As if they were sentient, influenza viruses use their spike proteins to roll around the human body with direction and constant velocity — like wheels powered by an engine — until they arrive at the perfect location for infection.
It's already been established that the influenza virus travels along the body's respiratory tract upon inhalation, but only recently have scientists discovered that the pathogen self-propels with its two spike proteins — and in a paper published May 28 in Physical Review Letters, physicists modeled the exact way the motor behaves.
"It was slightly scary when I saw this experimental rolling virus," study author Falko Ziebert, a physicist at Heidelberg University in Germany, told The Academic Times. "But if you have understood it, you can try to prevent it from doing so."
While the team specifically studied the mechanism of the influenza virus, Ziebert noted that some variants of the coronavirus have two spike proteins as well — even though the primary variant has only one — meaning the findings could potentially be extended to the pathogen driving the worldwide pandemic.
"Corona, it's typically inside droplets and you exhale it and that's how it's transmitted, but maybe it can roll," he said, while emphasizing that such a conjecture remains to be proven.
Depicted as actual "spikes" on the often spherical virus, influenza's two spike proteins have separate jobs. Hemagglutinin, or HA, is in charge of firmly attaching to sugars on the cell membrane; neuraminidase, or NA, is an enzyme responsible for cutting that attachment.
"You need the hemagglutinin to get attached to the surface — but maybe it's not a good position — so you need the other spike to cut it again and then you can explore the space," Ziebert said. "We found that the two together can bring the virus in a state where it's actually rolling."
With the virus's spikes quickly alternating — sort of in a single-file pattern — between attachment and release, it rolls in one direction like a wheel. Eventually, the pathogen reaches a cell's entrance point, becomes engulfed and starts infection.
"I think it makes sense that viruses can create directional motility," Ziebert said. "It has to go for meters into our lungs — in the small branches of it to infect lung cells."
Viruses are not believed to be living objects, but rather an encapsulated clump of DNA that diffuses through the body at random. Realizing that these pathogens have the ability to self-propel and generate motion — as well as laying out exactly how they do so — opens the door to several new possibilities for vaccine development or even treatment.
"If you are to — so to say — mess up the spike proteins, the virus cannot enter the cell, but probably it can also not move so efficiently," Ziebert said. "Whatever you do, you will probably destroy this balance."
Ziebert explained how if HA is attacked, then the virus wouldn't be able to latch onto the cell membrane and could then be washed away by mucus. On the other hand, if NA is targeted by medication, then the rolling process wouldn't happen and the virus would be stuck somewhere along the membrane, unable to enter the cell.
"We found that this is a bi-stable system — so that's where dynamics comes in," he said. "It's also possible that we have a stage where it's rolling with constant velocity; this doesn't break torque [or] balance. It doesn't break thermodynamic laws."
The team additionally found that even if a cell membrane has a limited amount of sugar for the spikes to interact with, the virus is still able to make do and roll. Going forward, Ziebert said, there are many more questions to be answered, such as, what causes the virus to stop rolling when infecting the cell?
"Bacteria, they can swim," Ziebert said. "That's why they can go over large distances and that's why they are dangerous."
"If viruses can also move," he continued, "one should think about whether it's really a big effect and one has to do something against it, or maybe it's irrelevant. This is now for the virologists to look at."
The study, "How influenza's spike motor works," published May 28 in Physical Review Letters, was authored by Falko Ziebert, Heidelberg University and Igor M. Kulić, CNRS Strasbourg, France and Leibniz-Institute of Polymer Research.
This story has been updated to include Igor Kulić's main affiliation.