Leporipoxvirus Cu-Zn Superoxide Dismutase Homologs Inhibit Cellular Superoxide Dismutase, but Are Not Essential for Virus Replication or Virulence

Abstract

Vertebrate poxviruses encode homologs of cellular cupro-zinc superoxide dismutases (Cu-Zn SOD). In this study we have examined the molecular genetic properties of two Cu-Zn SOD homologs encoded by the Shope fibroma virus (SFV) and myxoma virus. These Leporipoxvirus proteins should be catalytically inactive as judged by the point mutations which alter a key catalytic arginine and restructure the predicted Cu-binding domain. This prediction was confirmed using in situ gel assays and recombinant proteins produced both in bacteria and in mammalian cells. Western blot analysis showed that these proteins are produced in abundance late in infection and can, upon exposure to oxidizing conditions, form disulfide cross-linked dimers. They are also virion components and not essential for growth in culture or virulence. Leporipoxvirus Cu-Zn SOD homologs affected two phenotypes. First, deletion of the myxoma M131R gene caused the mutant virus to grow better ( approximately 10-fold) in culture than does the wild-type parent. Second, expression of either native or recombinant Leporipoxvirus proteins is accompanied by a decline in cellular Cu-Zn SOD activity. We concluded that these gene products can somehow modulate the activity of host Cu-Zn SODs, but what advantage is thus gained by the virus remains to be established.

Amsacta mooreiEntomopoxvirus Expresses an Active Superoxide Dismutase

“…1). As noted previously (12,13,50), the vaccinia virus and myxoma virus SODs are both clearly missing key residues required for full function. We hypothesized from the sequence alignments that the SOD from AmEPV should be an enzymatically active protein, and we chose to express AMVSOD in E. coli and analyze the purified protein for SOD activity.…”

“…It has been proposed that the nonactive SODs of the vertebrate poxviruses may operate by interacting with the relevant chaperones of the host cell to withhold critical copper residues and thereby reduce endogenous Cu-Zn SOD activity (55); however, deletions of sod from Shope fibroma virus and vaccinia virus have little effect in animal models (4,13). On the other hand, the exact role that an active SOD plays during an infection also remains to be determined, and we propose several possibilities.…”

“…Many of the poxviruses, including variola virus, the causative agent of smallpox, and vaccinia virus, which is used for immunization against smallpox, contain genes exhibiting sequence homology to those for Cu-Zn SODs. Two recent studies examined the role of SOD in vaccinia virus and the leporipoxviruses Shope fibroma virus and myxoma virus (4,13). In both cases, the protein produced from the sod gene was found to be nonfunctional as a superoxide dismutase.…”

“…Myxoma virus and Shope fibroma virus are closely related, and their SODs are more similar to bovine SOD than is vaccinia virus SOD; however, the leporipoxvirus SODs do not contain the catalytic arginine and lack one of the cysteines necessary for intrasubunit disulfide bond formation. As with the vaccinia virus SOD, the protein is packaged into the mature virion, and deletion of the myxoma virus sod gene has only a slight effect on the spread of the virus in rabbits, with no difference in final disease state (13). More recently, Teoh et al (55) demonstrated that the Shope fibroma virus SOD complexes with the host cellular copper chaperone and thus may inhibit cellular enzyme function by preventing the addition of the required copper to the cellular SOD enzyme.…”

Comparative Analysis of the Complete Genome Sequence of the California MSW Strain of Myxoma Virus Reveals Potential Host Adaptations

“…M131R encodes an inactive homologue of mammalian Zn/Cu superoxide dismutase (SOD) and is predicted to interfere with the activity of cellular SOD (29)(30)(31). Deletion of the M131R gene had no effect on the virulence of MYXV in European rabbits.…”