Leporipoxvirus Cu,Zn-superoxide dismutase (SOD) homologs are catalytically inert decoy proteins that bind copper chaperone for SOD.
Many Chordopoxviruses encode catalytically inactive homologs of cellular Cu-Zn superoxide dismutase (SOD). The biological function of these proteins is unknown, although the proteins encoded by Leporipoxviruses have been shown to promote a slow decline in the level of superoxide dismutase activity in virus-infected cells. To gain more insights into their function, we have further characterized the enzymatic and biochemical properties of a SOD homolog encoded by Shope fibroma virus. Shope fibroma virus SOD has retained the zinc binding properties of its cellular homolog, but cannot bind copper. Site-directed mutagenesis showed that it requires at least four amino acid substitutions to partially restore copper binding activity, but even these changes still did not restore catalytic activity. Reciprocal co-immunoprecipitation experiments showed that recombinant Shope fibroma virus SOD forms very stable complexes with cellular copper chaperones for SOD and these observations were confirmed using glutathione-S-transferase tagged proteins. Similar viral SOD/chaperone complexes were formed in cells infected with a closely related myxoma virus, where we also noted that some of the SOD antigen co-localizes with mitochondrial markers using confocal fluorescence microscopy. About 2% of the viral SOD was subsequently detected in gradient-purified mitochondria extracted from virus-infected cells. These poxviral SOD homologs do not form stable complexes with cellular Cu,Zn-SOD or affect its concentration. We suggest that Leporipoxvirus SOD homologs are catalytically inert decoy proteins that are designed to interfere in the proper metallation and activation of cellular Cu,Zn-SOD. This reaction might be advantageous for tumorigenic poxviruses, since higher levels of superoxide have been proposed to have anti-apoptotic and tumorigenic activity.
Teoh
Melissa L T
ML
Department of Molecular Biology & Genetics, University of Guelph, Guelph, Ontario N1G 2W1, Canada.
Walasek
Paula J
PJ
Evans
David H
DH
D000595:Amino Acid Sequence; D015153:Blotting, Western; D002384:Catalysis; D003300:Copper / Q000378:metabolism*; D004591:Electrophoresis, Polyacrylamide Gel; D012776:Fibroma Virus, Rabbit / Q000201:enzymology; D005982:Glutathione Transferase / Q000378:metabolism; D006801:Humans; D018152:Leporipoxvirus / Q000201:enzymology*; D008670:Metals / Q000494:pharmacology; D018613:Microscopy, Confocal; D008928:Mitochondria / Q000378:metabolism; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D009232:Myxoma / Q000378:metabolism; D009233:Myxoma virus / Q000201:enzymology; D010802:Phylogeny; D010957:Plasmids / Q000378:metabolism; D011233:Precipitin Tests; D011485:Protein Binding; D011994:Recombinant Proteins / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D013482:Superoxide Dismutase / Q000737:chemistry* / Q000378:metabolism; D015032:Zinc / Q000378:metabolism
D008670:Metals; D011994:Recombinant Proteins; D003300:Copper; D013482:Superoxide Dismutase; D005982:Glutathione Transferase; D015032:Zinc