Penn State researchers unravel key facts about how a virus exits a host cell

November 19, 2000

Researchers at the Penn State College of Medicine have identified a component of the little-understood machinery that allows retroviruses to exit or bud from the cell surface before re-infecting a new cell, which may offer a new target for the development of a single antiviral drug for a wide spectrum of viral diseases such as AIDS, rabies, ebola, measles, mumps or influenza.

Specifically, the research points to a role for ubiquitin, a 76-amino acid polypeptide, as crucial to budding (the maturation and release of a virus from the cell surface). The paper, "Ubiquitin Is Part Of The Retrovirus Budding Machinery," by Penn State scientists Akash Patnaik, an M.D./Ph.D. candidate; Vincent Chau, professor of cellular and molecular physiology; and John Wills, professor of microbiology and immunology, is one of a trio of papers published on Nov. 21 in the Proceedings of the National Academy of Science (PNAS).

All three papers show that ubiquitin is part of the machinery required for retrovirus budding. PNAS reviewers call the results "important and ground breaking."

Wills, professor of microbiology and immunology at Penn State College of Medicine, says a virus must mature and "pinch off" a cell membrane before it becomes infectious. This pinching off process is called budding. In previous research, the Penn State researchers identified the Gag (groups specific antigen) protein as the viral encoded machinery that produces buds.

By inserting the Gag gene into cells, Wills and his colleagues had the ability to recreate viral particles in the lab and identify the parts of Gag that actually drive the budding process. Unfortunately, understanding how Gag works is half the story. Scientists also needed to know what the cell requires for budding to take place. This current research shows that cells require ubiquitin for budding to take place.

The possibility of interrupting the budding process marks the significance of this recently published research.

Working independently, scientists at Penn State College of Medicine, Harvard's Dana-Farber Cancer Institute and the National Institute of Allergies and Infectious Diseases (NIAID) of the National Institutes of Health published complementary papers on the role of the ubiquitination machinery in late steps of retrovirus budding. The data show the functional connection between ubiquitin and Gag (Penn State), the recruitment of the ubiquitin ligase, which encourages the virus to assemble, to the site of budding (Harvard), and the maturation of Gag, the final stage in which the virus becomes infectious (NIAID). Each paper presents a slightly different view of these connections.

This study follows research published 10 years ago that showed that Rous sarcoma virus (RSV), a retrovirus common to chickens, contained unusually large amounts of ubiquitin. In the intervening decade, other scientists have identified ubiquitin in other retroviruses including HIV. Wills believes that RSV, HIV and other enveloped retroviruses such as Ebola and rabies, use ubiquitin in similar manners.

"If these viruses use a common mechanism for virus budding, this raises the possibility of developing a single antiviral drug for a wide spectrum of viral diseases. This is rather like the idea behind broad-spectrum antibiotics, where one drug can be effective against more than one strain of bacteria," he says.

Wills of Penn State points out that the mechanistic details of ubiquitin's role in budding needs to be determined. "Our data and those of the accompanying papers suggest that the protein ubiquitin can modify Gag in a manner necessary for retrovirus budding. This is significant, however, because so little is known about how enveloped viruses bud," he says.

Gag proteins alone are not capable of triggering virus-cell separation. Viral buds, which emerge on the cell's surface, are released by the late or L domain segment of Gag, a portion that develops with cell maturation. L domain proteins are thought to recruit the cellular machinery needed for virus cell separation from the plasma membrane. The research shows an apparent correlation between L domains and ubiquitin.

The simplest interpretation of the Penn State research is that budding requires ubiquitination of the RSV Gag protein.
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Penn State

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