In the vast majority of influenza A viruses characterized to date, hemagglutinin 
(HA) is the receptor-binding and fusion protein, whereas neuraminidase (NA) is a 
receptor-cleaving protein that facilitates viral release but is expendable for 
entry. However, the NAs of some recent human H3N2 isolates have acquired 
receptor-binding activity via the mutation D151G, although these isolates also 
appear to retain the ability to bind receptors via HA. We report here the 
laboratory generation of a mutation (G147R) that enables an N1 NA to completely 
co-opt the receptor-binding function normally performed by HA. Viruses with this 
mutant NA grow to high titers even in the presence of extensive mutations to 
conserved residues in HA's receptor-binding pocket. When the receptor-binding NA 
is paired with this binding-deficient HA, viral infectivity and red blood cell 
agglutination are blocked by NA inhibitors. Furthermore, virus-like particles 
expressing only the receptor-binding NA agglutinate red blood cells in an 
NA-dependent manner. Although the G147R NA receptor-binding mutant virus that we 
characterize is a laboratory creation, this same mutation is found in several 
natural clusters of H1N1 and H5N1 viruses. Our results demonstrate that, at 
least in tissue culture, influenza virus receptor-binding activity can be 
entirely shifted from HA to NA.