Uncovering the Viral Mechanics of Croup.

Researchers say they have solved the crystal structure of a key protein that allows the paramyxoviruses--the major cause of croup and other diseases--to attach to cells, which in turn permits the viruses to invade cells and cause respiratory illness. The new research, published in the November 2000 issue of Nature Structural Biology, may provide a basis for developing drugs to fight these diseases.

Members of the paramyxovirus family include the parainfluenza viruses (which cause respiratory infections), the mumps virus, and the Newcastle disease virus (which affects birds). "Most people who are infected with parainfluenza get symptoms similar to a bad cold," says Susan Crennell, the postdoctoral student at the University of Bath in England who used X-ray crystallographic techniques to identify the protein. "In very young children, this can develop into life-threatening respiratory disease including pneumonia." She adds, "Paramyxoviruses can be very dangerous to people--especially children--with compromised immune function."

The multifunctional paramyxovirus protein hemagglutinin-neuraminidase (HN) is critical to the invasion process that creates illness, Crennell says. HN, a protein on the surface of the virus, binds to cells, permitting a cascade of molecular events that results in the virus fusing to the target cell and then incorporating itself into the cell. Once inside the cell, the virus then has established a base of operations from which it can infect other cells.

Crennell says the project was made difficult because HN is a flexible protein that changes shape with different periods of the cell's life cycle. Before HN binds to the cell, it has a different shape than after it attaches. Once HN has bound to the target, the structure of the cell prevents a mediator--a drug, for example--from interfering with HN's work setting the infectious stage of development into motion. Crennell says the key to preventing HN from working is to know its shape prior to binding so it can be attacked before initiating the events that lead to fusion.

The study was performed using a form of the Newcastle disease virus. Crennell says that particular virus was used because scientists found they could get a crystalline structure of the Newcastle HN protein, something that has so far evaded researchers using the viruses that cause human disease. "The HN protein on this virus we believe is very close to that found on the human virus," says Crennell. With the crystal form available, Crennell exposed the structure to X rays. Computers analyzed the subtle changes in the signals produced by the X rays in order to show the shape of the protein.

Crennell says the structure provides the basis for the structure-based design of inhibitors for a range of paramyxovirus-induced diseases. Other researchers, including principal investigator Garry Taylor of the University of St. Andrews in Fife, Scotland, are just beginning to use the crystal structure of HN to design drugs that can prevent binding and keep the virus from causing illness.

"We have no vaccines or effective drugs to stop viruses like parainfluenza," says Fran Rubin, program officer for respiratory diseases at the National Institute of Allergy and Infectious Diseases, which partially funded Crennell's study. "This study opens new possibilities for drug development." She adds, "This report is the most exciting discovery in the field in quite some time."