Thirty thousand years ago mammoths and other giant fauna roamed Earth. So, too, did a kind of giant virus that is so large it can be seen under a light microscope. Yet unlike many giant land animals this megavirus, Pithovirus sibericum, is still around.
It is not the first megavirus discovered, but at 1.5 µm in length, it is the largest. Two other families of giant viruses—whose large genomes can in some cases encode more proteins than bacteria—have been found in locations as diverse as Chile and Australia. Researchers in France plucked P. sibericum from the Siberian permafrost.
Discovery of this new family of Pithoviruses reveals that giant viruses are “much more diverse than initially assumed,” argue Chantal Abergel and Jean-Michel Claverie at the Structural & Genomic Information Laboratory at Aix-Marseille University, in France (Proc. Nat. Acad. Sci. USA, 2014, DOI: 10.1073/pnas.1320670111).
P. sibericum doesn’t have a well-organized protein capsid shell, which is common to many smaller viruses and to the members of Megaviridae, the first family of megaviruses, discovered in 2003. Instead, P. sibericum DNA is enclosed by a thick membranelike envelope, similar to that encasing Pandoravirus, the megavirus discovered in 2011. Although P. sibericum is 50% larger than the 1-µm-long Pandoravirus, its genome encodes only about 500 proteins, one-fifth of Pandoravirus’s approximately 2,500 proteins.
A majority of all megavirus proteins are of unknown structure and function, a bounty of hundreds of ancient proteins “that simply don’t resemble anything we’ve seen before,” Abergel says. “There are more proteins here for structural biologists to study than is possible in a lifetime of work.”
“We also need to study their molecular mechanisms of infection,” comments Eugene V. Koonin, who studies megaviruses at the National Center for Biotechnology Information, in Bethesda, Md. Pithovirus, for example, appears to have a plug in its membranous envelope that must be “uncorked” for infection to occur, he adds.