STUB1 (CHIP) is a cochaperone E3 ligase containing three tandem repeats of tetratricopeptide (TPR) motifs and a C-terminal U-box domain separated by a charged coiled-coil region (Paul I & Ghosh MK 2014). STUB1 functions as a negative co-chaperone for the HSP90/HSP70 chaperone to regulate protein quality control by targeting unfolded or misfolded proteins for proteasomal degradation. STUB1 (CHIP) also targets many mature proteins for ubiquitination and degradation or degradation-independent regulation (Paul I & Ghosh MK 2014). Structural studies suggest that STUB1 functions as homodimer (Zhang M et al. 2005)

Receptor-interacting serine/threonine protein kinase 3 (RIPK3) functions as a key regulator of necroptosis. STUB1, as an E3 ligase, mediates ubiquitylation of RIPK3 at Lys55 and Lys363 and targets it to lysosomal degradation in human non-small cell lung carcinoma (H1299) cells (Seo J et al. 2016). Treatment with geldanamycin (an inhibitor of HSP90) induced the degradation of RIPK3 in mouse fibroblasts L929 cells even under STUB1-depleted conditions, suggesting that HSP90 might not be involved in the STUB1-mediated degradation of RIPK3 (Seo J et al. 2016). Further, RIP3 kinase activity was not required for its interaction with STUB1 in human embryonic kidney 293 (HEK293) cells, suggesting that STUB1-mediated regulation is independent of RIPK3 phosphorylation status (Choi SW et al. 2018). Moreover, Chip(-/-) mouse embryonic fibroblasts, CHIP-depleted L929 and human colorectal adenocarcinoma (HT-29) cells exhibited higher levels of RIPK3 expression, resulting in increased sensitivity to necroptosis induced by TNFα (Seo J et al. 2016). Supporting these findings, in vivo studies demonstrated that the inflammatory and lethal phenotypes of Chip−/− mice were rescued by crossing with Ripk3 knockout mice (Seo J et al. 2016). The ubiquitin E3 ligase function of STUB1 was also essential for the degradation of RIPK3 in mouse neuroblastoma N2a cell line. Ansiomycin, an inhibitor of protein synthesis, attenuated necroptosis by upregulating STUB1 in oxygen-glucose deprivation (OGD)-challenged N2a cells and primary cultured mouse hippocampal neurons (Tang MB et al. 2018). These data suggest that STUB1 (CHIP) can negatively regulate necroptosis by ubiquitylation-mediated degradation of RIPK3.