Viral Motors that Package DNA

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Image: Purdue News Service. Graphics courtesy of CLARK GEDNEY

Viruses are like blueprints in a tube: they consist of little more than DNA plans, protected by an outer shell. So to build new copies of themselves, or replicate, they must co-opt machinery within the cells they infect. But researchers from Purdue University and the University of Minnesota have identified one vital tool that viruses appear to bring with them to the construction site. In this week's issue of Nature, Purdue biologist Michael Rossmann and his colleagues offer an atom-by-atom description of a tiny motor found within one virus--the phi29 bacteriophage--that packages new viral DNA into new shells. "This study provides the first knowledge of a DNA packaging motor," Rossmann says. "Though other motor systems have been studied in biology, this is the first motor known to translocate genetic material."

The team used an array of micro-imaging techniques to get a good look at the DNA packaging motor, and found that it consisted of three main parts: an elongated prohead; a doughnut-shaped connector that positions itself at the opening of a new viral shell and threads DNA into it; and a unique RNA-ATPase complex that powers the packaging process. The connector itself contains 12 protein subunits, which the researchers suggest may work as cylinders in the motor (see illustration). "Our results suggest that the prohead and connector comprise a rotary motor, with the head and the ATPase complex acting as a stator and the DNA acting as a spindle," Rossmann explains. They hope the result will shed light on how DNA is packaged in other viruses such as Herpesvirus, the pathogen behind Herpes simplex, chicken pox and shingles. Any additional insight could led to new therapeutic targets for preventing viral infections.

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