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Giant viruses able to replicate in Acanthamoeba castellanii penetrate their host through phagocytosis. After capsid opening, a fusion between the internal membranes of the virion and the phagocytic vacuole triggers the transfer in the cytoplasm of the viral DNA together with the DNA repair enzymes and the transcription machinery present in the particles. In addition, the proteome analysis of purified mimivirus virions revealed the presence of many enzymes meant to resist oxidative stress and conserved in the Mimiviridae. Megavirus chilensis encodes a predicted copper, zinc superoxide dismutase (Cu,Zn-SOD), an enzyme known to detoxify reactive oxygen species released in the course of host defense reactions. While it was thought that the metal ions are required for the formation of the active-site lid and dimer stabilization, megavirus chilensis SOD forms a very stable metal-free dimer. We used electron paramagnetic resonance (EPR) analysis and activity measurements to show that the supplementation of the bacterial culture with copper and zinc during the recombinant expression of Mg277 is sufficient to restore a fully active holoenzyme. These results demonstrate that the viral enzyme's activation is independent of a chaperone both for disulfide bridge formation and for copper incorporation and suggest that its assembly may not be as regulated as that of its cellular counterparts. A SOD protein is encoded by a variety of DNA viruses but is absent from mimivirus. As in poxviruses, the enzyme might be dispensable when the virus infects Acanthamoeba cells but may allow megavirus chilensis to infect a broad range of eukaryotic hosts.

journal of virology

The Megavirus Chilensis Cu,Zn-Superoxide Dismutase: the First Viral Structure of a Typical Cellular Copper Chaperone-Independent Hyperstable Dimeric Enzyme