NIH Research Festival
Influenza A virus, a major human respiratory pathogen, enters the host cell by receptor mediated endocytosis. In the endosome, the influenza transmembrane protein hemagglutinin (HA) changes conformation at low pH and catalyzes membrane fusion between the viral and endosomal membrane. Membrane fusion is thermodynamically neutral but has a high kinetic barrier due to hydration repulsion of phospholipids. To overcome the initial energetic barrier, membrane fusion is thought to proceed through a low energy intermediate called hemifusion. Theoretical models predict two different structures: hemifusion stalk and hemifusion diaphragm. However, neither of the hemifusion structures has been directly observed in viral induced fusion. Here we study fusion intermediates catalyzed by either by wild-type HA or by hemifusion G1S mutant HA incorporated into virus-like particles using Volta-induced phase-plate for cryo-electron tomography. We show the structure of the G1S mutant HA mediated hemifusion diaphragms as well as induced free membrane edges. The average diameter of the hemifusion diaphragm is independent on the presence of the influenza matrix layer. Remarkably, in addition to complete fusion products, the free membrane edges and formation of HD are also prevalent in wild-type HA. Thus, the hemifusion diaphragm is a common, stable intermediate to membrane fusion formed by a pathway involving rupture of host membrane. This data offer hints of a novel alternate pathway of direct, hemagglutinin-mediated hemifusion diaphragm formation, omitting the intermediate of the hemifusion stalk.
Scientific Focus Area: Virology
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