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Studies Shed Light on CMV Stealth TacticsFor millions of years, viruses--scrappy creatures consisting of a handful of genes sheathed in protein--have stayed alive by plundering host cells. Using their modest genetic resources, they divert the host cell's equipment--enzymes, ribosomes, and other cellular machinery--for their own purposes, namely making more virus.
Several years ago, Hidde Ploegh and his colleagues showed that cytomegalovirus (CMV), a particularly nasty pathogen, does this by short-circuiting the host cell's alarm system. Normally, cells alert passing immune cells to the presence of an invader by displaying on their surface a molecular red flag--a tiny piece of pathogen hoisted on the shoulders of another protein, part of the MHC Class I complex. CMV subverts this system by secreting two proteins that prevent the MHC Class I complex from reaching the cell surface. But details of the process remained sketchy. For example, the researchers knew that the two viral proteins, US2 and US11, bind to MHC Class I in the endoplasmic reticulum (ER), and that from there, the complex is sent to the bowels of the cell--specifically, the proteasome--where it is destroyed. What was not clear was whether these two henchman proteins were acting alone or with other proteins. Nor was it clear how exactly the MHC Class I complex was being dislocated: was it pushed out by proteins inside the ER, or was it pulled out by its tail, which protrudes from the ER into the cytoplasm? It now appears that the tail is critical for dislocation. In a series of experiments reported in the June 20 Proceedings of the National Academy of Sciences, Ploegh and his colleagues found that tail-less versions of the MHC Class I complex did not leave the ER, even in the presence of US2 and US11. "This paper shows, indeed, this little piece is somehow essential. If you delete it, you cannot pull MHC Class I out," says Ploegh, the Edward Mallinckrodt Jr. professor of immunopathology. His co-authors are Craig Story, research fellow, and Margo Furman, a graduate student, both in pathology. As for the question of accomplices, while it is possible that chopping off the MHC Class I tail affects the ability of US2 and US11 to dislocate the complex, it is more likely that other proteins are involved in the process of extrication, says Ploegh. He and his colleagues are currently looking for such proteins. Peering into the Viral MirrorUntil relatively recently, Ploegh was studying the workings of the MHC Class I complex directly, paying little attention to viruses. In 1991, a graduate student in his lab in Holland was curious about CMV and tried infecting fibroblasts with the virus. The MHC complex disappeared from the surface of the cells. "We just couldn't find it--it was gone," he says. Soon after, a CMV geneticist homed in on two viral genes, US2 and US11, that were causing the trouble.Having figured out that the viral pair was helping to divert MHC Class I to the proteasome--and mindful that viruses tend to co-opt the host's own mechanisms--Ploegh wondered whether US2 and US11 might be mirroring a normal cellular process. Many proteins besides MHC Class I are manufactured and assembled in the ER, but not released until they are properly folded. Not all proteins reach their proper conformation--some are misfolded or damaged. Biologists had long wondered how these junk proteins were removed from the ER. Ploegh and graduate student Johannes Huppa discovered that at least one protein, a component of the T cell receptor, is also sent from the ER to the proteasome when incompletely assembled. "That was a conceptual breakthrough," says Ploegh. He and his colleagues were also excited to discover that MHC Class I and possibly the T cell receptor protein leave the ER through the same opening, or channel, they use to enter the ER. "So the channel involved in protein import can actually be used in the reverse orientation," says Ploegh. "This is probably the most controversial part of our proposal." The recent discovery that the MHC Class I complex is pulled through that channel by its tail was made by expressing a gene for a tail-less version of MHC Class I--along with the US2 and US11 genes--in cells that normally do not make the proteins. The researchers also inactivated the proteasome, reasoning that if MHC Class I was being sent there, it would not be disposed of but would accumulate in the cytoplasm instead. They found that MHC Class I was absent from the cytoplasm, as well as from the surface of the cells, suggesting it was being detained in the ER by US2 and US11 but not dislocated. It is not yet clear how--or if--CMV actually deploys US2 and US11 during an infection. "In a viral infection, there is a highly ordered sequence of events that must unfold for it to be successful. It is possible that other proteins join the fray and somehow modify US2 and US11," Ploegh says. CMV is a nimble virus, capable of infecting almost any cell--indeed it is notorious for infecting the retinas of AIDS patients and causing widespread damage in transplant patients taking immunosuppressive drugs. Understanding how CMV hides its tracks could someday lead to new ways to flush out the virus and, possibly, nip CMV infections in the bud. "Know your enemy," Ploegh advises. "The more intelligence you have on these foreign invaders, the better able you will be to defend yourself." But the findings have implications beyond the obvious one of treating CMV infection. For example, one of the big problems in gene therapy is designing viral vectors that can deliver genetic cargo without being attacked by the immune system. By borrowing features of CMV's stealth system--adding US2 or US11 or other yet-to-be discovered genes--vectors might be made temporarily invisible. In fact, this approach has already been tried, though Ploegh has not heard of any successes so far. What is more promising at the moment, he says, is the light CMV is shedding on the cell's other secrets. "Viruses usually co-opt mechanisms of the host and twist them to their own advantage," says Ploegh. "So they really capitalize on what the cell's capabilities are to begin with. Studying them, we can get new insight into aspects of cell biology." --Misia Landau |
