Caltech and Purdue scientists determinestructure of the Dengue fever virus
Reporting in the March 8 issue of the journal Cell, Caltech biology professor James H. Strauss, lead author Richard J. Kuhn of Purdue (a former postdoctoral scholar in Strauss's lab), and Michael G. Rossman and Timothy S. Baker, both of Purdue, describe the structure of the virus they obtained with a cryoelectron microscope. The detailed electron-density map shows the inner RNA core of the virus as well as the other spherical layers that cover it. At the surface is the glycoprotein scaffolding thought to allow the virus to interact with the receptor and invade a host cell.
This is the first time the structure of one of the flaviviruses has been described, Strauss says. The flaviviruses are a class of viruses that include the yellow fever, West Nile, tick-borne encephalitis, and Japanese encephalitis viruses. All are enclosed with a glycoprotein outer layer that includes minor projections out of the lipid layer due to the geometry of the scaffolding.
"Most viruses that cause serious illness are enveloped, including influenza, hantaviruses, West Nile virus, smallpox, and herpes—though not polio," Strauss says.
The surprise for the researchers was the unusual manner in which the glycoproteins are arranged. Details from the Caltech and Purdue computer-generated images show a highly variegated structure of glycoprotein molecules that are evenly dispersed, but with a surprisingly complicated pattern.
"It's symmetrical, but not with the obvious symmetry of most symmetric viruses," Strauss explains. "This was not an expected result."
Strauss says it's still unclear what the odd symmetry will ultimately mean for future research aimed at controlling the disease, because the precise function of the different structural domains of the glycoproteins are still not known. Those that have been false-colored blue in the image are the domain of glycoproteins thought to be involved in receptor binding—and thus responsible for the virus's entry into a cell. The glycoprotein structures coded yellow are an elongated domain thought to be responsible for holding the scaffolding together; and the ones coded red have a function that is not yet known.
But a more detailed view of these structures is the beginning of a more informed strategy for a focused medical or pharmaceutical attack, Strauss says. "You can think of the protease inhibitors for HIV. Those in large part came from knowing the structure of the HIV enzymes you were trying to interfere with."
Thus, the new work could lead to drugs that will bind to the virus to prevent it from entering the cell, or perhaps from reassembling once it is already inside the cell.
Dengue fever is a mosquito-spread disease that has been known for centuries, but was first isolated in the 1940s after it became a significant health concern for American forces in the Pacific theater. A worldwide problem, the disease is found throughout Latin America, the Caribbeans, Southeast Asia, and India, and is currently at epidemic levels in Hawaii.
Especially virulent is the closely related dengue hemorrhagic fever, which is responsible for most of the deaths. The disease is a leading cause of infant mortality in Thailand, where there is an especially vigorous program to find an effective vaccine.
More information can be found on the Center for Disease Control Web site at http://www.cdc.gov/ncidod/dvbid/dengue/index.htm.
Contact: Robert Tindol (626) 395-3631