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Researchers at the University of Texas Medical Branch at Galveston have discovered a key tactic that the Rift Valley fever virus uses to disarm the defenses of infected cells. The mosquito-borne African virus causes fever in humans, inflicting liver damage, blindness and even death on a small percentage of the people it infects. Rift Valley fever also afflicts cattle, goats and sheep, resulting in a nearly 100 percent abortion rate in these animals. Its outbreaks periodically cause economic devastation in parts of Kenya, Somalia, Sudan and Zimbabwe, and bioterrorism experts warn that its introduction to the United States would cripple the North American beef industry.
"It's really important to know how this virus causes disease, and that's what we're doing here, working to understand its pathogenicity at the molecular level," said assistant professor Tetsuro Ikegami, lead author of a paper on the discovery now appearing in PLoS Pathogens.
Ikegami and his collaborators focused on a viral protein called NSs. The protein was already known to be a major factor in making Rift Valley fever more virulent; earlier research had shown that it penetrated cell nuclei and disrupted the coding of RNA messages, including those ordering the production of the antiviral protein interferon beta.
"We didn't know what the mechanism was, but we suspected NSs had some additional function that would promote viral replication," Ikegami said. So — starting with an already weakened strain of Rift Valley fever virus produced as part of a vaccine development project — he created a genetically engineered form of the virus that lacked the genes for NSs.
Safety precautions make working with natural, "wild-type" Rift Valley fever virus difficult; at UTMB, investigations are restricted to a tightly secured biosafety level 4 lab, where researchers work in protective, full-body "spacesuits." By contrast, the vaccine strain of the virus that Ikegami modified, known as MP-12, can safely be handled inside a standard biosafety cabinet.
Using the NSs-free mutant virus to perform a series of cell-culture experiments, the researchers found that NSs does in fact have a second function. It attacks a protein called PKR, the beginning of a chain of biochemical reactions leading to the accumulation of a molecular complex known as phosphorylated eIF2-alpha. Phosphorylated eIF2-alpha suppresses overall protein production. Unblocked, it would prevent Rift Valley fever virus from using cellular protein synthesis machinery to make the proteins it needs to replicate itself. But since NSs prevents the phosphorylation of eIF2-alpha by taking out PKR, the virus is free to copy itself within host cells without interference.
"It's amazing that the virus evolved to use one protein to do two jobs, to use its very limited genetic information to perform these very different functions," said microbiology and immunology professor Shinji Makino, senior author of the paper. "It's really interesting, and it's also important, because these types of experiments are critical to learning how to control this virus."
The paper's other authors include senior research scientist Krishna Narayanan, graduate student Sungyong Won, postdoctoral fellow Wataru Kamitani and pathology and microbiology and immunology professor C.J. Peters. This research was supported by grants from the National Institutes of Health, the James W. McLaughlin Foundation and UTMB's Sealy Center for Vaccine Development.