Structure of mouse Mx1 protein. Molecular assembly and GTP-dependent conformational change.

Mouse Mx1 protein is an interferon-inducible nuclear protein and confers resistance to influenza virus infection. The Mx1 protein purified from interferon-induced A2G mouse liver exhibited GTPase activity as did the Mx1 protein purified from the Mx1 cDNA-expressing Escherichia coli (Nakayama, M., Nagata, K., Kato, A., and Ishihama, A. (1991) J. Biol. Chem. 266, 21404-21408; Nakayama, M., Nagata, K., and Ishihama, A. (1992) Virus Res. 22, 227-234). The Mx1 protein purified from both mouse liver and Mx1-cDNA expressing E. coli was found to exist as assembled polymeric states judged from gel filtration pattern. By making a set of deletion derivatives of the Mx1 cDNA, the main motif for self-assembly of the Mx1 protein was mapped between amino acid residues 51-99. This motif is highly conserved not only in the Mx family of proteins but also in Mx-related proteins. The polymeric form of Mx1 from E. coli was observed as "horseshoe"-like structure by negative staining microscopy. When the Mx1 protein was incubated with GTP, this horseshoe structure was transformed to larger and tightly stacked helical forms. Electron microscopic analysis of immunostained liver of the interferon-induced mice indicated that the Mx1 protein exists in nuclei, forming giant complexes of about half the size of nucleoli.