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1.  Membrane aberrancy and unfolded proteins activate the endoplasmic reticulum stress sensor Ire1 in different ways 
Molecular Biology of the Cell  2011;22(18):3520-3532.
In contrast to the classical model, in which unfolded proteins accumulated in the endoplasmic reticulum trigger the unfolded-protein response (UPR), we show that membrane aberrancy also evokes this protective cellular event. This finding may explain UPR activation under various physiological conditions.
Eukaryotic cells activate the unfolded-protein response (UPR) upon endoplasmic reticulum (ER) stress, where the stress is assumed to be the accumulation of unfolded proteins in the ER. Consistent with previous in vitro studies of the ER-luminal domain of the mutant UPR initiator Ire1, our study show its association with a model unfolded protein in yeast cells. An Ire1 luminal domain mutation that compromises Ire1's unfolded-protein–associating ability weakens its ability to respond to stress stimuli, likely resulting in the accumulation of unfolded proteins in the ER. In contrast, this mutant was activated like wild-type Ire1 by depletion of the membrane lipid component inositol or by deletion of genes involved in lipid homeostasis. Another Ire1 mutant lacking the authentic luminal domain was up-regulated by inositol depletion as strongly as wild-type Ire1. We therefore conclude that the cytosolic (or transmembrane) domain of Ire1 senses membrane aberrancy, while, as proposed previously, unfolded proteins accumulating in the ER interact with and activate Ire1.
doi:10.1091/mbc.E11-04-0295
PMCID: PMC3172275  PMID: 21775630
2.  Saccharomyces cerevisiae Rot1 Is an Essential Molecular Chaperone in the Endoplasmic Reticulum 
Molecular Biology of the Cell  2008;19(8):3514-3525.
Molecular chaperones prevent aggregation of denatured proteins in vitro and are thought to support folding of diverse proteins in vivo. Chaperones may have some selectivity for their substrate proteins, but knowledge of particular in vivo substrates is still poor. We here show that yeast Rot1, an essential, type-I ER membrane protein functions as a chaperone. Recombinant Rot1 exhibited antiaggregation activity in vitro, which was partly impaired by a temperature-sensitive rot1-2 mutation. In vivo, the rot1-2 mutation caused accelerated degradation of five proteins in the secretory pathway via ER-associated degradation, resulting in a decrease in their cellular levels. Furthermore, we demonstrate a physical and probably transient interaction of Rot1 with four of these proteins. Collectively, these results indicate that Rot1 functions as a chaperone in vivo supporting the folding of those proteins. Their folding also requires BiP, and one of these proteins was simultaneously associated with both Rot1 and BiP, suggesting that they can cooperate to facilitate protein folding. The Rot1-dependent proteins include a soluble, type I and II, and polytopic membrane proteins, and they do not share structural similarities. In addition, their dependency on Rot1 appeared different. We therefore propose that Rot1 is a general chaperone with some substrate specificity.
doi:10.1091/mbc.E07-12-1289
PMCID: PMC2488298  PMID: 18508919
3.  Two regulatory steps of ER-stress sensor Ire1 involving its cluster formation and interaction with unfolded proteins 
The Journal of Cell Biology  2007;179(1):75-86.
Chaperone protein BiP binds to Ire1 and dissociates in response to endoplasmic reticulum (ER) stress. However, it remains unclear how the signal transducer Ire1 senses ER stress and is subsequently activated. The crystal structure of the core stress-sensing region (CSSR) of yeast Ire1 luminal domain led to the controversial suggestion that the molecule can bind to unfolded proteins. We demonstrate that, upon ER stress, Ire1 clusters and actually interacts with unfolded proteins. Ire1 mutations that affect these phenomena reveal that Ire1 is activated via two steps, both of which are ER stress regulated, albeit in different ways. In the first step, BiP dissociation from Ire1 leads to its cluster formation. In the second step, direct interaction of unfolded proteins with the CSSR orients the cytosolic effector domains of clustered Ire1 molecules.
doi:10.1083/jcb.200704166
PMCID: PMC2064738  PMID: 17923530
4.  A role for BiP as an adjustor for the endoplasmic reticulum stress-sensing protein Ire1 
The Journal of Cell Biology  2004;167(3):445-456.
In the unfolded protein response, the type I transmembrane protein Ire1 transmits an endoplasmic reticulum (ER) stress signal to the cytoplasm. We previously reported that under nonstressed conditions, the ER chaperone BiP binds and represses Ire1. It is still unclear how this event contributes to the overall regulation of Ire1. The present Ire1 mutation study shows that the luminal domain possesses two subregions that seem indispensable for activity. The BiP-binding site was assigned not to these subregions, but to a region neighboring the transmembrane domain. Phenotypic comparison of several Ire1 mutants carrying deletions in the indispensable subregions suggests these subregions are responsible for multiple events that are prerequisites for activation of the overall Ire1 proteins. Unexpectedly, deletion of the BiP-binding site rendered Ire1 unaltered in ER stress inducibility, but hypersensitive to ethanol and high temperature. We conclude that in the ER stress-sensory system BiP is not the principal determinant of Ire1 activity, but an adjustor for sensitivity to various stresses.
doi:10.1083/jcb.200405153
PMCID: PMC2172501  PMID: 15520230
5.  Genetic Evidence for a Role of BiP/Kar2 That Regulates Ire1 in Response to Accumulation of Unfolded Proteins 
Molecular Biology of the Cell  2003;14(6):2559-2569.
In the unfolded protein response (UPR) signaling pathway, accumulation of unfolded proteins in the endoplasmic reticulum (ER) activates a transmembrane kinase/ribonuclease Ire1, which causes the transcriptional induction of ER-resident chaperones, including BiP/Kar2. It was previously hypothesized that BiP/Kar2 plays a direct role in the signaling mechanism. In this model, association of BiP/Kar2 with Ire1 represses the UPR pathway while under conditions of ER stress, BiP/Kar2 dissociation leads to activation. To test this model, we analyzed five temperature-sensitive alleles of the yeast KAR2 gene. When cells carrying a mutation in the Kar2 substrate-binding domain were incubated at the restrictive temperature, association of Kar2 to Ire1 was disrupted, and the UPR pathway was activated even in the absence of extrinsic ER stress. Conversely, cells carrying a mutation in the Kar2 ATPase domain, in which Kar2 poorly dissociated from Ire1 even in the presence of tunicamycin, a potent inducer of ER stress, were unable to activate the pathway. Our findings provide strong evidence in support of BiP/Kar2-dependent Ire1 regulation model and suggest that Ire1 associates with Kar2 as a chaperone substrate. We speculate that recognition of unfolded proteins is based on their competition with Ire1 for binding with BiP/Kar2.
doi:10.1091/mbc.E02-11-0708
PMCID: PMC194903  PMID: 12808051

Results 1-5 (5)