SPATA2 regulates the activation of RIPK1 by modulating linear ubiquitination

Stimulation of cells with TNFα leads to the formation of the TNF-R1 signaling complex (TNF-RSC) to mediate downstream cellular fate decision. Activation of the TNF-RSC is modulated by different types of ubiquitination and may lead to cell death, including apoptosis and necroptosis, in both RIPK1-dependent and RIPK1-independent manners. Spata2 (spermatogenesis-associated 2) is an adaptor protein recruited into the TNF-RSC to modulate the interaction between the linear ubiquitin chain assembly complex (LUBAC) and the deubiquitinase CYLD (cylindromatosis). However, the mechanism by which Spata2 regulates the activation of RIPK1 is unclear. Here, we report that Spata2-deficient cells show resistance to RIPK1-dependent apoptosis and necroptosis and are also partially protected against RIPK1-independent apoptosis. Spata2 deficiency promotes M1 ubiquitination of RIPK1 to inhibit RIPK1 kinase activity. Furthermore, we provide biochemical evidence for the USP domain of CYLD and the PUB domain of the SPATA2 complex preferentially deubiquitinating the M1 ubiquitin chain in vitro. Spata2 deficiency also promotes the activation of MKK4 and JNK and cytokine production independently of RIPK1 kinase activity. Spata2 deficiency sensitizes mice to systemic inflammatory response syndrome (SIRS) induced by TNFα, which can be suppressed by RIPK1 inhibitor Nec-1s. Thus, Spata2 can regulate inflammatory response and cell death in both RIPK1-dependent and RIPK1-independent manners.