No enhanced influenza virus resistance of murine and avian cells expressing cloned duck Mx protein.

We used polymerase chain reaction techniques to isolate a highly conserved genomic fragment of the Mx gene of a wild duck. This probe recognized the transcripts of two distinct Mx alleles of virus-infected duck embryo cells, which we cloned and sequenced. Duck Mx mRNA was not detectable in uninduced cells, but it accumulated to a high level after infection with Newcastle disease virus or after treatment of cultured duck embryo cells with double-stranded RNA. Duck Mx protein exhibited the characteristic structural features of interferon-induced mammalian Mx proteins, including GTP-binding and leucine zipper motifs. Western blot analysis revealed multiple forms of duck Mx protein with molecular masses between 72 to 82 kDa. In induced duck embryo cells as well as in murine or avian cells permanently transfected with cloned duck Mx cDNAs, duck Mx protein accumulated predominantly in the cell nucleus. Its immuno-staining pattern differed markedly from that of the nuclear mouse and rat Mx1 proteins. A significant portion of duck Mx protein was also found in the cytoplasm where it formed large granules. Duck Mx proteins failed to render permanently transfected mouse or chick cells resistant to different strains of influenza virus. This unexpected result suggests that influenza virus resistance of ducks is not dependent on interferon-induced Mx proteins.