Quaternary structure of influenza virus hemagglutinin after acid treatment.

Hemagglutinin (HA), a trimeric spike glycoprotein of influenza virus, mediates fusion between the viral envelope and the membrane of an endosome during virus entry. Fusion is triggered by low pH, which induces an irreversible conformational change in the protein. Several studies have indicated that intersubunit contacts along the trimer interfaces may be broken during this alteration. To determine whether HA dissociates into individual subunits as a consequence of the conformational change, we used velocity gradient sedimentation in the presence of Triton X-100. We also determined the resistance of acid-treated HA to dissociation by sodium dodecyl sulfate, a property of the HA trimer. At pH 7.0, isolated HA sedimented as a 9S trimer and gave the characteristic trimer pattern after sodium dodecyl sulfate-polyacrylamide gel electrophoresis. After acidification the HA remained trimeric irrespective of whether it was exposed to acid in intact virus particles or in solubilized form. Only when very low concentrations of HA were acidified did a fraction dissociate to dimers and monomers. In contrast, the water-soluble ectodomain fragment of HA (BHA) readily dissociated under a variety of conditions. Negative-stain electron microscopy supported the notion that HA molecules in virus particles do not dissociate upon acidification and may form larger oligomeric structures in the plane of the viral membrane. Taken together, the results suggested that it is the trimeric HA, or higher-order structures thereof, that are active in the acid-induced fusion reaction. Further, the results emphasized the role of the transmembrane anchors of HA in preventing dissociation of the trimer.