As the avian influenza A virus seeks to infect its bird hosts, it brings a special weapon to the fray – four tiny amino acids that hang off the end of a viral protein called NS1, said researchers from Baylor College of Medicine and The University of Texas at Austin in a report that appears in the current Journal of Virology.

This collection of molecules known as the PDZ binding-motif help make the avian virus – known to experts at H5N1 – particularly virulent and a threat to human populations everywhere.

Fast-spreading viruses

"The recent pandemic of H1N1 influenza virus that circled the globe in short order demonstrates how quickly a new influenza virus to which we do not have previous exposure can spread," said Dr. Andrew P. Rice, the Nancy Chang Professor of Molecular Virology and Microbiology at BCM. "While that virus turned out to be mildly pathogenic, the avian virus called H5N1 is highly pathogenic. When it infects humans, about 60 percent of them die."

The virus has yet to be transmitted efficiently from human to human. Most people infected with it were in close contact with infected birds.

"One factor that is believed to contribute to the virulence of H5N1 is nonstructural protein 1, or NS1," said Rice. In general, this protein inhibits the cell’s production of interferon, an innate immune response to the immediate invasion by the virus.

Focus on scribble

Rice and his colleagues found that this NS1 PDZ binding-motif targets a whole set of proteins. They decided to focus on one protein called Scribble, which promotes apoptosis, or cell death, in infected cells.

"In the context of viral infection, Scribble can sense a virus and promotes cell suicide in order to limit virus replication," said Rice. In other words, Scribble is a protective protein that helps the organism rid itself of infected cells and prevent their spread.

The PDZ binding-motif enables the avian NS1 protein to bind to Scribble, preventing it from promoting the death of infected cells.

"In our experiments, this ability to inactivate Scribble let the virus replicate four-fold more in the laboratory," said Rice.

In the future, he and his colleagues plan to study the other proteins targeted by the PDZ binding-motif to understand the consequences of viral NS1 binding to them. They also plan to study the proteins bound by the human form of influenza virus.

Scribble and other PDZ proteins targeted by viruses play important roles in the cell. Some regulate apoptosis; others deal with cell polarity that enables the cell to sense it orientation. Viruses seek to perturb this normal function, he said.

Others who took part in this work include Hongbing Liu, Lisa Golebiewski, Eugene C. Dow and Ronald T. Javier of BCM, and Robert M. Krug of UT Austin. Funding for this work came from the National Institutes of Health and the Western Regional Center of Excellence for Biodefense and Emerging Infectious Diseases Research.