Nuclear localization of mouse Mx1 protein is necessary for inhibition of influenza virus

The interferon-induced Mx1 protein of mice confers selective resistance to influenza virus. It inhibits viral mRNA synthesis in the nucleus of influenza virus-infected cells. The related human MxA protein is localized in the cytoplasm and can inhibit influenza virus and vesicular stomatitis virus but not other viruses. MxA blocks a poorly defined cytoplasmic multiplication step of influenza virus that follows primary transcription of the viral genome. We previously showed that nuclear variants of MxA that carry an artificial nuclear translocation signal were also active against influenza virus. However, these variants blocked primary transcription of influenza virus. In the present study, we addressed the question of whether cytoplasmic forms of Mx1 were capable of mimicking the antiviral action of MxA by determining the antiviral activities of mutant mouse Mx1 protein. Cytoplasmic Mx1(E614), which differs from wild-type Mx1 by a single amino acid substitution in its nuclear transport signal, failed to inhibit the multiplication of influenza virus and vesicular stomatitis virus. Relocation of Mx1(E614) to the nucleus with the help of the simian virus 40 large T nuclear translocation signal attached to its amino terminus restored the influenza virus-inhibiting activity. Other changes in the carboxy-terminal region of Mx1 also abolished transport to the nucleus and simultaneously abolished antiviral activity. One of these variants, Mx1/A, gained activity against influenza virus upon relocation to the nucleus. These results demonstrate that unlike human MxA, the mouse Mx1 protein can function only in the nucleus. This finding has important implications regarding the mechanistic details of Mx protein action.