Arabidopsis Bax inhibitor-1: A rheostat for ER stress-induced programmed cell death

Haptophyte-infecting viruses change the genome condensing proteins of dinoflagellates

Giant viruses are extraordinary members of the virosphere due to their structural complexity and high diversity in gene content. Haptophytes are ecologically important primary producers in the ocean, and all known viruses that infect haptophytes are giant viruses. However, little is known about the specifics of their infection cycles and the responses they trigger in their host cells. Our in-depth electron microscopic, phylogenomic and virion proteomic analyses of two haptophyte-infecting giant viruses, Haptolina ericina virus RF02 (HeV RF02) and Prymnesium kappa virus RF02 (PkV RF02), unravel their large capacity for host manipulation and arsenals that function during the infection cycle from virus entry to release. The virus infection induces significant morphological changes in the host cell that is manipulated to build a virus proliferation factory. Both viruses' genomes encode a putative nucleoprotein (dinoflagellate/viral nucleoprotein; DVNP), which was also found in the virion proteome of PkV RF02. Phylogenetic analysis suggests that DVNPs are widespread in marine giant metaviromes. Furthermore, the analysis shows that the dinoflagellate homologues were possibly acquired from viruses of the order Imitervirales. These findings enhance our understanding of how viruses impact the biology of microalgae, providing insights into evolutionary biology, ecosystem dynamics, and nutrient cycling in the ocean.

Characterization of the Bax Inhibitor-1 Family in Cauliflower and Functional Analysis of BobBIL4

The Bax inhibitor-1 (BI-1) gene family, which is important for plant growth, development, and stress tolerance, remains largely unexplored in cauliflower. In this study, we identified and characterized cauliflower BI-1 family genes. Based on aligned homologous sequences and collinearity with Arabidopsis genes, we identified nine cauliflower BI-1 genes, which encode proteins that varied in length, molecular weight, isoelectric point, and predicted subcellular localization, including the Golgi apparatus, plasma membrane, and various compartments within the chloroplast. Phylogenetic analyses detected evolutionary conservation and divergence among these genes. Ten structural motifs were identified, with Motif 5 found to be crucial for inhibiting apoptosis. According to the cis-regulatory elements in their promoters, these genes likely influence hormone signaling and stress responses. Expression profiles among tissues highlighted the functional diversity of these genes, with particularly high expression levels observed in the silique and root. Focusing on BobBIL4, we investigated its role in brassinosteroid (BR)-mediated root development and salt stress tolerance. BobBIL4 expression levels increased in response to BR and salt treatments. The functional characterization of this gene in Arabidopsis revealed that it enhances root growth and salinity tolerance. These findings provide insights into BI-1 gene functions in cauliflower while also highlighting the potential utility of BobBIL4 for improving crop stress resistance.

Regulation of plant epigenetic memory in response to cold and heat stress: towards climate resilient agriculture

Plants have evolved to adapt and grow in hot and cold climatic conditions. Some also adapt to daily and seasonal temperature changes. Epigenetic modifications play an important role in regulating plant tolerance under such conditions. DNA methylation and post-translational modifications of histone proteins influence gene expression during plant developmental stages and under stress conditions, including cold and heat stress. While short-term modifications are common, some modifications may persist and result in stress memory that can be inherited by subsequent generations. Understanding the mechanisms of epigenomes responding to stress and the factors that trigger stress memory is crucial for developing climate-resilient agriculture, but such an integrated view is currently limited. This review focuses on the plant epigenetic stress memory during cold and heat stress. It also discusses the potential of machine learning to modify stress memory through epigenetics to develop climate-resilient crops.

The Rice Malectin Regulates Plant Cell Death and Disease Resistance by Participating in Glycoprotein Quality Control

In animals, malectin is well known to play an essential role in endoplasmic reticulum quality control (ERQC) by interacting with ribophorin I, one unit of the oligosaccharyltransferase (OST) complex. However, the functions of malectin in plants remain largely unknown. Here, we demonstrate the rice OsMLD1 is an ER- and Golgi-associated malectin protein and physically interacts with rice homolog of ribophorin I (OsRpn1), and its disruption leads to spontaneous lesion mimic lesions, enhanced disease resistance, and prolonged ER stress. In addition, there are many more N-glycosites and N-glycoproteins identified from the mld1 mutant than wildtype. Furthermore, OsSERK1 and OsSERK2, which have more N-glycosites in mld1, were demonstrated to interact with OsMLD1. OsMLD1 can suppress OsSERK1- or OsSERK2-induced cell death. Thus, OsMLD1 may play a similar role to its mammalian homologs in glycoprotein quality control, thereby regulating cell death and immunity of rice, which uncovers the function of malectin in plants.

