Viral Mug Shot
If a picture is worth a thousand words, then Rice University’s precise new image of a virus’s protective coat is seriously undervalued.
More than three years in the making, the image contains some 5 million atoms — each in precisely the right place — and it could help scientists find better ways to both fight viral infections and design new gene therapies.
Painstakingly created from hundreds of high-energy X-ray diffraction images, the image paints the clearest picture yet of the viruses’ genome-encasing shell, called a “capsid.”
The stunning image, which debuted in the Proceedings of the National Academy of Sciences, reveals the structure of a type of protein coat shared by hundreds of known viruses containing double-stranded RNA genomes. Painstakingly created from hundreds of high-energy X-ray diffraction images, the image paints the clearest picture yet of the viruses’ genome-encasing shell, called a “capsid.”
Capsids come into play because viruses can reproduce themselves only by invading a host cell and hijacking its biochemical machinery. But when they invade, viruses need to seal off their genetic payload to prevent it from being destroyed by the cell’s protective mechanisms.
“When these viruses invade cells, the capsids get taken inside and never completely break apart,” said lead researcher Jane Tao, assistant professor of biochemistry and cell biology at Rice.
Though there are more than 5,000 known viruses, including whole families that are marked by wide variations in genetic payload and other characteristics, most of them use either a helical or a spherical capsid.
In their attempt to precisely map the spherical variety, Tao and lead author Junhua Pan, a postdoctoral research associate at Rice, first had to create a crystalline form of the capsid that could be X-rayed. They chose penicillium stoloniferum virus F, or PsV-F, a virus that infects the fungus that makes penicillin. Although PsV-F does not infect humans, it is similar to others that do. By analyzing the way the X-rays scattered when they struck the crystals, the team created a precise 3-D image of the spherical capsid.
Previous studies had shown that spherical capsids contain dozens of copies of the capsid protein, or CP, in an interlocking arrangement. The new research identified the sphere’s basic building block: a four-piece arrangement of CP molecules called a tetramer, which could also be building blocks for other viruses’ protein coats. By deciphering both the arrangement and the basic building block, the research team hopes to learn more about the capsid-forming process.
“Because many viruses use this type of capsid, understanding how it forms could lead to new approaches for antiviral therapies,” Tao said. “It could also aid researchers who are trying to create designer viruses and other tools that can deliver therapeutic genes into cells.”
The research was supported by the National Institutes of Health, the USDA, The Welch Foundation, the Kresge Science Initiative endowment fund, the Agouron Foundation and the San Diego Supercomputer Center.