Biomolecular Condensates of Interferon-Inducible Human and Murine Antiviral Mx Proteins

Type I (IFN-α/β) and Type III (IFN-λ) interferon-inducible myxovirus resistance (Mx) proteins have long been recognized to inhibit a broad spectrum of RNA- and DNA-containing viruses (1-5). Mx proteins are dynamin-family GTPases of size 60-70 kDa (1-3) with different subcellular localization and antiviral activities. In the last 5 years there has been a paradigm shift in our understanding of the structures formed by human and murine Mx proteins in the cytoplasm and nucleus. We now recognize that the major human MxA protein forms structures in the cytoplasm and the major murine Mx1 forms structure in the nucleus which comprise liquid-liquid phase separated (LLPS) biomolecular condensates [membraneless organelles (MLOs)] with a gel-like internal consistency. The second human Mx protein, MxB, which had been previously observed to associate with the cytoplasmic face of nuclear pores, was also recently confirmed to represent cytoplasmic biomolecular condensates. Our overall focus now is to understand the formation, dynamics and antiviral function of biomolecular condensates of different Mx proteins in the cytoplasm and nucleus in cells of different species. We highlight the metastability and rapidly reversible hypotonicity sensing properties of MxA condensates in human cells, especially as applicable to environmental stress inflicted on the oral mucosa.