|
||||||||
|
STRUCTURAL BIOLOGY Image from Epidemic Dengue Virus Reveals Drug TargetStructural biologists at Children's Hospital have identified a vulnerable pocket in the protein that coats the dengue virus, a deadly mosquito-transmitted pathogen in tropical climates, including regions of the U.S. The pocket is located at a pivotal hinge point that could be filled with a small molecule to prevent the protein from folding into the shape that enables the virus to infect cells. Revealed in a crystal structure, the discovery could speed the search for a drug to treat the disease, according to a report in the Proceedings of the National Academy of Sciences Early Edition, online May 20.
In this X-ray crystallography close-up of an envelope protein of dengue virus type 2, the small detergent molecule beta-octyl glucoside (red and gray) fills a deep pocket at a hinge where the protein folds to fuse and infect cells in the body. (Image courtesy of Proceedings of the National Academy of Sciences) The Danger of DengueDengue fever and its potentially lethal complication, dengue hemorrhagic fever, have emerged as major international public health problems in 100 countries, infecting as many as 100 million people annually. The Centers for Disease Control and Prevention calls the disorder the most important mosquito-borne viral disease affecting humans. Outbreaks are possible in Texas and the southeastern United States, where cases have been reported and where mosquitoes that transmit the virus breed in standing water. No vaccine is available.
More specifically, in an infected person's bloodstream, the E-protein shell of a virus latches onto a cell, nuzzles up closer with its viral load, fuses the viral membrane with the cell membrane, and inserts the viral genes. The dengue E protein achieves all this by performing gymnastic contortions, first stretching and then bending. But if its range of motion were limited by filling the pocket, the virus might be unable to fuse with the target cell. "Inhibiting fusion is a sensible way of inhibiting viral replication," said senior author Stephen Harrison, a Howard Hughes investigator at Children's Hospital, whose group is also working on preventing another sort of viral fusion involving HIV. In fact, he said, the latest HIV drug on the market targets viral fusion. "This may turn out to be a false lead, but it suggests an interesting avenue to screen for compounds to inhibit dengue fusion," said Harrison, who is also a professor of biological chemistry and molecular pharmacology at the Medical School. "In the case of dengue and HIV, we can use structural insights to find small-molecule fusion inhibitors." The findings might apply to the three other types of dengue virus and to different viruses in the same family with similar envelope proteins, like yellow fever, tick-borne encephalitis, and Japanese encephalitis.
Yorgo Modis, Stephen Harrison (l to r), and their collaborators propose that this pocket might help scientists find small molecules to inhibit dengue-virus infection. (Photo by Phil Farnsworth) Crystal CultureThe pocket was discovered in structural biology experiments designed to reveal more details about how the E protein works; they were funded by the international Human Frontier Science Program, the National Institutes of Health, and the Howard Hughes Medical Institute. These efforts have three big bottlenecks. The first step is getting a cooperative protein. The next is growing crystals of the protein that can stand up to the punishing X-rays used to examine its nooks and crannies in atomic detail. A trick sometimes used to improve crystal quality proved to be the key to this study. Yorgo Modis, a postdoctoral fellow and first author of the paper, used a detergent to grow better crystals in his early attempts. Later, he grew crystals without the detergent.In the final step, structure determination, Modis and his colleagues found a detergent molecule in a pocket near the E protein hinge. A flap in the protein appears to hide the pocket normally. When the pocket is filled, the flap stays open and keeps the protein propped open at the hinge. The structure of dengue E confirms predictions based on similar gene sequences for the envelope proteins of other viruses in the same family. Further supporting evidence comes from studies that show genetic mutations in the hinge and pocket region interfere with fusion under some conditions. "We didn't know detergents could insert and stabilize the protein assembly," Modis said. "From a practical point of view, detergents don't make good drugs. But knowing the size and shape of the pocket should advance the search for a drug." Modis is now studying the postfusion structure to learn more details about how the protein's shape change enables the virus to infect cells. Harrison is leading a new effort at HMS, the Center for Molecular and Cellular Dynamics (see Focus, Jan. 24, 2003), which aims to integrate this kind of information into a coherent picture of how atoms, small molecules, and protein superstructures move, transfer information, and reconfigure in the crowded, fast-paced life of a cell. The center reflects the trend in biology of finding meaningful ways to integrate the growing "parts list" of genes, proteins, and molecular structures to improve understanding of human health and disease and to display it in useful ways. --Carol Cruzan Morton |