Stress response proteins NRP1 and NRP2 are pro-survival factors that inhibit cell death during ER stress

Environmental stresses cause an increased number of unfolded or misfolded proteins to accumulate in the endoplasmic reticulum (ER), resulting in ER stress. To restore ER homeostasis and survive, plants initiate an orchestrated signaling pathway known as the unfolded protein response (UPR). Asparagine-rich protein (NRP) 1 and NRP2, two homologous proteins harboring a Development and Cell Death domain, are associated with various stress responses in Arabidopsis (Arabidopsis thaliana), but the relevant molecular mechanism remains obscure. Here, we show that NRP1 and NRP2 act as key pro-survival factors during the ER stress response and that they inhibit cell death. Loss-of-function of NRP1 and NRP2 results in decreased tolerance to the ER stress inducer tunicamycin (TM), accelerating cell death. NRP2 is constitutively expressed while NRP1 is induced in plants under ER stress. In Arabidopsis, basic leucine zipper protein (bZIP) 28 and bZIP60 are important transcription factors in the UPR that activates the expression of many ER stress-related genes. Notably, under ER stress, bZIP60 activates NRP1 by directly binding to the UPRE-I element in the NRP1 promoter. These findings reveal a pro-survival strategy in plants wherein the bZIP60-NRPs cascade suppresses cell death signal transmission, improving survival under adverse conditions.

Bax inhibitor-1 from orange spotted grouper, Epinephelus coioides involved in viral infection

Bax inhibitor-1 (BI-1) is a conserved anti-apoptotic protein that suppresses endoplasmic reticulum (ER) stress-induced cell death. However, the function of fish BI-1 is not quite clear. In the present study, a bi-1 homolog (Ecbi-1) from orange spotted grouper, Epinephelus coioides was identified and its roles in viral infection were investigated. EcBI-1 encoded 237 amino acids protein, contained six transmembrane regions and a conservative C-terminus motif. Ecbi-1 predominantly expressed in kidney and spleen of healthy grouper. After SGIV stimulation, Ecbi-1 transcript was significantly increased in vitro. Subcellular localization analysis revealed that EcBI-1 was localized throughout the cytoplasm and co-localized with ER. Furthermore, overexpression of EcBI-1 suppressed SGIV infection induced cell death, caspase-3 activity and viral genes transcription. And C-terminus motif was critical for regulation roles of EcBI-1 during SGIV infection. In addition, EcBI-1 could interact with EcBNIP3 in vitro. Together, our data firstly demonstrated that fish BI-1 play important roles in response to viral infection.

The IRE1/bZIP60 Pathway and Bax Inhibitor 1 Suppress Systemic Accumulation of Potyviruses and Potexviruses in Arabidopsis and Nicotiana benthamiana Plants

The inositol requiring enzyme (IRE1) is an endoplasmic reticulum (ER) stress sensor. When activated, it splices the bZIP60 mRNA, producing a truncated transcription factor that upregulates genes involved in the unfolded protein response. Bax inhibitor 1 (BI-1) is another ER stress sensor that regulates cell death in response to environmental assaults. The potyvirus 6K2 and potexvirus TGB3 proteins are known to reside in the ER, serving, respectively, as anchors for the viral replicase and movement protein complex. This study used green fluorescent protein (GFP)-tagged Turnip mosaic virus (TuMV), Plantago asiatica mosaic virus (PlAMV), Potato virus Y (PVY), and Potato virus X (PVX) to determine that the IRE1/bZIP60 pathway and BI-1 machinery are induced early in virus infection in Arabidopsis thaliana, Nicotiana benthamiana, and Solanum tuberosum. Agrodelivery of only the potyvirus 6K2 or TGB3 genes into plant cells activated bZIP60 and BI-1 expression in Arabidopsis thaliana, N. benthamiana, and S. tuberosum. Homozygous ire1a-2, ire1b-4, and ire1a-2/ire1b-4 mutant Arabidopsis plants were inoculated with TuMV-GFP or PlAMV-GFP. PlAMV accumulates to a higher level in ire1a-2 or ire1a-2/ire1b-4 mutant plants than in ire1b-4 or wild-type plants. TuMV-GFP accumulates to a higher level in ire1a-2, ire1b-4, or ire1a-2/ire1b-4 compared with wild-type plants, suggesting that both isoforms contribute to TuMV-GFP infection. Gene silencing was used to knock down bZIP60 and BI-1 expression in N. benthamiana. PVX-GFP and PVY-GFP accumulation was significantly elevated in these silenced plants compared with control plants. This study demonstrates that two ER stress pathways, namely IRE1/bZIP60 and the BI-1 pathway, limit systemic accumulation of potyvirus and potexvirus infection. Silencing BI-1 expression also resulted in systemic necrosis. These data suggest that ER stress-activated pathways, led by IRE1 and BI-1, respond to invading potyvirus and potexviruses to restrict virus infection and enable physiological changes enabling plants to tolerate virus assault.

