Evilevitch studies how viruses release DNA to infect cells

Alex Evilevitch, associate professor physics, studies the mechanisms behind viral infection. (credit: Alex Evilevitch) Alex Evilevitch, associate professor physics, studies the mechanisms behind viral infection. (credit: Alex Evilevitch)

In today’s society, the term virus is widely understood due to its role in common, well-known diseases. Nevertheless, there is still much that is unknown about the mechanisms that control viral function. Recent findings by Alex Evilevitch, associate professor of physics at Carnegie Mellon University, have shed light on several of the mechanisms behind viral infection.

Many viruses consist of double-stranded DNA surrounded by a protein coat called a capsid. For replication of the DNA to occur, the DNA is injected into a host cell. After the cell has replicated the DNA, it is once again packaged back into the capsid of the virus. Evilevitch, along with Fred Homa, herpes virologist at the University of Pittsburgh School of Medicine, examined the mechanism behind the transfer of viral DNA from the capsid into the host cell.

In prior research, Evilevitch and his team measured the pressure inside bacteriophages, which are viruses that infect bacteria cells. The team discovered that the force driving DNA from the virus into the host cell is a mechanical pressure, which suggests that large physical forces could be involved in viral infection.

“That was the breakthrough measurement,” Evilevitch said. “I was then interested to see if similar mechanisms can be found in eukaryotic viruses or, more importantly, human viruses.” The researchers found that the herpes virus was the human virus most similar to the bacteriophages they had been studying, as it had a very similar structure. This discovery led the researchers to measure the pressure inside the herpes virus. Their results ended up being very similar to the phage measurement, suggesting that it followed the same mechanism of DNA release into a cell as the bacteriophage.

This discovery led to many more questions, namely about the physical structure of the viral DNA. The researchers were aware that DNA, at the density required in a virus, is packed in a rigid crystalline structure. They had also measured the rate of DNA release from the herpes virus and determined that the release was an extremely quick process.

“The big question was: If DNA is packaged like a solid crystal, how can it be released so quickly at tens of thousands of base pairs per second into the cell and cause infection?” Evilevitch said. To answer this question, the team used atomic force microscopy and small angle X-ray scattering to image the virus at different temperatures and to determine the stiffness of DNA packaged inside the virus, which in turn, allowed them to measure the fluidity of the DNA.

“We were interested to see whether biological temperature would have an effect on the structure of the DNA that could potentially explain how DNA is released in the cell during infection,” Evilevitch explained. “We found that DNA stiffness in the capsid undergoes a rapid drop at the temperature of infection. This showed us that at that point, DNA doesn’t show any resistance and is completely fluid, which suggests that it can be freely released and infect the cell.” Evilevitch explained that this result clearly illustrates the concept of viral metastability, the idea that a virus needs to be stable in between infections, but unstable during infection in order to transfer its DNA to the host cell.

“Previous studies tried to show that the capsid was metastable. We found for the first time that it’s actually also the genome that’s metastable,” Evilevitch said. “Inside the virus when the environment is not favorable for infection, such as at lower temperatures, it behaves like a solid crystal. When the physical environment is optimized for infection, such as at infection temperature, DNA can freely flow from the virus into the cell nucleus and initiate infection.”
Evilevitch explained that the research had a dual purpose, to further understand the mechanism behind viral infections and to develop potential treatments that inhibit those mechanisms.

If methods of inhibiting the viral DNA phase transition can be developed, the virus becomes essentially noninfectious. “Herpes is a serious health issue — it’s the second largest human pathogen after influenza,” Evilevitch noted. “These studies suggest a completely new approach to the development of antiviral herpes treatments.”

The research, supported by the Swedish Research Council, the National Science Foundation, the National Institutes of Health, and the McWilliams Fellowship at Carnegie Mellon, was published in Nature Chemical Biology and the Proceedings of the National Academy of Sciences.