A Mimivirus Enzyme Close-Up

Mimivirus is so enormous—its genome is 1.2 million base pairs long—that researchers who discovered it in a British water tower initially thought it had to be a bacterium. Also, the virus’s icosahedral capsid is covered in 125-nm-long protein-carbohydrate fibers, a coat more commonly observed in bacteria than viruses. Researchers have suspected that these fibers help the virus infect its host. Although the virus’s preferred prey has not been identified, it is known to infect amoebas in a pinch. Now, a research team led by Purdue University’s Michael G. Rossmann has solved the structure of R135, a protein found abundantly in the viral fibers (Structure 2015, DOI: 10.1016/j.str.2015.03.023). By using X-ray crystallography, the team found that the 652-amino-acid-long protein’s overall shape is similar to that of enzymes that carry out oxidation-reduction reactions, and its amino acid sequence is similar to that of an oxidase involved in plant lignin degradation. They contend that Mimivirus might typically prey on seaweed or algae that contain lignin in their cell walls because enzymes that degrade lignin would help the microorganism penetrate these cells.