics, including an additional cavity adjacent to their active sites formed by novel 340-loop conformations. Comparative analysis also revealed new structural insights into the N-glycosylation, calcium binding, and second sialic acid binding site of influenza virus NA. This comprehensive study is critical for understanding the complexity of the most successful influenza drug target and for the structure-based design of novel influenza inhibitors. IMPORTANCE Influenza viruses impose a great burden on society, by the human-adapted seasonal types as well as by variants that occasionallyjump from the avian reservoir to infect humans. The surface glycoprotein neuraminidase (NA) is essential for the propagation of the virus and currently the most successfully drug-targeted molecule. Therefore, the structural and functional analysis of NA is critical for the prevention and control of influenza infections. There are nine subtypes of influenza A virus NA (N1 to N9). In addition, influenza B virus also contains NA, and there are two influenza NA-like molecules, N10 and N11, which were recently identified in bats. Crystal structures for all of these proteins have been solved and reported with the exception of N7 and N6. The structural analysis of influenza virus N7 and N6 presented in this study therefore completes the puzzle and adds to a comprehensive understanding of influenza virus NA.

Molecular Characterizations of Surface Proteins Hemagglutinin and Neuraminidase from Recent H5Nx Avian Influenza Viruses

“…The N6 NA possesses a unique 340-cavity similar to the previously published N6 and N7 structures (Fig. 8) (25). This small cavity has not been observed in any other NA structures, and given its close proximity to the active site and the conserved Ca 2ϩ binding site, it is likely to play a role in NA function.…”

“…Finally, while N8 NA has additional glycosylation sites at Asn86, Asn295, and Asn400, none of them had interpretable glycan density. There is a suggestion that N-glycosylation might be one of the factors to differentiate individual NAs within each subtype (25); however, more data are needed to confirm this correlation.…”

“…6), the overall NA structure is very similar, with the typical "box-shaped" tetrameric association of identical monomers, containing six four-stranded, antiparallel ␤-sheets that form a propeller-like arrangement (Fig. 7A) (25,(61)(62)(63)(64)(65)(66)(67)(68). The nine subtypes of influenza A virus NA found in birds have been classified into two groups according to their sequences (69).…”

“…The N1, N2, N6 and N8 structures were solved using NA structures from the influenza A/Brevig Mission/1/1918 (H1N1) (PDB code 3B7E), A/RI/5ϩ/1957 (H2N2) (PDB code 3TIA), A/chicken/Nanchang/7-010/ 2000 (H3N6) (PDB code 4QN4), and A/harbor seal/Massachusetts/1/ 2011 (H3N8) (PDB code 4WA3) viruses, respectively, as search models (15,(24)(25)(26). The NA structures were refined using the same strategy as the HA structures.…”

Resistance to Mutant Group 2 Influenza Virus Neuraminidases of an Oseltamivir-Zanamivir Hybrid Inhibitor

“…So far, the structures of all subtypes have been solved (13)(14)(15)(16)(17)(18)(19). Influenza viruses containing N1 and N2 (i.e., H1N1, H3N2, and H2N2) are the most common types that infect humans and are therefore a great threat to public health (20)(21)(22)(23).…”