Mimivirus up close - Scientists investigate structural details of the largest known virus

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.

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...

What's really interesting is that we hardly understand this stuff.

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Mimivirus possesses many characteristics which place it at the boundary of living and non-living. It is as large as several bacterial species, such as Rickettsia conorii and Tropheryma whipplei, possesses a genome of comparable size to several bacteria, including those above, and codes for products previously not thought to be encoded by viruses. In addition, mimivirus possesses genes coding for nucleotide and amino acid synthesis, which even some small obligate intracellular bacteria lack. This means that unlike these bacteria, mimivirus is not dependent on the host cell genome for coding the metabolic pathways for these products. They do however, lack genes for ribosomal proteins, making mimivirus dependent for protein translation and energy metabolism. These factors combined have thrown scientists into debate over whether mimivirus is a distinct form of life, comparable on a domain scale to Eukarya, Archaea and Bacteria. Nevertheless, mimivirus does not exhibit the following characteristics, all of which are part of many conventional definitions of life: homeostasis, response to stimuli, growth in the normal sense of the term (instead replicating via self-assembly of individual components) or undergoing cellular division. Because its lineage is very old and could have emerged prior to cellular organisms, mimivirus has added to the debate over the origins of life. Some genes unique to mimivirus, including those coding for the capsid, have been conserved in a variety of viruses which infect organisms from all domains - Eukarya, Archaea and Bacteria. This has been used to suggest that mimivirus is related to a type of DNA virus that emerged before cellular organisms and played a key role in the development of all life on Earth. An alternative hypothesis is that there were three distinct types of DNA viruses that were involved in generating the three known domains of life.

...They do however, lack genes for ribosomal proteins, making mimivirus dependent for protein translation and energy metabolism. These factors combined have thrown scientists into debate over whether mimivirus is a distinct form of life...

As far as I know, there is no set definition of "life". But it would seem that with organisms (using the term as having internal organization or structure) such as the mimivirus or even AI, it behooves us to create a useful construct for "life".

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