The molecular-level events that take place when a flu virus fuses itself to a host cell and starts a new infection were recently revealed in stark resolution by University of Washington researchers.
Scientists at the UW School of Pharmacy used advanced electron microscopy to examine and sequence the stages of membrane fusion. The Journal of Virologypublished the breakthrough in late May.
“For the first time we are able to look at these processes at high resolution and see the proteins, the membranes and how they become contorted and remodeled as the virus carries out membrane fusion,” said Kelly Lee, associate professor of medicinal chemistry and the paper’s lead author.
Centers for Disease Control and Prevention
(Click to enlarge.) Illustration of the influenza virus, with protruding spikes that latch onto normal cells.
Illustration of the influenza virus
Understanding how the influenza virus aggressively pries its way into cells could inform research of new therapies for the flu and viruses such as HIV, herpes, dengue and Zika, he said.
Lee’s team captured three-dimensional, nanoscopic images with cryo-electron microscopy. These powerful electron microscopes and next-generation cameras afford unprecedented glimpses into structures of protein machinery and viruses.
Researchers take the protein and virus in a water buffer and, without staining the specimen, flash-freeze it in liquid ethane. The sample freezes before water can crystallize, which preserves the proteins and membranes in their native states. As with an x-ray or computed tomography (CT) scan, researchers tilt the samples for different views of the fusion process and then use computational methods to reconstruct the 3-D image.
Flu viruses are “enveloped” – that is, they have a lipid membrane that protects their genetic material. To deliver its genome into cells, the virus must merge its membrane with the host’s cellular membrane.
Flu viruses’ external membranes also are decorated with fusion protein, called “spike complexes” for their shape, and as the virus makes its way through a body, those spikes can grab and then draw the target membrane towards the virus, creating a dimple in the target surface. When the target membrane is pulled tight against the virus’ surface, a fusion occurs and a channel is opened, through which the virus delivers its genome to the cell.
Strong demand exists among researchers in many areas of biological science to use electron microscopes and to “harness their capabilities to help us understand cellular processes at the nanoscopic level,” Lee said.
Membrane fusion also is at the heart of basic cell biology, and is a critical step in sperm-egg fertilization and signaling across nerve synapses, for example. Lee hopes that his team’s work on the influenza virus case may help to improve the understanding of these fundamental cell functions.