How the flu gets cells to crack open its shell

Researchers were surprised how long it takes to crack open the shell, or capsid, of the flu virus: the process lasts around 20 to 30 minutes. The total infection period—from docking onto the cell's surface to the RNA entering the cell nucleus—is two hours. (Credit: Lotte Grønkjær/Flickr)

The flu virus has a clever way to trick cells into cracking open its shell and releasing its genetic code.

Until now, very little has been known about how the capsid of the flu virus is cracked open.

Scientists say they’ve uncovered the details: the virus disguises itself as a bundle of waste, called an aggresome. Cells react by activating their “waste disposal system” and tearing open the capsid.

The waste disposal system of a cell is essential for eliminating protein garbage. If the cell fails to dispose of these waste proteins (caused by stress or heat) quickly enough, the waste starts to aggregate.

To get rid of these aggregates, the cell activates its machinery, which dismantles the clumps and breaks them down into smaller pieces, so that they can be degraded. It is precisely this mechanism that the influenza virus exploits.

How viruses infect

Viruses cannot multiply without cellular machinery. The pathogen infiltrates the host cells and uses their replication and protein production machinery to multiply.


The virus has to overcome the initial barrier by docking on the surface of the cell membrane.

The cell engulfs the virus in a bubble and transports it towards the cell nucleus. During this journey, the solution inside the bubble becomes increasingly acidic. The acidic pH value is ultimately what causes the virus’s outer shell to melt into the membrane of the bubble.

The flu virus has to overcome a further obstacle before releasing its genetic code: the few pieces of RNA that make up the genome of the flu virus are packed into a capsid, which keeps the virus stable when moving from cell to cell. The capsid also protects the viral genes against degradation.

The disguise

The virus capsid carries cellular waste “labels” on its surface. These labels, called unanchored ubiquitin, call into action an enzyme known as histone deacetylase (HDAC6), which binds to ubiquitin.

At the same time, HDAC6 also binds to scaffolding motor proteins, pulling the perceived “garbage bundle” apart so that it can be degraded. This mechanical stress causes the capsid to tear, releasing the genetic material of the virus.

The viral RNA molecules pass through the pores of the cell nucleus, again with the help of cellular transport factors. Once within the nucleus, the cell starts to reproduce the viral genome and build new virus proteins.

Total infection time

The researchers were also surprised by how long the opening of the capsid takes, with the process lasting around 20 to 30 minutes. The total infection period—from docking onto the cell’s surface to the RNA entering the cell nucleus—is two hours.

“The process is gradual and more complex than we thought,” says Yohei Yamauchi, a former postdoc with ETH Zurich professor Ari Helenius, who detected HDAC6 by screening human proteins for their involvement in viral infection.

In a follow-up study, lead author Indranil Banerjee confirmed how the flu virus is programmed to trick HDAC6 into opening its capsid.

A mouse model provided encouraging proof. If the protein HDAC6 was absent, the flu infection was significantly weaker than in wild-type mice: the flu viruses did not have a central docking point for binding to the waste disposal system.

Possible therapy?

Little research has previously been conducted on how an animal virus opens its capsid. This is one of the most important stages during infection.

“We did, however, underestimate the complexity associated with unpacking the capsid,” admits Helenius.

Whether there are therapeutic applications for the findings remains to be seen as an absence of HDAC6 merely moderates the infection rather than prevents it. The known HDAC6 inhibitors target its two active areas.

Blocking the enzymatic activity does not help prevent HDAC6 from binding to ubiquitin, but rather supports the virus by stabilizing the cell’s framework.

“We would need a substance that prevents HDAC6 from binding to ubiquitin, without touching the enzyme,” says Yamauchi.

Nevertheless, the structure of HDAC6 indicates that this is possible and follow-up experiments are already planned. The researchers have filed a patent for this purpose.

Researchers from the Friedrich Miescher Institute for Biomedical Research in Basel and the Biological Research Center in Szeged (Hungary) also collaborate on the study, which is published in the journal Science.

Source: ETH Zurich