The Cell Detective

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Ploegh's group discovered that that's exactly how HCMV operates: it targets the protein that carries snippets of other proteins up to the cell surface for display. The carrier protein, called MHC class I, functions as the cell's flag bearer. It hangs around the place in the cell where proteins are made and destroyed, grabbing onto whatever snippets it finds and hoisting them to the cell surface. Researchers in Ploegh's lab have isolated a cluster of HCMV genes that destroy or detain MHC.

This work is giving immunologists a peek into herpes­viruses' bag of tricks--and illuminating the quotidian activities of normal human cells. "The virus has hijacked what we now believe is an essential pathway for protein quality control," Ploegh says. Cells are very careful when copying their DNA, because any mistakes will be passed on to future generations. But protein production is sloppy, with an error rate of about 10 percent. "That garbage needs to be cleaned out of the cell," Ploegh says; indeed, "part of the process of synthesis is also the breakdown of misfits." Biologists aren't sure how misfit proteins are recognized, but once they are, they are given a pass that lets them into a proteasome, which Ploegh compares to a "meat grinder." After exiting a proteasome, minced proteins, be they viral or the cell's own, pass into the compartment where MHC lies in wait, whereupon it rushes them to the surface for inspection by T cells.

Ploegh and his students studied two HCMV genes in human cells and showed that either one can disrupt the flag-bearing process. "Rather than work with infected cells, you can simply install in your cell this single gene and see the entire pathway unfold," he says.

Joana Loureiro, a graduate student from the University of Lisbon in Portugal who is conducting her doctoral research in Ploegh's lab, is studying one of these genes, called US2 (the other, called US11, has similar effects). Using a technique called pulse-chase, Loureiro can track MHC, US2's victim. First, she inundates, or "pulses," human cells with radioactively labeled protein building blocks--so many that the MHCs made during the pulse period will predominantly be radioactive and trackable. Then she "chases" the first set of building blocks with a second set that's unlabeled. Loureiro can thus track a group of proteins made only during a certain period. This lets her see the timing of events: in the presence of the viral US2 protein, MHC pokes out of its usual compartment and is then chewed up by the cell's meat grinder.

HCMV has other ways of disabling MHC--for example, proteins that simply drag it down like an anchor so that it cannot reach the cell surface. But Ploegh says the "most spectacular example" of the virus's ingenuity is the one Loureiro is studying, in which the virus turns the cell's own quality-control machinery against itself. The Ploegh lab has shown that HCMV can disable MHC in literally minutes; the infected cell simply has no time to send out a warning to the immune system.

HCMV may be mimicking a normal protein-processing mechanism in organisms from yeast to humans. Yeasts, those simplest of fungi whose genetic workings have proved very much like our own, contain genes that define pathways similar to those used by US2 and US11 in human cells. This similarity, Ploegh says, suggests "a link that can be made between escape from immune [system] detection by viruses and very basic, normal pathways." He adds, "We think the pathways used by US2 and US11 are emblematic of how your typical mammalian cell deals with protein garbage." By studying how native human proteins help the viral protein destroy MHC, Loureiro hopes to uncover how the normal pathway works.