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Mimivirus, the largest known virus, bears a starfish-shaped structure (A and B) that covers an opening in the virus coat through which DNA might be expelled when infecting a host. The DNA is enveloped in a membrane, seen in gray in these renderings reconstructed from cryo-electron microscopy images (C and D). This membrane is concave beneath the starfish-covered opening.Rossmann et al., PLoS Biology 2009

Scientists have zoomed in on mimivirus, the enormous virus with the delicate name that has perplexed scientists since 1992, when it was found living in an amoeba in a water tower in England.

“This is like landing on the moon,” says Michael Rossmann of Purdue University in West Lafayette, Ind. Rossmann and an international team of scientists report the results of their reconnaissance online April 27 in PLoS Biology.

Mimivirus, full name Acanthamoeba polyphaga Mimivirus, is the largest known virus in the world. Its mass is more than 100 times that of the virus that causes the common cold, says Rossmann. Because of its size and other features, mimivirus blurs the lines used to determine what is alive. The new work may help scientists understand if and how the virus could cause disease in humans.

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Atomic force microscopy reveals cracks outlining one triangular face on a mimivirus coat sheared of its surface fibers (left). The cracks aren’t consistent from face to face, suggesting some instability in the coat. The rendering at right shows unusual surface depressions (numbered 1-18) that, upon closer inspection, are actually holes.Rossmann et al., PLoS Biology 2009

The research team used cryo-electron microscopy, a technique that took thousands of snapshots of the beastie and averaged them together, revealing unseen structural details. This allowed a closer look at a starfish-shaped structure on mimivirus’s outer shell, or capsid, which appears to plug an opening through which the virus could eject DNA into its host. Using atomic force microscopy, the researchers also found tiny, regularly spaced holes in the virus’s outer shell, which aren’t typically seen in a virus of this shape.

The new structural finds, along with previous genetic and morphological work, confirm that mimivirus is an odd mix of genes and parts found in viruses, bacteria and even eukaryotes, the organisms that sequester their DNA in a nucleus.

“We’re looking at the boundary of something alive and something dead,” Rossmann says.

Viruses aren’t usually considered alive — they can’t replicate without a host’s machinery — but mimivirus has many traits that fall outside the virus box.

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