Murine Cytomegalovirus Virion-Associated Protein M45 Mediates Rapid NF-κB Activation after Infection

Murine cytomegalovirus (MCMV) rapidly induces activation of nuclear factor kappa B (NF-kappa B) upon infection of host cells. After a transient phase of activation, the MCMV M45 protein blocks all canonical NF-kappa B-activating pathways by inducing the degradation of the gamma subunit of the inhibitor of kappa B kinase complex (IKK gamma; commonly referred to as the NF-kappa B essential modulator [NEMO]). Here we show that the viral M45 protein also mediates rapid NF-kappa B activation immediately after infection. MCMV mutants lacking M45 or expressing C-terminally truncated M45 proteins induced neither NF-kappa B activation nor transcription of NF-kappa B-dependent genes within the first 3 h of infection. Rapid NF-kappa B activation was absent in MCMV-infected NEMO-deficient fibroblasts, indicating that activation occurs at or upstream of the IKK complex. NF-kappa B 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-kappa B 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-kappa B, 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-kappa B activation. IMPORTANCE Transcription factor NF-kappa B 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-kappa B signaling by stimulating or inhibiting the activation of this transcription factor. Cytomegaloviruses, opportunistic pathogens that cause lifelong infections in their hosts, activate NF-kappa B rapidly and transiently upon infection but block NF-kappa B 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-kappa B signaling. It not only blocks NF-kappa B signaling later in infection but also triggers the rapid activation of NF-kappa B 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-kappa B signaling are likely to be found in other viral proteins.