Mx proteins: antiviral gatekeepers that restrain the uninvited

Abstract

Fifty years after the discovery of the mouse Mx1 gene, researchers are still trying to understand the molecular details of the antiviral mechanisms mediated by Mx proteins. Mx proteins are evolutionarily conserved dynamin-like large GTPases, and GTPase activity is required for their antiviral activity. The expression of Mx genes is controlled by type I and type III interferons. A phylogenetic analysis revealed that Mx genes are present in almost all vertebrates, usually in one to three copies. Mx proteins are best known for inhibiting negative-stranded RNA viruses, but they also inhibit other virus families. Recent structural analyses provide hints about the antiviral mechanisms of Mx proteins, but it is not known how they can suppress such a wide variety of viruses lacking an obvious common molecular pattern. Perhaps they interact with a (partially) symmetrical invading oligomeric structure, such as a viral ribonucleoprotein complex. Such an interaction may be of a fairly low affinity, in line with the broad target specificity of Mx proteins, yet it would be strong enough to instigate Mx oligomerization and ring assembly. Such a model is compatible with the broad "substrate" specificity of Mx proteins: depending on the size of the invading viral ribonucleoprotein complexes that need to be wrapped, the assembly process would consume the necessary amount of Mx precursor molecules. These Mx ring structures might then act as energy-consuming wrenches to disassemble the viral target structure.

Fig 1

Structure of Mx proteins. (A) The different domains of Mx proteins delineated by their primary sequence and their relationship to the domains determined in the 3D structure of Mx proteins. The Mx domains in the primary sequence include the GTPase domain, the middle domain (MD), and the GTPase effector domain (GED). The domains in the 3D structure include the G domain, the bundle signaling element (BSE), and the stalk domain. The amino acid sequences of domains important for the antiviral activity of Mx proteins are aligned. These domains include the GTP-binding motif, the dynamin signature, the C-terminal leucine zipper, and the nuclear localization signal (NLS). The conserved amino acids in the leucine zipper are indicated with an asterisk. The position of the epitope recognized by the monoclonal antibody 2C12, which can neutralize Mx antiviral activity, is indicated above the MD. The 2C12 epitope and loop L4 are important for viral target recognition. (B) 3D structure of Mx proteins (PDB file MxA 3SZR [73]). (C) 3D structure of MxA tetramer showing the interfaces (1 to 3 and loop L4) involved in oligomer formation (65, 73, 109). The images in panels B and C were generated with PyMOL.

Fig 2

Phylogenetic tree based on primary amino acid sequences of Mx proteins. The Mx protein sequences were aligned in ClustalX2 (using the protein weight matrix PAM350), and the nonconserved N terminus was trimmed using the program Jalview (81, 143). This alignment was then used to build a bootstrap neighbor-joining tree in ClustalX2 with 1,000 bootstrap replicates. The deduced phylogenetic tree, visualized with Dendroscope (144), allows the identification of different subfamilies of Mx proteins: (i) fish Mx proteins (purple); (ii) Mx proteins of birds, reptiles, and amphibians (green); (iii) nonrodent mammalian Mx2 proteins (yellow); (iv) rodent Mx1/Mx2 proteins (blue); and (v) nonrodent mammalian Mx1 proteins (orange).

Fig 3

Genomic organization of Mx genes. The genomic organization of the different Mx genes of the different organisms was retrieved from the UCSC, NCBI Map Viewer, and Ensembl genome browsers; their organization with respect to their flanking genes FAM3B and TMPRSS2 is shown. The chromosome (Chr), scaffold (sc), or contig number is given.

Fig 4

Antiviral mechanisms of Mx proteins. Steps in the life cycles of different viruses that are inhibited by Mx proteins are shown. Human MxA, mouse Mx2, and porcine Mx1 inhibit Thogoto virus (THOV), vesicular stomatitis virus (VSV), La Crosse virus (LACV), and influenza viruses in the cytoplasm. Mouse Mx1 inhibits THOV and influenza viruses in the nucleus.

Fig 5

Amino acid residues in influenza A virus nucleoprotein associated with sensitivity to Mx. Left, 3D structure of an NP monomer (PDB file 2Q06 [145]). The residues important for Mx1 and MxA resistance are highlighted in red. Residues that contribute in only a minor way to Mx1 resistance are indicated in light red (95). Right, these important residues are also exposed on the surface of viral ribonucleoprotein (RNP) complexes (PDB file 4BBL [146]). The RNA strand is depicted in green. The images were generated with PyMOL.