Activation of autophagy by unfolded proteins during endoplasmic reticulum stress

Endoplasmic reticulum stress is defined as the accumulation of unfolded proteins in the endoplasmic reticulum, and is caused by conditions such as heat or agents that cause endoplasmic reticulum stress, including tunicamycin and dithiothreitol. Autophagy, a major pathway for degradation of macromolecules in the vacuole, is activated by these stress agents in a manner dependent on inositol-requiring enzyme 1b (IRE1b), and delivers endoplasmic reticulum fragments to the vacuole for degradation. In this study, we examined the mechanism for activation of autophagy during endoplasmic reticulum stress in Arabidopsis thaliana. The chemical chaperones sodium 4-phenylbutyrate and tauroursodeoxycholic acid were found to reduce tunicamycin- or dithiothreitol-induced autophagy, but not autophagy caused by unrelated stresses. Similarly, over-expression of BINDING IMMUNOGLOBULIN PROTEIN (BIP), encoding a heat shock protein 70 (HSP70) molecular chaperone, reduced autophagy. Autophagy activated by heat stress was also found to be partially dependent on IRE1b and to be inhibited by sodium 4-phenylbutyrate, suggesting that heat-induced autophagy is due to accumulation of unfolded proteins in the endoplasmic reticulum. Expression in Arabidopsis of the misfolded protein mimics zeolin or a mutated form of carboxypeptidase Y (CPY*) also induced autophagy in an IRE1b-dependent manner. Moreover, zeolin and CPY* partially co-localized with the autophagic body marker GFP-ATG8e, indicating delivery to the vacuole by autophagy. We conclude that accumulation of unfolded proteins in the endoplasmic reticulum is a trigger for autophagy under conditions that cause endoplasmic reticulum stress.

Bcl-2 suppresses activation of VPEs by inhibiting cytosolic Ca²⁺ level with elevated K⁺ efflux in NaCl-induced PCD in rice

Bcl-2 is one of the most important antiapoptotic members in mammals and prevents many forms of apoptosis in a variety of cell types. Our previous study revealed that overexpression of Bcl-2 significantly suppressed H2O2/NaCl-induced programmed cell death via inhibiting the transcriptional activation of OsVPE2 and OsVPE3 in transgenic rice. However, Ca(2+) and K(+) homeostasis of this process remains largely unknown. In the present study, we investigate whether nonselective cation channels (NSCC) blockers affect Bcl-2 function in rice under salt stress and how Bcl-2 affects ion homeostasis in salt stress-induced PCD. The results showed that overexpression of Bcl-2 significantly decreased transient elevations in the cytosolic Ca(2+) levels, inhibited NaCl-induced K(+) efflux but not H(+) efflux across the plasma membrane, and further suppressed the expression levels of OsVPE2 and OsVPE3, leading to the inhibition of salt-induced PCD and increase of tolerance to salt stress in transgenic rice. During the NaCl-induced PCD, the effects of a NSCC blocker La(3+) on ion homeostasis and VPEs expression in wild-type were similar to the effects of Bcl-2 overexpression in transgenic line. However, a synergistic effect of Bcl-2 and La(3+) was not obviously detectable. Our results suggested that Bcl-2 played an important role in suppression of NaCl-induced PCD by disruption of ion homeostasis, providing an insight into the mechanistic study of plant VPEs, cytosolic Ca(2+) level and K(+) efflux.

