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Nuclear localization of mouse Mx1 protein is necessary for inhibition of influenza virus
T Zürcher1, J Pavlovic, P Staeheli
1Institute for Immunology and Virology, University of Zurich, Switzerland.
Abstract:
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.
Insights
Mouse Mx1 protein inhibits influenza virus replication exclusively in the nucleus. Unlike human MxA, cytoplasmic mouse Mx1 variants lack antiviral activity, highlighting species-specific differences in Mx protein function.
Area of Science:
- Virology
- Immunology
- Molecular Biology
Background:
- The interferon-induced Mx1 protein in mice provides resistance to influenza virus by inhibiting viral mRNA synthesis in the nucleus.
- Human MxA protein, localized in the cytoplasm, inhibits influenza and vesicular stomatitis viruses but not others, acting on a post-transcriptional cytoplasmic step.
- Previous studies showed nuclear-localized human MxA variants inhibit influenza virus by blocking primary transcription.
Purpose of the Study:
- To investigate whether cytoplasmic forms of mouse Mx1 can mimic the antiviral action of human MxA.
- To determine the antiviral activities of mutant mouse Mx1 proteins with altered localization signals.
Main Methods:
- Generation and testing of mutant mouse Mx1 proteins with altered nuclear transport signals.
- Assessing the antiviral activity of these mutants against influenza virus and vesicular stomatitis virus.
- Utilizing simian virus 40 large T nuclear translocation signal to redirect cytoplasmic variants to the nucleus.
Main Results:
- Cytoplasmic mouse Mx1(E614) mutant failed to inhibit influenza virus and vesicular stomatitis virus multiplication.
- Relocating Mx1(E614) to the nucleus restored its influenza virus-inhibiting activity.
- Other carboxy-terminal Mx1 variants that lost nuclear transport also lost antiviral activity, but gained activity upon nuclear relocation.
Conclusions:
- Mouse Mx1 protein functions exclusively in the nucleus to inhibit influenza virus, unlike human MxA.
- Species-specific localization is critical for the antiviral mechanism of Mx proteins.
- These findings offer insights into the mechanistic details of Mx protein-mediated antiviral action.