Murine GFP-Mx1 forms phase-separated nuclear condensates and associates with cytoplasmic intermediate filaments: novel antiviral activity against vesicular stomatitis virus

Type I and III interferons (IFNs) induce expression of the "myxovirus resistance proteins" MxA in human cells and its ortholog Mx1 in murine cells. Human MxA forms cytoplasmic structures, some tethered to intermediate filaments. In contrast, murine Mx1 mainly forms nuclear bodies. Both HuMxA and MuMx1 are antiviral towards influenza A virus (FLUAV) (an orthomyxovirus). However, it has long been considered that HuMxA, but not MuMx1, was antiviral towards vesicular stomatitis virus (VSV) (a rhabdovirus). We previously reported that the cytoplasmic human GFP-MxA structures in Huh7 hepatoma cells were phase-separated membraneless organelles (MLOs) ("biomolecular condensates"). In the present study we investigated whether nuclear murine Mx1 structures might also represent phase-separated biomolecular condensates. The transient expression of murine GFP-Mx1 in Huh7 hepatoma and Mich-2H6 melanoma cells led to the appearance of Mx1 nuclear bodies. These GFP-MuMx1 nuclear bodies were rapidly disassembled by exposing cells to 1, 6-hexanediol (5% w/v), or to hypotonic buffer (40-50 mosM), consistent with properties of membraneless phase-separated condensates. FRAP assays revealed that the GFP-MuMx1 nuclear bodies upon photobleaching showed a slow partial recovery (mobile fraction: [~]18%) suggestive of a gel-like consistency. Surprisingly, expression of GFP-MuMx1 in Huh7 cells also led to the appearance of MuMx1 in a novel cytoplasmic giantin-based intermediate filament meshwork and in cytoplasmic structures in 20-30% of transiently transfected Huh7 cells. Remarkably, Huh7 cells with cytoplasmic murine GFP-MuMx1 filaments, but not those with only nuclear bodies, showed antiviral activity towards VSV. Thus, murine GFP-Mx1 nuclear bodies comprised phase-separated condensates. Unexpectedly, GFP-MuMx1 associated with cytoplasmic giantin-based intermediate filaments in a subset of Huh7 cells, and, such cells showed antiviral activity towards VSV.