Zika virus exposed
Image: US-based Purdue University researcher scrutinises Zika virus [Purdue University]
US-based researchers have used cryo-electron microscopy to determine the structure of the Zika virus, revealing insights critical to the development of antiviral treatments and vaccines.
Crucially, the researchers identified regions within the virus structure that differ from Zika's family of viruses - including dengue and yellow fever - which will help to explain how the virus is transmitted and manifests as a disease.
"The structure of the virus provides a map that shows potential regions of the virus to target," says Richard Kuhn, director of the Purdue Institute for Inflammation, Immunology and Infectious Diseases. "Determining the structure... illuminates the most promising areas for further testing and research to combat infection."
A representation of the surface of the Zika virus [Purdue University/Rossmann/Kuhn]
A preparation of the Zika virus, isolated from an infected patient during the French Polynesia epidemic in 2013-14, was first frozen onto Lacey carbon electron microscopy grids.
Then, using an FEI Titan Krios EM equipped with a Gatan K2 Summit detector, the researchers examined the sample at x14,000 in 'super-resolution' mode, yielding a pixel size of 1.04Å.
Kuhn and colleagues collected 2,974 images, and using EM image processing and particle analysis software, selected 11,842 particles to generate a cryo-EM map of the virus structure, with an average resolution of 4.2Å. Image filtering boosted overall resolution to 3.8Å.
Michael Rossmann (right) and Richard Kuhn with the cryo-electron microscope used to determine the structure of the Zika virus. [Purdue University photo/Mark Simons]
As Kuhn's colleague, Professor Michael Rossmann from Biological Sciences highlights, at this near-atomic resolution, key features of the virus structure could be observed.
What's more, groups of atoms that form specific chemical entities, such as those that represent one of 20 naturally occurring amino acids, were recognised.
“We were able to determine through cryo-electron microscopy the virus structure at a resolution that previously would only have been possible through X-ray crystallography,” says Rossman. "However, X-ray crystallography requires a relatively large amount of virus, which isn’t always available; can be very difficult to do, especially for viruses like Zika that have a lipid membrane and don’t organize accurately in a crystal; and takes a long time."
"Now, we can do this through electron microscopy and view the virus in a more native state," he adds. "This was unthinkable only a few years ago."
A representative cryo-EM image of frozen Zika virus shows different virion phenotypes. [Purdue University/Rossmann/Kuhn]
The researchers found the structure to be very similar to that of Zika virus family members, known as flaviviruses; an RNA genome is surrounded by a lipid membrane inside an icosahedral protein shell.
However, the researchers also discovered that the glycosylation site within the Zika virus shell was very different to that of related viruses.
"The structure of Zika virus is similar to other known flavivirus structures except for the ~10 amino acids that surround the Asn154 glycosylation site found in each of the 180 envelope glycoproteins that make up the [virus] shell," says Rossman. "The carbohydrate moiety associated with this residue, recognisable in the cryo-EM electron density, may function as an attachment site of the virus to host cells."
“If this site... is involved in attachment to human cells, it could be a good spot to target an antiviral compound,” he adds. “If this is the case, perhaps an inhibitor could be designed to block this function and keep the virus from attaching to and infecting human cells.”
The researchers now intend to evaluate the different regions as targets for treatment and develop potential therapeutic molecules.
Research is published in Science.