Morphology of the isolated hemagglutinin and neuraminidase subunits of influenza virus

The BEL (A0) strain of influenza virus was disrupted with sodium dodecyl sulphate (SDS), and the hemagglutinin subunits were isolated by electrophoresis on cellulose acetate strips. Neuraminidase subunits were isolated from an A 0-A 2 recombinant influenza virus (X-7F1) in a similar way. Electron micrographs of the hemagglutinin subunits (in the presence of SDS) showed rods approximately 40 Å wide and 140 Å long. The neuraminidase subunits had a quite different appearance. In the presence of SDS, these were seen as oblong structures about 85 Å long and 50 Å wide with a centrally attached fibre 100 Å long possessing what appeared to be a diffuse tail or small knob about 40 Å in diameter at its end. When SDS was removed, the subunits aggregated. The hemagglutinin formed clusters of radiating rods; the neuraminidase subunits aggregated by the tips of their tails and formed structures similar in appearance to the heads of seeding dandelions. When SDS was removed from mixtures of the subunits, mixed clusters containing both kinds of subunits were formed. In the presence of SDS, the hemagglutinin subunits had a sedimentation coefficient of 7.5 S. They adsorbed to, but did not agglutinate red cells and thus appeared to be “monovalent.” Hemagglutinin activity was obtained only after removal of the SDS, and it is assumed that this activity was associated with the aggregates of rods seen in electron micrographs. The neuraminidase subunits in the presence of SDS sedimented faster (8.5 S) than the hemagglutinin subunits and possessed enzyme activity both before and after removal of the SDS. Both the hemagglutinin and neuraminidase, after the removal of SDS, sedimented more rapidly and as broader bands than in the presence of SDS, suggesting that they had formed aggregates—thus confirming the electron microscopic observations. These results suggest that the “spikes” seen on the surface of influenza virus particles are of two morphologically distinct kinds, one of which is associated with neuraminidase activity, and the other with the hemagglutinin.