The Other Sputnik: The First Known Virophage

By: Helen Beilinson

For every organism on earth, there are a multitude of viruses that can infect it. Every. Single. Organism. There are viruses that infect everything from bacteria to fish to humans, and anything in between. I’m going to tell you about one of my personal favorite viruses, Sputnik.

But first, let me introduce our host, the amoeba. Amoebae are single celled organisms, more closely resembling our cells than those of bacteria. Unlike humans, they don’t have an extensive immune system that protects them from harmful pathogens in their environment. Their main mechanism of protection is ‘eating’ things in their environment and breaking them down, in a process called ‘phagocytosis’. Although this defense works great often enough, there is not much the amoeba can do to protect itself once it’s infected with a virus.

One such virus is the Mamavirus. Mamavirus is a HUGE virus at around 750 nm in diameter. This may seem small, but that’s about half the size of the bacterium E. coli and it’s almost eight times as large as the infamous influenza virus. You can even see it under a regular light microscope, the kind of microscope used in many high school science labs, compared to most other smaller viruses that you need a high powered microscope to see them. It was first found almost seven years ago living inside amoeba in a cooling tower. Mamavirus lives and replicates in the cytoplasm of amoeba, where it sets up a “replication factory” that takes over the cell. This replication factory is where the virus replicates its DNA genome and makes the proteins necessary to build the outer shell in which it keeps its genome. Once Mamavirus replicates to a high degree, the viruses break down the amoeba cell, a process called ‘lysis’, to be released. As I’m sure is clear, this isn’t a great fate for the amoeba who has little defense against the virus once it starts replicating. That’s where Sputnik comes in.

Sputnik is a very small DNA virus, only about 50 nm in size, about fifteen times smaller than Mamavirus. It became fairly famous after its discovery as the first known virophage, a virus that infects viruses. That’s right—Sputnik is a virus that infects viruses. In fact, it infects Mamaviruses. Although Sputnik can infect amoeba on its own, it’s not that great at replicating by itself. However, when the amoeba is also infected with Mamavirus, Sputnik replicates like crazy. It takes over Mamavirus’ replication factory, which is a perfect little nook for the smaller virus to replicate. Although it might sound kind of unpleasant to be infected with two viruses at once, it’s actually a benefit for the amoeba. By hijacking the Mamavirus’ machinery, Sputnik reduces the ability for the big virus to replicate and make proper virions (or virus particles). It can reduce the Mamavirus load up to 70% and decreases amoeba lysis by threefold. Turns out the old proverb “the enemy of my enemy is my friend” really is the case with the amoeba and Sputnik.

Since the discovery of Sputnik, many more virophages have been discovered, which predominantly attack the large viruses that infect amoeba and zooplankton (a marine microorganism). Virophages have increased our knowledge of the deep complexity of the microbial world. As these critters are essentially brand new to our understanding, there is still a lot to learn about how they influence our ecosystems. There has been an ongoing battle amongst scientists about whether or not viruses are living things. Without getting too much into the details of things, the major point of contention is whether we can consider something living if it has to live within another organism’s cells to replicate. Maybe we need to reconsider whether we should classify viruses are living organisms or not. If they can also be infected with viruses, just like us and our pets and the bacteria that live in our guts—why can’t they be alive?