MIND bomb 2 prevents RIPK1 kinase activity-dependent and -independent apoptosis through ubiquitylation of cFLIPL

Mind bomb 2 (MIB2) is an E3 ligase involved in Notch signalling and attenuates TNF-induced apoptosis through ubiquitylation of receptor-interacting protein kinase 1 (RIPK1) and cylindromatosis. Here we show that MIB2 bound and conjugated K48- and K63-linked polyubiquitin chains to a long-form of cellular FLICE-inhibitory protein (cFLIP(L)), a catalytically inactive homologue of caspase 8. Deletion of MIB2 did not impair the TNF-induced complex I formation that mediates NF-kappa B activation but significantly enhanced formation of cytosolic death-inducing signalling complex II. TNF-induced RIPK1 Ser(166) phosphorylation, a hallmark of RIPK1 death-inducing activity, was enhanced in MIB2 knockout cells, as was RIPK1 kinase activity-dependent and -independent apoptosis. Moreover, RIPK1 kinase activity-independent apoptosis was induced in cells expressing cFLIP(L) mutants lacking MIB2-dependent ubiquitylation. Together, these results suggest that MIB2 suppresses both RIPK1 kinase activity-dependent and -independent apoptosis, through suppression of RIPK1 kinase activity and ubiquitylation of cFLIP(L), respectively. Nakabayashi et al find that the E3 ligase MIB2 ubiquitylates the apoptosis-inhibitor cFLIP and that deletion of MIB2 enhances both RIPK1 kinase-dependent and -independent apoptosis through an increase in RIPK1 kinase activity and impairment of ubiquitylation of cFLIP(L), respectively.

Automated summary

The E3 ligase MIB2 ubiquitylates apoptosis-inhibitor cFLIP, suppressing RIPK1 kinase activity-dependent and -independent apoptosis. Deletion of MIB2 enhances RIPK1 kinase activity and impairs ubiquitylation of cFLIP(L), resulting in increased apoptosis.