Nodulin 22, a novel small heat-shock protein of the endoplasmic reticulum, is linked to the unfolded protein response in common bean

The importance of plant small heat shock proteins (sHsp) in multiple cellular processes has been evidenced by their unusual abundance and diversity; however, little is known about their biological role. Here, we characterized the in vitro chaperone activity and subcellular localization of nodulin 22 of Phaseolus vulgaris (PvNod22; common bean) and explored its cellular function through a virus-induced gene silencing-based reverse genetics approach. We established that PvNod22 facilitated the refolding of a model substrate in vitro, suggesting that it acts as a molecular chaperone in the cell. Through microscopy analyses of PvNod22, we determined its localization in the endoplasmic reticulum (ER). Furthermore, we found that silencing of PvNod22 resulted in necrotic lesions in the aerial organs of P. vulgaris plants cultivated under optimal conditions and that downregulation of PvNod22 activated the ER-unfolded protein response (UPR) and cell death. We also established that PvNod22 expression in wild-type bean plants was modulated by abiotic stress but not by chemicals that trigger the UPR, indicating PvNod22 is not under UPR control. Our results suggest that the ability of PvNod22 to suppress protein aggregation contributes to the maintenance of ER homeostasis, thus preventing the induction of cell death via UPR in response to oxidative stress during plant-microbe interactions.

Deletion of the human cytomegalovirus US17 gene increases the ratio of genomes per infectious unit and alters regulation of immune and endoplasmic reticulum stress response genes at early and late times after infection

Human cytomegalovirus (HCMV) employs numerous strategies to combat, subvert, or co-opt host immunity. One evolutionary strategy for this involves capture of a host gene and then its successive duplication and divergence, forming a family of genes, many of which have immunomodulatory activities. The HCMV US12 family consists of 10 tandemly arranged sequence-related genes in the unique short (US) region of the HCMV genome (US12 to US21). Each gene encodes a protein possessing seven predicted transmembrane domains, patches of sequence similarity with cellular G-protein-coupled receptors, and the Bax inhibitor 1 family of antiapoptotic proteins. We show that one member, US17, plays an important role during virion maturation. Microarray analysis of cells infected with a recombinant HCMV isolate with a US17 deletion (the ΔUS17 mutant virus) revealed blunted host innate and interferon responses at early times after infection (12 h postinfection [hpi]), a pattern opposite that previously seen in the absence of the immunomodulatory tegument protein pp65 (pUL83). Although the ΔUS17 mutant virus produced numbers of infectious particles in fibroblasts equal to the numbers produced by the parental virus, it produced >3-fold more genome-containing noninfectious viral particles and delivered increased amounts of pp65 to newly infected cells. These results suggest that US17 has evolved to control virion composition, to elicit an appropriately balanced host immune response. At later time points (96 hpi), ΔUS17 mutant-infected cells displayed aberrant expression of several host endoplasmic reticulum stress response genes and chaperones, some of which are important for the final stages of virion assembly and egress. Our results suggest that US17 modulates host pathways to enable production of virions that elicit an appropriately balanced host immune response.

The effect of translationally controlled tumour protein (TCTP) on programmed cell death in plants

BACKGROUND

Translationally controlled tumour protein (TCTP), a well known protein of the animal kingdom, was shown to be a Ca(2+)-binding protein with important functions in many different cellular processes (e.g. protection against stress and apoptosis, cell growth, cell cycle progression, and microtubule organization). However, only little is known about TCTP in plants. Transcript and protein levels of plant TCTPs were shown to be altered by various stress conditions (e.g. cold, salt, draught, aluminium, and pathogen infection), and Arabidopsis thaliana TCTP (AtTCTP) was described as an important regulator of growth. The aim of this study was to further characterize plant TCTP relating to one of its major functions in animals: the protection against cell death.

RESULTS

We used two different activators of programmed cell death (PCD) in plants: the mammalian pro-apoptotic protein BAX and tunicamycin, an inhibitor of glycosylation and trigger of unfolded protein response (UPR). Over-expression of AtTCTP significantly decreased cell death in tobacco leaf discs in both studies. A (45)Ca overlay assay showed AtTCTP to be a Ca(2+)-binding protein and localization experiments revealed cytosolic distribution of AtTCTP-GFP in Arabidopsis seedlings.

CONCLUSIONS

Our study showed cytoprotective effects of plant TCTP for the first time. Furthermore, we showed the ability of AtTCTP to bind to Ca(2+) and its cytosolic distribution within the cell. If these results are combined, two putative modes of action can be assumed: 1) AtTCTP acts as Ca(2+) sequester, preventing PCD by reducing cytosolic Ca(2+) levels as described for animals. 2) AtTCTP could directly or indirectly interact with other cytosolic or membrane-bound proteins of the cell death machinery, thereby inhibiting cell death progression. As no homologous proteins of the anti-apoptotic machinery of animals were found in plants, and functional homologues still remain to be elucidated, future work will provide more insight.

