Role of GTPase activity of murine Mxl protein in nuclear localization and anti-influenza virus activity

Murine Mxl protein is an interferon-inducible GTPase which localizes in nuclei and inhibits influenza virus infection. Wild-type Mxl and two mutant Mxl proteins, each carrying a single mutation either in the GTP-binding motif (S50I) or in the self-assembly motif (C71S), were expressed in MDCK cells. Wild-type Mxl localized in nuclei, forming small granules with minute dots, and inhibited influenza virus growth. Mutant SSOI, which had no GTP-binding or GTPase activities, formed linear structures in nuclei and lacked anti-viral activity, while C71S appeared iffuse in nuclei as minute dots without granules, but retained the inhibitory activity against influenza virus growth. A correlation existed between GTPase activity, intranuclear distribution and antiviral activity. We concluded that GTPase activity is essential for expression of the biological activity of Mxl protein. Murine Mxl protein is an interferon-inducible nuclear protein present in the A2G mouse strain, which is resistant to influenza virus infection. Expression of the Mxl protein in cells and transgenic animals permissive for influenza virus infection induces an anti-influenza virus state in the absence of interferon, indicating that Mxl itself inhibits influenza virus growth (Staeheli et al., 1986; Kolb et al., 1992). Biochemical analysis indicated that Mxl is a GTP-binding protein with GTPase activit