A study conducted by Michigan University (USA) is shedding new light on giant viruses – enigmatic viruses that are huge compared to their well-known “relatives” (on average ten times bigger) and that can also survive in extreme environments such as the Siberian permafrost or beneath the Antarctic ice. With the help of imaging technology and state-of-the-art microscopes, the US researchers were able to identify and characterize the different proteins that play a key role in the mechanisms used by these viruses to orchestrate infections. The results of the study were published in the scientific journal Cell.
As we said before, these viruses are gigantic compared to the average size of “normal” viruses. We are actually talking about tiny biological entities, measuring just 300 nanometers (i.e. billionths of a meter), but this is certainly much bigger than the typical 30 nanometers, for example, of rhinoviruses, responsible for the common cold. “Giant viruses are gargantuan in size and complexity”, explained Kristin Plant, coordinator of the research, “The giant viruses recently discovered in Siberia retained the ability to infect after 30,000 years in permafrost.” How can they survive for millennia? Because they have a very sturdy outer protein capsule (or capsid, to use the technical term) that protects the viral genome inside, even in harsh environments. The capsules of the species analyzed by the biologists from Michigan State University (Mimivirus, Antarctica virus, Samba virus and the newly discovered Tupanviruses – typically found in deep ocean waters) are icosahedral, or shaped like a twenty-sided die.
These viruses use unique ways to attach to cells and to inject their viral genome into them. However, such mechanisms are difficult to accurately identify. To solve this problem, the US researchers decided to reproduce several “environmental” characteristics typical of giant viruses (acidity and little oxygen) in lab conditions and then studied their reproduction mechanisms using highly-advanced apparatus. In particular, the researchers used so-called cry microscopes, which work at very low temperatures and make it possible to identify biomolecular structures at almost atomic level. They also used scanning electron microscopes, which enable mechanical processes and movements to be photographed and studied. In the end, for the first time ever, they were able to describe how giant viruses transfer their genetic material to the host cell, through a series of starfish-shaped protrusions, which allow the material to be released slowly. The researchers also “characterized” different proteins that take part in this replication. Such information may be very useful for developing substances capable of blocking these viruses in the future.
But can (or could) giant viruses, which were only discovered around twenty years ago, also infect humans? This is still being debated by experts, and new studies will be needed. For the time being, we cannot answer this question…