PP6 negatively modulates LUBAC-mediated M1-ubiquitination of RIPK1 and c-FLIPL to promote TNFα-mediated cell death

Activation of TNFR1 by TNFα induces the formation of a membrane-associated, intracellular complex termed complex I. Complex I
orchestrates a complex pattern of modifications on key regulators of TNF signaling that collectively determines the cell fate by
activating pro-survival or executing cell death programs. However, the regulatory mechanism of complex I in cell-fate decision is
not fully understood. Here we identify protein phosphatase-6 (PP6) as a previously unidentified component of complex I. Loss of
PP6 protects cells from TNFα-mediated cell death. The role of PP6 in regulating cell death requires its phosphatase activity and
regulatory subunits. Further mechanistic studies show that PP6 modulates LUBAC-mediated M1-ubiquitination of RIPK1 and c-FLIPL
to promote RIPK1 activation and c-FLIPL degradation. We also show that melanoma-associated PP6 inactivating mutants offer
resistance to cell death due to the loss of sensitivity to TNFα. Thus, our study provides a potential mechanism by which melanomarelated
PP6 inactivating mutations promote cancer progression.