The health measures enacted to combat the coronavirus pandemic, in part, led to a record low number of influenza cases in the US for the 2020–21 flu season. As restrictions are lifted, however, the flu may resume its typical infection rates—the 2019–20 flu season, which the Centers for Disease Control and Prevention classed as moderate, saw a total of 38 million people sick and 22 000 deaths. Despite the disease’s typical ubiquity, some basic properties of the virus still aren’t well understood. For example, until a few years ago, researchers didn’t know that flu viruses could propel themselves across the surface of cells, and they still don’t know how or why the viruses perform that rolling motion.
Now Falko Ziebert of Heidelberg University in Germany and Igor Kulić of the Charles Sadron Institute in Strasbourg, France, have developed a model for how a flu virus rolls. Their proposed mechanism, which produces persistent and nearly inevitable motion, should apply to a range of related systems.
The flu virus (gray ring in the figure) is covered in two types of spike proteins, hemagglutinin (HA, blue) and neuraminidase (NA, red). The approximately 10 nm spikes serve different functions: HA temporarily binds to the sialic acid heads of the cell’s springy glycan coating (green), whereas NA snips off the sialic acid to sever that contact.
Glycan chains, which Ziebert and Kulić treat as ideal springs, gain free energy when they bond to HA proteins. But they pay for it in stretching energies that vary with position relative to the virus; glycan chains at the center of the virus stretch less than those toward the edges. The balance of those energies determines how many bonds form at the base of the virus. At the same time, NA proteins lop off sialic acid at some rate and thus reduce the number of binding sites available.
The stretched glycans apply torques on the cylindrical virus, and in the absence of NA proteins, those torques are balanced. The activity of the NA proteins reduces the torques and leads to an imbalance that tugs the virus in one direction. Once the virus is in motion, the NA reactions keep the virus headed in the same direction because glycan chains at the back of the virus are more likely to be cut due to their longer contact time with NA. The researchers calculate a typical flu virus’s steady-state rolling velocity as 0.4 s−1, which is in the range of experimental measurements. And the introduction of stochastic reaction kinetics to the simulation still results in persistent rolling even in the face of 100 nm gaps in the glycan layer.
The same model should apply to relatives of influenza that have similar spike proteins, including toroviruses and betacoronaviruses (but not SARS-CoV-2), and to recently developed DNA monowheels—silica particles coated with DNA that roll on RNA-covered surfaces. DNA monowheels could transport microscopic cargo, and the distance they travel is a sensitive measure of the RNA sequence they’re rolling on. (F. Ziebert, I. M. Kulić, Phys. Rev. Lett. 126, 218101, 2021.)
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June 30, 2021 at 12:51AM
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The flu virus keeps on rolling - Physics Today
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