Just how the virus succeeds in pushing out the membrane into a "bud" has been clarified by researchers at Northwestern University in elaborate experiments involving changing the virus' RNA.
The virus as it buds from the cell normally has two thread-like proteins that extend out from the cell and small portions, called "tails," that extend inside the cell membrane. It had been theorized that the "tails" of these two proteins pull the cell membrane outward, at the same time as a third protein, called M1, pushes against the cell wall.
When the researchers eliminated the "tails" of these two proteins, the viral particles were severely mutated --- demonstrating the crucial role the tails play in forming new viral particles. The research was described in the March 17 issue of the EMBO Journal, a publication of the European Molecular Biology Organization.
"This clearly demonstrates the importance of these two protein parts in the development of the virus," said Robert A. Lamb, John Evans Professor of Cell Biology at Northwestern and an Investigator of the Howard Hughes Medical Institute. "It had been theorized that they played an important role. Now we know that they control the particles' shape," he added.
"This is a very important study because Dr. Lamb's group has shown for the first time that there are redundant signals in the cytoplasmic tails of these influenza (proteins) that determine virus shape," said John K. Rose, professor in the departments of pathology and cell biology at the Yale University School of Medicine.
"It is critical that we understand the mechanics of viral assembly before we can develop methods to block it," Rose added. "This is an important step in that direction."
This kind of virus gets itself engulfed by the cell, and once inside makes new proteins by using its own RNA and the cell's protein-making machinery. When it replicates itself, several of these proteins move to the cell's outer membrane and cause it to "bud," pushing out a portion of the membrane until it is pinched off, forming a round new viral particle that incorporates a portion of the cell's membrane. This particle then goes on to infect other cells.
Three proteins involved in this process are believed to be M1, HA (for hemagglutinin) and NA (for neuraminidase). It has been theorized that M1 pushes against the cell wall from the inside, while HA and NA pull from the outside. By eliminating the "tails" of the HA and NA proteins, they were no longer able to perform this pulling function, the experiment indicated.
A normal influenza viral particle is circular and measures about 80 nanometers across ( a nanometer is a billionth of a meter).
Lamb's group found that deleting the tails of the HA protein resulted in minimal deformation of the viral particles, and deleting the tails of the NA protein caused more deformation, but deleting both tails caused a far greater deformation, suggesting that both play a similar and crucial role in the budding process. The severely deformed viral particles contain many partial viral buds strung along a lengthy rope-like structure. The length measures up to 700 nanometers.
The experiments provide strong evidence that "the normal 'pull' of the ... tails provides a driving force to pinch off virions," the paper said.
Subsequent analysis showed that the mutated virus particles were still infectious, but less so, and only about one-tenth as many were produced. Images of the mutated viruses, taken with an electron transmission microscope, were included in the EMBO paper.
Other authors of the paper were Hong Jin, of the department of biochemistry, cell biology and molecular biology and the Howard Hughes Medical Institute; George Leser, a research associate; and Jie Zhang, a graduate student in the department of biochemistry, cell biology and molecular biology.
(An image of a mutated virus is available at Lamb's home page at