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Type I and III interferon-regulated Mx genes encode evolutionary conserved antiviral restriction factors which inhibit a broad range of RNA viruses in most vertebrate species. In mice, the Mx1 locus confers resistance to members of the Orthomyxoviridae, such as influenza A and Thogoto viruses. Mice of strain A2G carry a functional Mx1 gene, while most other laboratory inbred mouse strains carry truncated and therefore defective Mx1 alleles. A recent study (Ferris MT et al. PLOS pathogens 9(2) 2013) investigating how genetic factors might contribute to influenza susceptibility showed that CAST/EiJ mice, an inbred strain derived from wild Mus musculus castaneus, are susceptible to influenza virus challenge although they carry a full-length Mx1 gene. Here we characterized CAST-derived Mx1 in order to better understand the molecular basis of its altered antiviral properties. We were able to show that CAST-derived Mx1 retains a low degree of anti-influenza activity but, surprisingly, is still able to confer full protection against Thogoto virus challenge. Sequencing revealed that CAST-derived Mx1 differs by two amino acids from A2G-derived Mx1 which are both localized in the GTPase (G) domain of the protein. These mutations lead to a reduction in GTPase activity. They further diminish Mx1 protein levels in organs of animals after interferon stimulation in vivo. Interestingly, these mutations have no effect on mRNA levels, suggesting that the reduced Mx1 protein levels might be attributed to differences in protein stability. In summary, our findings are able to explain the unexpected virus susceptibility of CAST/EiJ mice. |
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