The ORF6 protein of the SARS-CoV-2 virus plays a crucial role in blocking the innate immune response of the infected cells by inhibiting interferon pathways. Additionally, it binds to and immobilises the RAE1 protein on the cytoplasmic membranes, thereby blocking mRNA transport from the nucleus to the cytoplasm. In all these cases, the host cell proteins are tethered by the flexible C-terminus of ORF6. A possible strategy to inhibit the biological activity of ORF6 is to bind its C-terminus with suitable ligands. Our in silico experiments suggest that hIFNγ binds the ORF6 protein with high affinity, thus impairing its interactions with RAE1 and, consequently, its activity in viral invasion. The in vitro studies reported here reveal a shift of the localisation of RAE1 in ORF6 overexpressing cells upon treatment with hIFNγ from predominantly cytoplasmic to mainly nuclear, resulting in the restoration of the export of mRNA from the nucleus. We also explored the expression of GFP in transfected-with-ORF6 cells by means of fluorescence microscopy and qRT-PCR, finding that treatment with hIFNγ unblocks the mRNA trafficking and reinstates the GFP expression level. The ability of the cytokine to block ORF6 is also reflected in minimising its negative effects on DNA replication by reducing accumulated RNA-DNA hybrids. Our results, therefore, suggest hIFNγ as a promising inhibitor of the most toxic SARS-CoV-2 protein.
Keywords: COVID-19; R-loops; RAE1; SARS-CoV-2 ORF6; computational modelling; human interferon-γ; immunofluorescence; innate immune response; mRNA transportation.