Projection of critical NA residues onto the tertiary structure. A, NA monomer ribbon diagram. Tri-arginyl residues (R118, R292 and R371) are labeled blue for reference of the enzyme active site. Residue substitution sites (N234, M241, M257, E286, and G345) are labeled red. B, NA monomer surface representation. N234 and E286 are shown in red. C, NA tetramer surface representation. View is looking up from the virion surface at the underside of the globular head. N234 and E286 are shown in red. D, side profile view of NA tetramer surface representation, parallel to the viral membrane. N234 and E286 are labeled red and the active site is labeled blue. The NA structures are displayed using PyMOL published by DeLano Scientific. PDB ID: 2HTY[34, 35].