Inadequate BiP availability defines endoplasmic reticulum stress

Abstract

How endoplasmic reticulum (ER) stress leads to cytotoxicity is ill-defined. Previously we showed that HeLa cells readjust homeostasis upon proteostatically driven ER stress, triggered by inducible bulk expression of secretory immunoglobulin M heavy chain (μs) thanks to the unfolded protein response (UPR; Bakunts et al., 2017). Here we show that conditions that prevent that an excess of the ER resident chaperone (and UPR target gene) BiP over µs is restored lead to µs-driven proteotoxicity, i.e. abrogation of HRD1-mediated ER-associated degradation (ERAD), or of the UPR, in particular the ATF6α branch. Such conditions are tolerated instead upon removal of the BiP-sequestering first constant domain (CH1) from µs. Thus, our data define proteostatic ER stress to be a specific consequence of inadequate BiP availability, which both the UPR and ERAD redeem.

Interaction mapping of endoplasmic reticulum ubiquitin ligases identifies modulators of innate immune signalling

“…Moreover, ER homeostatic maintenance did not require any individual ER-E3 since siRNA-mediated knockdowns of endogenous isoforms were not sufficient to induce the splicing of XBP1 ( Figure 3—figure supplement 1A ), consistent with CRISPRi screens for ER stress induction ( Adamson et al, 2016 ). Taken together, these findings are consistent with the unique positioning of Hrd1 among E3s to resolve proteotoxic ER stress ( Vitale et al, 2019 ).…”

“…Moreover, there were only 24 HCIPs among known UPR target genes ( Bergmann et al, 2018 ) with at least a quarter belonging to the Hrd1 complex ( Supplementary file 1 , Table 7, Christianson et al, 2012 ). Thus, evolutionary expansion of E3s in the mammalian ER has not extensively supplemented adaptive stress-resolving capabilities functionally redundant with Hrd1, consistent with ubiquitination by the Hrd1 complex being essential for survival under proteotoxic ER stress conditions ( Yagishita et al, 2005 ; Vitale et al, 2019 ). With misfolding of secreted rather than membrane proteins being the principal instigators of ER stress, an exclusivity of access to lumenal substrates via SEL1L offers an explanation why Hrd1 appears indispensable.…”

Interaction mapping of endoplasmic reticulum ubiquitin ligases identifies modulators of innate immune signalling

“…3a), consistent with findings from CRISPRi screens for ER stress induction 59 . Taken together, these findings are consistent with unique positioning of Hrd1 among E3s to resolve proteotoxic ER stress 60 . 10…”

“…Moreover, there were only 24 HCIPs among known 20 UPR target genes 58 with at least a quarter belonging to the Hrd1 complex (Extended Data 24 ). Thus, evolutionary expansion of E3s in the mammalian ER has not extensively supplemented adaptive stress resolving capabilities functionally redundant with Hrd1, consistent with ubiquitination by the Hrd1 complex being essential for survival under proteotoxic ER stress conditions 60,118 . With misfolding of secreted rather than membrane 25 proteins being the principal instigators of ER stress, an exclusivity of access to lumenal substrates via SEL1L offers an explanation why Hrd1 appears indispensable.…”

Proteotoxic Stress and Cell Death in Cancer Cells

“…Clearly, the availability of a pool of chaperones is a critical condition. Particularly, for ER stress, the ER resident chaperone BIP is a key factor during the switch from proteostasis to proteotoxicity [174]. HSF1 is the master regulator of chaperone expression in response to proteotoxic stress.…”