title: Murine cytomegalovirus virion-associated protein M45 mediates rapid NF-κB activation after infection.
abstract: Murine cytomegalovirus (MCMV) rapidly induces activation of nuclear factor κB (NF-κB) upon infection of host cells. After a transient phase of activation, the MCMV M45 protein blocks all canonical NF-κB-activating pathways by inducing the degradation of the gamma subunit of the inhibitor of κB kinase complex (IKKγ; commonly referred to as the NF-κB essential modulator [NEMO]). Here we show that the viral M45 protein also mediates rapid NF-κB activation immediately after infection. MCMV mutants lacking M45 or expressing C-terminally truncated M45 proteins induced neither NF-κB activation nor transcription of NF-κB-dependent genes within the first 3 h of infection. Rapid NF-κB activation was absent in MCMV-infected NEMO-deficient fibroblasts, indicating that activation occurs at or upstream of the IKK complex. NF-κB activation was strongly reduced in murine fibroblasts lacking receptor-interacting protein 1 (RIP1), a known M45-interacting protein, but was restored upon complementation with murine RIP1. However, the ability of M45 to interact with RIP1 and NEMO was not sufficient to induce NF-κB activation upon infection. In addition, incorporation of the M45 protein into virions was required. This was dependent on a C-terminal region of M45, which is not required for interaction with RIP1 and NEMO. We propose a model in which M45 delivered by viral particles activates NF-κB, presumably involving an interaction with RIP1 and NEMO. Later in infection, expression of M45 induces the degradation of NEMO and the shutdown of canonical NF-κB activation.
  
  
  OBJECTIVE
  Transcription factor NF-κB is an important regulator of innate and adaptive immunity. Its activation can be beneficial or detrimental for viral pathogens. Therefore, many viruses interfere with NF-κB signaling by stimulating or inhibiting the activation of this transcription factor. Cytomegaloviruses, opportunistic pathogens that cause lifelong infections in their hosts, activate NF-κB rapidly and transiently upon infection but block NF-κB signaling soon thereafter. Here we report the surprising finding that the murine cytomegalovirus protein M45, a component of viral particles, plays a dual role in NF-κB signaling. It not only blocks NF-κB signaling later in infection but also triggers the rapid activation of NF-κB immediately following virus entry into host cells. Both activation and inhibition involve M45 interaction with the cellular signaling mediators RIP1 and NEMO. Similar dual functions in NF-κB signaling are likely to be found in other viral proteins.
authors.affiliation: Heinrich Pette Institute, Leibniz Institute for Experimental Virology, Hamburg, Germany.
authors.forename: Eva
authors.initials: E
authors.lastname: Krause
authors.affiliation: Department of Medicine, University of Cambridge, Cambridge, United Kingdom.
authors.forename: Miranda
authors.initials: M
authors.lastname: de Graaf
authors.affiliation: Heinrich Pette Institute, Leibniz Institute for Experimental Virology, Hamburg, Germany.
authors.forename: Patricia M
authors.initials: PM
authors.lastname: Fliss
authors.affiliation: Department of Medicine, University of Cambridge, Cambridge, United Kingdom.
authors.forename: Lars
authors.initials: L
authors.lastname: Dölken
authors.affiliation: Heinrich Pette Institute, Leibniz Institute for Experimental Virology, Hamburg, Germany wolfram.brune@hpi.uni-hamburg.de.
authors.forename: Wolfram
authors.initials: W
authors.lastname: Brune
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D020690:GTPase-Activating Proteins / Q000378:metabolism*; D005816:Genetic Complementation Test; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism*; D051379:Mice; D018345:Mice, Knockout; D018146:Muromegalovirus / Q000276:immunology*; D016328:NF-kappa B / Q000378:metabolism*; D025941:Protein Interaction Mapping; D012264:Ribonucleotide Reductases / Q000378:metabolism*; D014764:Viral Proteins / Q000378:metabolism*
chemical_list: D020690:GTPase-Activating Proteins; D047908:Intracellular Signaling Peptides and Proteins; C514981:NEMO protein, mouse; D016328:NF-kappa B; C490620:Ralbp1 protein, mouse; D014764:Viral Proteins; D012264:Ribonucleotide Reductases; C422747:m45 protein, Mouse cytomegalovirus 1