NIH Research Festival
Influenza A virus, a major human respiratory pathogen, enters the host cell by receptor mediated endocytosis. The influenza transmembrane protein hemagglutinin (HA) changes conformation at low pH and catalyzes membrane fusion pore formation which allows release of the genome and subsequent formation of virus progeny. Membrane fusion is thought to proceed through an intermediate called hemifusion; yet for viral fusion the hemifusion structure has never been characterized. To capture the hemifusion intermediate we used influenza hemagglutinin (HA) incorporated into influenza virus-like particles (VLP). We studied the VLP fusion with liposomes mimicking the endosomal membrane at low pH using Volta phase-plate cryo-electron tomography which provides unprecedented signal-to-noise ratio close to focus. We found two distinct structures: a hemifusion diaphragm (HD) and a novel structure termed lipidic junction. Liposomes with lipidic junctions were ruptured with membrane edges stabilized by HA spikes. The rupture frequency and HD diameter was dependent on the level of cholesterol present in the liposomal membrane. At low cholesterol levels, ruptured membranes and HD with large diameters (~ 20 nm) were abundant. In contrast, at higher cholesterol concentrations closer to physiological conditions, ruptured liposomes were rare and HD with small diameters (~9 nm) were detected as predicted by theoretical calculations. We propose that cholesterol concentration in the endosomal membrane functions as a switch between two independent pathways: one leading to membrane rupture and lipidic junction as dead-end products and other leading to fusion pore formation. The latter is relevant under the conditions of influenza virus infection of cells.
Scientific Focus Area: Virology
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