A single sequence change destabilizes the influenza virus neuraminidase tetramer

A single change (E119G) in the influenza A virus N9 neuraminidase (NA) results in resistance of the enzyme to the NA inhibitor 4-Guanidino-Neu5Ac2en (4-GuDANA). This change causes a salt link between Glu119, which sits in a pocket in the bottom of the active site of the enzyme, and the 4-guanidinium moiety of the inhibitor to be lost. NA "heads" of the resistant enzyme produced only a few small crystals under conditions in which the wild-type enzyme readily formed large crystals. These small crystals were of sufficient quality to yield X-ray crystallographic data which confirmed the E119G change and demonstrated the presence of electron density representing either a strong structural-water molecule or an anionic species in place of the glutamate carboxylate. NA heads of the resistant enzyme also have greatly reduced NA activity per milligram of total protein. We have now found that the mutant NA heads consist predominantly of monomers with a few dimers and tetramers, as determined by electron microscopic analysis of the protein. The low level of enzymatic activity as well as the small number of crystals obtained were probably from the few tetramers remaining intact in the preparation. The purified wild-type and 4-GuDANA-resistant enzymes were treated with the homobifunctional NHS-ester cross linker, DTSSP. SDS-PAGE analysis of the treated enzymes clearly revealed cross-linked dimers of the wild-type enzyme. In contrast, only a small proportion of the 4-GuDANA-resistant neuraminidase was cross-linked. An examination of the known X-ray crystallographic structure of the wild-type NA reveals a salt bridge between Glu119 and Arg156 of the same monomer. Arg156 is a conserved amino acid that is situated at the interface between monomers, and a salt link between this amino acid and Glu119 may contribute to the stability of enzyme tetramers. It is suggested that the E119G alteration in the 4-GuDANA-resistant NA leads to the abrogation of this interaction and thus to the instability of the NA tetramers.