BAX inhibitor-1 silencing suppresses white spot syndrome virus replication in red swamp crayfish, Procambarus clarkii

BAX inhibitor-1 (BI-1) was originally described as an anti-apoptotic protein in both animal and plant cells. BI-1 overexpression suppresses ER stress-induced apoptosis in animal cells. Inhibition of BI-1 activity could induce the cell death in mammals and plants. However, the function of BI-1 in crustacean immunity was unclear. In this paper, the full-length cDNA of a BI-1 protein in red swamp crayfish, Procambarus clarkii (PcBI-1) was cloned and its expression profiles in normal and infected crayfish were analyzed. The results showed that PcBI-1 was expressed in hemocytes, heart, hepatopancreas, gills, stomach, and intestines of the crayfish and was upregulated after challenged with Vibrio anguillarum and with white spot syndrome virus (WSSV). To determine the function of PcBI-1 in the innate immunity of the crayfish, the RNA interference against PcBI-1 was performed and the results indicated the hemocyte programmed cell death rate was increased significantly and WSSV replication was declined after PcBI-1 knocked down. Altogether, PcBI-1 plays an anti-apoptotic role, wherein high PcBI-1 expression suppresses programmed cell death, which is beneficial for WSSW replication in crayfish.

TGBp3 triggers the unfolded protein response and SKP1-dependent programmed cell death

The Potato virus X (PVX) triple gene block protein 3 (TGBp3), an 8-kDa membrane binding protein, aids virus movement and induces the unfolded protein response (UPR) during PVX infection. TGBp3 was expressed from the Tobacco mosaic virus (TMV) genome (TMV-p3), and we noted the up-regulation of SKP1 and several endoplasmic reticulum (ER)-resident chaperones, including the ER luminal binding protein (BiP), protein disulphide isomerase (PDI), calreticulin (CRT) and calmodulin (CAM). Local lesions were seen on leaves inoculated with TMV-p3, but not TMV or PVX. Such lesions were the result of TGBp3-elicited programmed cell death (PCD), as shown by an increase in reactive oxygen species, DNA fragmentation and induction of SKP1 expression. UPR-related gene expression occurred within 8 h of TMV-p3 inoculation and declined before the onset of PCD. TGBp3-mediated cell death was suppressed in plants that overexpressed BiP, indicating that UPR induction by TGBp3 is a pro-survival mechanism. Anti-apoptotic genes Bcl-xl, CED-9 and Op-IAP were expressed in transgenic plants and suppressed N gene-mediated resistance to TMV, but failed to alleviate TGBp3-induced PCD. However, TGBp3-mediated cell death was reduced in SKP1-silenced Nicotiana benthamiana plants. The combined data suggest that TGBp3 triggers the UPR and elicits PCD in plants.

TGBp3 triggers the unfolded protein response and SKP1-dependent programmed cell death

The Potato virus X (PVX) triple gene block protein 3 (TGBp3), an 8-kDa membrane binding protein, aids virus movement and induces the unfolded protein response (UPR) during PVX infection. TGBp3 was expressed from the Tobacco mosaic virus (TMV) genome (TMV-p3), and we noted the up-regulation of SKP1 and several endoplasmic reticulum (ER)-resident chaperones, including the ER luminal binding protein (BiP), protein disulphide isomerase (PDI), calreticulin (CRT) and calmodulin (CAM). Local lesions were seen on leaves inoculated with TMV-p3, but not TMV or PVX. Such lesions were the result of TGBp3-elicited programmed cell death (PCD), as shown by an increase in reactive oxygen species, DNA fragmentation and induction of SKP1 expression. UPR-related gene expression occurred within 8 h of TMV-p3 inoculation and declined before the onset of PCD. TGBp3-mediated cell death was suppressed in plants that overexpressed BiP, indicating that UPR induction by TGBp3 is a pro-survival mechanism. Anti-apoptotic genes Bcl-xl, CED-9 and Op-IAP were expressed in transgenic plants and suppressed N gene-mediated resistance to TMV, but failed to alleviate TGBp3-induced PCD. However, TGBp3-mediated cell death was reduced in SKP1-silenced Nicotiana benthamiana plants. The combined data suggest that TGBp3 triggers the UPR and elicits PCD in plants.

