| Summary: | The linear ubiquitin chain assembly complex (LUBAC) modulates signalling outcomes of the tumour necrosis factor receptor 1 signalling complex (TNFR1-SC) by placing linear ubiquitin linkages on target proteins in this complex, which in turn leads to recruitment of effector and regulatory molecules. Thereby, LUBAC promotes TNF-induced gene-activatory signalling. Furthermore, LUBAC activity is crucial for the protection from TNF-induced cell death by limiting the formation of the death-inducing cytoplasmic signalling complex II. This thesis demonstrates that HOIP, the enzymatically active subunit of LUBAC, in its function as an adaptor or via its enzymatic activity, orchestrates the recruitment of a multitude of TNFR1-SC components and thereby specifically regulates TNFR1-SC signalling outcome with respect to gene-activatory signalling and cell death. The deubiquitinases CYLD and A20, respectively recruited by LUBAC or linear ubiquitin chains, cooperatively restrict gene activation but exert opposing effects on M1-linked ubiquitin chain stability and, thereby also on cell death. Whilst CYLD-mediated cleavage of M1 chains sensitizes cells to TNF-induced cell death, A20 binding to them prevents their hydrolysis which inhibits cell death. Furthermore, it is shown that the IKK-related kinases TBK1 and IKKε are recruited to the TNFR1-SC in a LUBAC activity-dependent manner. Whilst TBK1 can be recruited via the adaptors TANK or NAP1, IKKε recruitment to the TNFR1-SC solely requires TANK. Importantly, inhibition of the kinase activity of TBK1 and IKKε or abrogation of their recruitment to the TNFR1-SC results in sensitization to TNF-induced RIP1-activity-mediated cell death. Mechanistically, these non-canonical kinases prevent RIP1 from transitioning to complex II and, thereby, from triggering apoptosis or necroptosis. In summary, this PhD thesis elucidates how LUBAC regulates TNF-induced signalling outcomes in terms of gene-activatory signalling and cell death. In doing so, it identifies a previously unrecognised molecular mechanism which facilitates the recruitment and activation of the non-canonical kinases TBK1 and IKKε at the TNFR1-SC.
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