New look at influenza virus

New images of the influenza virus have prompted scientists to rethink previous ideas of how the virus escapes from cells, according to a study by researchers from Japan, Sweden, and the U.S.

The flu virus carries its genes in eight segments of RNA and proteins. Before escaping from a host cell, it has to assemble the right pieces and package them into a complete virus. Scientists think that shuffling these segments allows flu viruses to vary from year to year, producing new infectious strains. One theory held that the eight genetic segments are packaged randomly into viral particles, but only those with the right eight segments are viable. That would be inefficient, yet would put a natural brake on the growth rate of the virus.

The new images, taken using a technique called cryoelectron microscopy, clearly show that the eight genetic segments are organized into a precise pattern, with seven segments surrounding a central one, before the virus particle wraps itself in an outer coating and membrane and subsequently buds out of the cell.

"It was thought that viral RNA packaging often fails, but this is not true," explains R. Holland Cheng, professor of molecular and cellular biology at the University of California, Davis. In conventional electron microscopy, samples are "fixed" with heavy metals that deflect electrons, providing the image contrast, but that means that only the surface can be imaged, Cheng points out. In cryoelectron microscopy, samples are fixed by rapid freezing to extremely low temperatures with liquid nitrogen or helium. Researchers then take pictures from different angles and reconstruct them with a computer to create a three-dimensional image. Cryoelectron microscopy can capture information that you cannot get with conventional electron microscopy.

Cheng's laboratory is using the technique to look deeper into the cell to find the "glue" that bundles together the genome segments of the virus.

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