Arabidopsis sphingolipid fatty acid 2-hydroxylases (AtFAH1 and AtFAH2) are functionally differentiated in fatty acid 2-hydroxylation and stress responses

2-Hydroxy fatty acids (2-HFAs) are predominantly present in sphingolipids and have important physicochemical and physiological functions in eukaryotic cells. Recent studies from our group demonstrated that sphingolipid fatty acid 2-hydroxylase (FAH) is required for the function of Arabidopsis (Arabidopsis thaliana) Bax inhibitor-1 (AtBI-1), which is an endoplasmic reticulum membrane-localized cell death suppressor. However, little is known about the function of two Arabidopsis FAH homologs (AtFAH1 and AtFAH2), and it remains unclear whether 2-HFAs participate in cell death regulation. In this study, we found that both AtFAH1 and AtFAH2 had FAH activity, and the interaction with Arabidopsis cytochrome b₅ was needed for the sufficient activity. 2-HFA analysis of AtFAH1 knockdown lines and atfah2 mutant showed that AtFAH1 mainly 2-hydroxylated very-long-chain fatty acid (VLCFA), whereas AtFAH2 selectively 2-hydroxylated palmitic acid in Arabidopsis. In addition, 2-HFAs were related to resistance to oxidative stress, and AtFAH1 or 2-hydroxy VLCFA showed particularly strong responses to oxidative stress. Furthermore, AtFAH1 interacted with AtBI-1 via cytochrome b₅ more preferentially than AtFAH2. Our results suggest that AtFAH1 and AtFAH2 are functionally different FAHs, and that AtFAH1 or 2-hydroxy VLCFA is a key factor in AtBI-1-mediated cell death suppression.

Yeast Bax inhibitor, Bxi1p, is an ER-localized protein that links the unfolded protein response and programmed cell death in Saccharomyces cerevisiae

Bax inhibitor-1 (BI-1) is an anti-apoptotic gene whose expression is upregulated in a wide range of human cancers. Studies in both mammalian and plant cells suggest that the BI-1 protein resides in the endoplasmic reticulum and is involved in the unfolded protein response (UPR) that is triggered by ER stress. It is thought to act via a mechanism involving altered calcium dynamics. In this paper, we provide evidence that the Saccharomyces cerevisiae protein encoded by the open reading frame, YNL305C, is a bona fide homolog for BI-1. First, we confirm that yeast cells from two different strain backgrounds lacking YNL305C, which we have renamed BXI1, are more sensitive to heat-shock induced cell death than wildtype controls even though they have indistinguishable growth rates at 30°C. They are also more susceptible both to ethanol-induced and to glucose-induced programmed cell death. Significantly, we show that Bxi1p-GFP colocalizes with the ER localized protein Sec63p-RFP. We have also discovered that Δbxi1 cells are not only more sensitive to drugs that induce ER stress, but also have a decreased unfolded protein response as measured with a UPRE-lacZ reporter. Finally, we have discovered that deleting BXI1 diminishes the calcium signaling response in response to the accumulation of unfolded proteins in the ER as measured by a calcineurin-dependent CDRE-lacZ reporter. In toto, our data suggests that the Bxi1p, like its metazoan homologs, is an ER-localized protein that links the unfolded protein response and programmed cell death.

Bifunctional apoptosis regulator (BAR), an endoplasmic reticulum (ER)-associated E3 ubiquitin ligase, modulates BI-1 protein stability and function in ER Stress

Accumulation of misfolded proteins in the endoplasmic reticulum (ER) causes ER stress and activates inositol-requiring protein-1 (IRE1), among other ER-associated signaling proteins of the unfolded protein response (UPR) in mammalian cells. IRE1 signaling becomes attenuated under prolonged ER stress. The mechanisms by which this occurs are not well understood. An ER resident protein, Bax inhibitor-1 (BI-1), interacts with IRE1 and directly inhibits IRE1 activity. However, little is known about regulation of the BI-1 protein. We show here that bifunctional apoptosis regulator (BAR) functions as an ER-associated RING-type E3 ligase, interacts with BI-1, and promotes proteasomal degradation of BI-1. Overexpression of BAR reduced BI-1 protein levels in a RING-dependent manner. Conversely, knockdown of endogenous BAR increased BI-1 protein levels and enhanced inhibition of IRE1 signaling during ER stress. We also found that the levels of endogenous BAR were reduced under prolonged ER stress. Our findings suggest that post-translational regulation of the BI-1 protein by E3 ligase BAR contributes to the dynamic control of IRE1 signaling during ER stress.