The Role Of Phosphatase Holoenzyme PP1/GADD34and Raf Kinase Inhibitor Protein RKIP In Innate Immunity

The molecular mechanisms that fine tune TLRs responses need to be fully elucidated. Protein phosphatase-1(PP1) has been shown to be important in cell death and differentiation. However, the roles of PP1in TLRs-triggered immune response remain unclear. In this study, we demonstrate that PP1inhibits the activation of the MAPK and NF-κB pathway and the production of TNF-a, IL-6in macrophages triggered by TLRs3, TLRs4, and TLRs9in a phosphatase-dependent manner. We further demonstrate that PP1negatively regulates TLRs-triggered signaling by targeting TGF-(3-activated kinase1(TAKl) serine412(Ser412) phosphorylation, which is required for activation of TAK1-mediated IL-1R and TLRs signaling. Mutation of TAK1Serine412to alanine (S412A) significantly inhibits TLRs/IL-1R-triggered NF-κB and MAPK activation and induction of proinflammatory cytokines in macrophage. DNA damage-inducible protein34(GADD34) specifies PP1to dephosphorylate TAKl at Ser412. GADD34depletion abolished the interaction between TAK1and PP1, and it relieved PP1overexpression-induced inhibition of TLRs signaling and proinflammatory cytokine production. Therefore, PP1, as a physiologic inhibitor, together with its regulatory subunit GADD34, tightly controls TLRs-induced TAK1Ser412phosphorylation, preventing excessive activation of TLRs and protecting the host from overwhelmed inflammatory immune responses. TANK-binding kinase1(TBK1) activation is a central event in type I interferon production in anti-virus innate immunity. However, the regulatory mechanism underlying TBK1activation remains unclear. Here we report that raf kinase inhibitory protein (RKIP) is essential for TBK1activation and type I interferon production triggered by virus infection. Upon virus infection, RKIP is phosphorylated at serine109by TBKl. Phosphorylation of RKIP enhances its interaction with TBKl and in turn promotes TBK1autophosphorylation. Mutation of RKIP serine109to Alanine abrogates the interaction between RKIP and TBK1, and the anti-viral function of RKIP. RKIP deficiency inhibits intracellular double-stranded RNA-or DNA-induced type I interferon production. Consistently, RKIP deficiency renders the mice more susceptible to vesicular stomatitis virus (VSV) infection. This study reveals a previously unrecognized positive feedback loop between RKIP and TBK1that is essential for type I interferon production in antiviral innate immunity.