Both residues form hydrogen bonds with the heavy
chain of the Fab of mAb Mem5, whereas residue 199 also contacts
the light chain. Mutation of either residue
198 or 199 profoundly impacted the binding of NA by the tested
mAbs, including Mem5, similar to that
observed with mutants m1G8-1 and m1G8-2 in the present study.
More interestingly, residue 338 in the N1 subtype of NA is critical
for the binding by a group of cross-reactive mAbs, which react with
the NA of both H5N1 and H1N1, including the 1918 and
2009 pandemic H1N1 as well as seasonal H1N1 viruses. It is of significance to investigate
whether epitopes involving residues 198/199 or 338 are also
present in other subtypes of NAs.