Uncovering the structure of the poxvirus ‘portal complex’, a channel used by the virus to release its genetic material into infected cells, could help develop new antiviral treatments.
Structure of the portal complex (left) containing proteins essential to infection by poxviruses. Portal complexes are located within the core of the virus, shown in cutaway model of the virus at (right). Background shows electron cryotomography image of a cell infected by vaccinia virus. Credit: Structural Biology of Cells and Viruses Laboratory and Cellular Signalling and Cytoskeletal Function Laboratory at the Francis Crick Institute.
Poxviruses have a long history in human disease as well as medicine: the first ever vaccine was developed to protect against smallpox, which has since been eradicated. Despite this, scientists still don’t fully understand how poxviruses infect and replicate inside our cells. Recent outbreaks of mpox highlight an urgent need to understand poxvirus biology better.
Peter Rosenthal leads the Structural Biology of Cells and Viruses Laboratory at the Crick, seeking to understand the 3D structures of viruses, often focusing on viruses such as influenza. “Poxviruses are bigger and have many more components. We’ve needed new methods to understand their organisation,” he says.
Peter recently joined forces with Michael Way, head of the Crick’s Cellular Signalling and Cytoskeletal Function Laboratory, who focuses on how the vaccinia poxvirus hijacks human cells. Both were interested in how poxviruses release genetic material and assemble new viruses.
“At the start of infection, poxviruses release their genetic material in two steps from their sealed viral core into the host cell,” says Michael. “First, messenger RNA transcripts are exported, to produce proteins that suppress the host’s immune and anti-viral response, followed by the virus’s DNA genome to allow replication of new virus.”
Until now, researchers didn’t know how any genetic material was released from the viral core. But they had identified channels embedded in the wall of the viral core, dubbed ‘portal complexes’, suspected to play a role in transporting the viral genetic material, like a molecular exit tunnel.
In a study published today in Nature, Tom Calcraft in Peter’s lab worked with Miguel Hernandez Gonzalez in Michael’s lab to determine the structure of the portal complex using cryo-electron tomography, a rapidly advancing technique that allows imaging of more complicated structures at a higher resolution.
“This powerful imaging technique gave us a ‘map’ of the portal complex, essentially a sketch of the different components that make up the core and all the structures surrounding it,” says Tom.
Tom and Miguel then used the AI software AlphaFold2 to predict the structures of all known proteins in the vaccinia virus. Each potential protein was then slotted into the portal map to see which best matched.
“We identified three crucial proteins that make up the portal complex,” says Miguel. “They’re all previously known to be essential for mRNA export and one is also involved in the assembly of new virus particles, showing how critical portal complexes are to the virus.
The study also showed how a viral enzyme responsible for the release of the DNA genome docks onto the portal complex. “This was so exciting: it was evidence that the portal complex works like a central hub, connecting core functions in the virus replication cycle,” says Tom.
“We’d known about these three proteins in isolation, but no one had expected them to work together like this,” adds Peter. “AI prediction tools allowed us to be less restrictive with our assumptions, and combined with more powerful imaging techniques, completely opened up our view of this group of important viruses.”
The portals must efficiently export large amounts of genetic material, but the virus’s reliance on this transportation channel could help stop it in its tracks.
Michael adds, “We now have a structural target to focus on. And because poxviruses are so similar, they will have similar portal complexes, so the same drug could work on different members of this virus family.”
Now leading a lab at the Pirbright Institute, Miguel is continuing his collaboration with Peter’s team to better understand how the genetic material is being transported through the portals. Their research will involve screening for potential drugs which could block the release of the genetic material, aiming to get closer to new treatment strategies.
