Virophage is the name coined for viruses such as Sputnik and Mavirus that can only replicate in cells infected with a helper virus, whose replication they inhibit. I’ve never liked the name – it means virus eater – and neither does Brent Johnson, a virologist at Brigham Young University:
“I believe the term ‘virophage’ is unfortunate because it implies one virus is infecting another virus and eating it. The small virus isn’t infecting another virus, it’s just using it to assist in replication, which is consistent with the needs of a defective virus.” he explains. He prefers calling the giant viruses “megaviruses,” and considers the name “Megavirus-Associated Virus (MAV) more consistent with currently accepted virus nomenclature.”
For more discussion, see the article by Marsha Stone in the July 2011 Microbe.
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Would Hepatitis Delta come under this classification (albeit on a smaller scale)? Isn’t delta usually referred to a “dependi virus”? It lacks it’s own envelope proteins and relies on HBV to provide this…
Sputnik type and Hep Delta are both satellite viruses, right? Would the terms “satellite virus” and “MAV” be interchangeable, then? As far as I can remember, Hepatitis Delta doesn’t inhibit Hepatitis B replication, at least not to the extent that Sputnik apparently does for Mimivirus. So that may be explain why it isn’t being considered a “virophage” (despite the misnomer).
By this reasoning the term ‘bacteriophage’ is equally unfortunate as viruses of bacteria do not ‘eat’ their hosts.
We know now of course that “bacteriophages” are viruses, that they don’t literally “eat” bacteria, and that virus is the preferable term. But at least there is history in the term bacteriophage, as they were originally discovered simply as filterable agents which cause bacterial cell death. Coming up with the new term “virophage” however, feels like we’re being inaccurate when we clearly know better, just so someone can coin a cute name.
A satellite replicates only in the presence of a helper virus, because it lacks genes required for replication. Satellites have small RNA genomes, 500-2000 nt long. Virophages are like satellites, except that they are much larger and inhibit production of the helper virus. I supposed that property is why they are officially considered satellites.
Hepatitis delta virus is considered a satellite (dependoviruses are single-stranded DNA viruses, the adeno-associated viruses). Correct, HDV requires HBV for encapsidation. See my response to ad169.
Thank you for bringing my article in Microbe magazine before an even larger audience and please note that Matthias Fischer has explained at the end of this article why Mavirus and Sputnick are fully functional viruses and do “infect” giant viruses. Matthias doesn’t mind calling these viruses “MAVs” but that just seems to lay another layer of confusion on things. It doesn’t appear that Sputnik and Mavirus are helper viruses, poor little things are victims
A 2009 paper by Jean-Michael Claverie and Chantal Abergel in Annual Reviews of Genetics might clarify things (Mimivirus and its Virophage) much better than I can because I’m just the writer.
Marcia Stone
NYC
OOPS -sorry, it’s Mimivirus and CroV who don’t seem to be helper viruses but rather victims. My internal editor was on break.
Marcia Stone