title: The SARS coronavirus papain like protease can inhibit IRF3 at a post activation step that requires deubiquitination activity.
abstract: BACKGROUND
  The outcome of a viral infection is regulated by complex interactions of viral and host factors. SARS coronavirus (SARS-CoV) engages and regulates several innate immune response pathways during infection. We have previously shown that the SARS-CoV Papain-like Protease (PLpro) inhibits type I interferon (IFN) by inhibiting IRF3 phosphorylation thereby blocking downstream Interferon induction. This finding prompted us to identify other potential mechanisms of inhibition of PLpro on IFN induction.
  
  
  METHODS
  We have used plasmids expressing PLpro and IRF3 including an IRF3 mutant that is constitutively active, called IRF3(5D). In these experiments we utilize transfections, chromatin immunoprecipitation, Electro-mobility Shift Assays (EMSA) and protein localization to identify where IRF3 and IRF3(5D) are inhibited by PLpro.
  
  
  RESULTS
  Here we show that PLpro also inhibits IRF3 activation at a step after phosphorylation and that this inhibition is dependent on the de-ubiquitination (DUB) activity of PLpro. We found that PLpro is able to block the type I IFN induction of a constitutively active IRF3, but does not inhibit IRF3 dimerization, nuclear localization or DNA binding. However, inhibition of PLpro's DUB activity by mutagenesis blocked the IRF3 inhibition activity of PLpro, suggesting a role for IRF3 ubiquitination in induction of a type I IFN innate immune response.
  
  
  CONCLUSIONS
  These results demonstrate an additional mechanism that PLpro is able to inhibit IRF3 signaling. These data suggest novel innate immune antagonism activities of PLpro that may contribute to SARS-CoV pathogenesis.
authors.affiliation: Department of Microbiology and Immunology, University of Maryland at Baltimore, 685 West Baltimore St., Room 380, Baltimore, MD, 21201, USA. kmatthews@som.umaryland.edu.
authors.forename: Krystal
authors.initials: K
authors.lastname: Matthews
authors.affiliation: Department of Epidemiology, University of North Carolina, 3304 Michael Hooker Research Building, Chapel Hill, NC, 27599, USA. aschaefe@email.unc.edu.
authors.forename: Alexandra
authors.initials: A
authors.lastname: Schäfer
authors.affiliation: Department of Microbiology, Icahn School of Medicine at Mt. Sinai, New York, NY, 10029, USA. alissapham@gmail.com.|Current Address: NYU Langone Medical Center, Department of Pathology, 538 Medical Science Building, New York, NY, 10016, USA. alissapham@gmail.com.
authors.forename: Alissa
authors.initials: A
authors.lastname: Pham
authors.affiliation: Department of Microbiology and Immunology, University of Maryland at Baltimore, 685 West Baltimore St., Room 380, Baltimore, MD, 21201, USA. mfrieman@som.umaryland.edu.
authors.forename: Matthew
authors.initials: M
authors.lastname: Frieman
mesh_terms: D002460:Cell Line; D047369:Chromatin Immunoprecipitation; D000086782:Coronavirus 3C Proteases; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D024202:Electrophoretic Mobility Shift Assay; D006801:Humans; D050838:Interferon Regulatory Factor-3 / Q000037:antagonists & inhibitors*; D045473:Severe acute respiratory syndrome-related coronavirus / Q000201:enzymology*; D014162:Transfection; D025801:Ubiquitin / Q000378:metabolism; D014764:Viral Proteins / Q000378:metabolism*
chemical_list: C494232:IRF3 protein, human; D050838:Interferon Regulatory Factor-3; D025801:Ubiquitin; D014764:Viral Proteins; D003546:Cysteine Endopeptidases; D000086782:Coronavirus 3C Proteases