Fighting disease atom by atom
Rice lab’s atomic map of hepatitis E may reveal strategies to fight it

BY MIKE WILLIAMS
Rice News staff

“We have a story in China,” said graduate student Tom Guu. “Three wise men were blindfolded and made to examine an elephant. The first man, touching the trunk, proclaimed with certainty that he held a snake. The next man, grasping a leg, was convinced he embraced an oak tree. The last man, encountering the elephant’s side, was certain he had found a mighty boulder.”

	JEFF FITLOW
	From left, Tom Guu, Yizhi Jane Tao and Qiaozhen Ye worked for months in Tao’s Rice lab to define the structure of the hepatitis E capsid, an important step toward finding ways to halt the disease.

Guu was discussing why researchers have had such a difficult time wrapping their proverbial arms around the hepatitis E virus (HEV), a particularly nasty form of viral hepatitis that flourishes in the developing world, where poor sanitation is common.

“About 10 years ago, researchers began to describe what the virus looks like,” Guu said. “They found protrusions and indentations on its surface. While it looked a bit like a buckyball, or a geodesic dome, researchers were still stuck in the same predicament as the blind wise men and the elephant.”

Without a more detailed description of the virus, it has been hard to design drugs to stop it. To do that, you have to look at it very closely, as Guu and colleagues in the Rice University lab of Yizhi Jane Tao have done.

The researchers have published a new paper in the Proceedings of the National Academy of Sciences that, for the first time, details the atomic structure of the protein shell that carries HEV’s genetic code, suggesting that new ways to stop the virus may come in the not-too-distant future.

Tao, an assistant professor of biochemistry and cell biology, led a team that included Guu, research scientist Qiaozhen Ye, then-undergraduate student Douglas Mata, Zheng Liu and Changcheng Yin of Peking University in China and Kunpeng Li and Jingqiang Zhang of Sun Yat-sen University in Guangzhou, China.

Using X-ray crystallography and sophisticated software, Rice graduate student Tom Guu created stunning images that show the atomic structure of HEV, with a capsid protein pentamer fitted into the viral envelope derived from three-dimensional electron microscopy reconstruction.

Tao’s lab specializes in X-ray crystallography, a powerful technique that can pinpoint the exact location of every atom in a biomacromolecule or a large biomacromolecular assembly. In this case, the biomacromolecular assembly was the viral capsid shell, made from a network of individual capsid proteins from a strain of hepatitis E virus that had been made in insect cells, then purified and crystallized.

After two years of intense study, Guu calculated the position of each of the approximately 500,000 atoms that make up the capsid, an icosahedron-shaped particle that roughly resembles a buckyball. The resulting 3-D computer model gives researchers the ability to identify the particle’s host-cell binding sites, through which HEV spreads.

”Dr. Tao has already identified potential sites on the new model,” Guu said. “If we can prove these sites to be correct, labs around the world can start to design drugs, called competitive inhibitors, to interrupt the binding process and prevent the virus from attaching to cell receptors in the first place.”

Guu compared the virus’s capsid protein to a hollowed-out watermelon. “You have the outer shell of the virus, but you take out its insides,” he said. “It retains its outside properties.” The empty capsid may still bind to a cell, but it contains no genetic material to transfer, rendering it noninfectious and therefore an excellent candidate for a vaccine.

”In fact, other researchers have used the empty viral shell to vaccinate monkeys, and even humans. Later on, when the researchers challenged them with the real virus, they discovered that prior exposure to this virus-like particle conferred some sort of protection,” Guu said.

“This virus has been less studied than others in the pathogenic human virus domain,” Tao said. “It has been rather difficult to generate a culture in the lab to study how the virus invades the cell. The only way to work with it is to overexpress the protein on its own.”

It took nearly six months for Ye to identify the right construct, isolate the protein and form the first crystals, after the project was initiated by an international collaboration with Zhang at Sun Yat-sen University in early 2006. Mata then successfully reproduced one of these crystallization conditions. They were surprised to discover that, in the crystallization process, HEV capsid proteins extracted from the cultured cells self-assembled into virus-like particles. This in turn may lead to another strategy: “If you can prevent the protein from assembling, you can stop the virus too,” Ye said.

Tao sees great potential for their discovery. “There are so many things you can do with the structure that I think it will be useful for many years to come,” she said.