Parental influenza virion nucleocapsids are efficiently transported into the nuclei of murine cells expressing the nuclear interferon-induced Mx protein.

The interferon-induced murine Mx1 protein, which is localized in the nucleus, most likely specifically blocks influenza virus replication by inhibiting nuclear viral mRNA synthesis, including the mRNA synthesis catalyzed by inoculum (parental) virion nucleocapsids (R. M. Krug, M. Shaw, B. Broni, G. Shapiro, and O. Haller, J. Virol. 56:201-206, 1985). We tested two possible mechanisms for this inhibition. First, we determined whether the transport of parental nucleocapsids into the nucleus was inhibited in murine cells expressing the nuclear Mx1 protein. To detect the Mx1 protein, we prepared rabbit antibodies against the Mx1 protein with a CheY-Mx fusion protein expressed in bacteria. The fate of parental nucleocapsids was monitored by immunofluorescence with an appropriate dilution of monoclonal antibody to the nucleocapsid protein. The protein synthesis inhibitor anisomycin was added to the cells 30 min prior to infection, so that the only nucleocapsids protein molecules in the cells were those associated with nucleocapsids of the parental virus. These nucleocapsids were efficiently transported into the nuclei of murine cells expressing the Mx1 protein, indicating that this protein most likely acts after the parental nucleocapsids enter the nucleus. The second possibility was that the murine Mx1 protein might act in the nucleus to inhibit viral mRNA synthesis indirectly via new cap-binding activities that sequestered cellular capped RNAs away from the viral RNA transcriptase. We show that the same array of nuclear cap-binding proteins was present in Mx-positive and Mx-negative cells treated with interferon. Interestingly, a large amount of a 43-kDa cap-binding activity appeared after interferon treatment of both Mx-positive and Mx-negative cells. Hence, the appearance of new cap-binding activities was unlikely to account for the Mx-specific inhibition of viral mRNA synthesis. These results are most consistent with the possibility that the Mx1 protein acts directly to inhibit the viral transcriptase in the nucleus.