Endoplasmic reticulum stress-sensing mechanisms in yeast and mammalian cells
Introduction
It was more than 20 years ago that Kozutsumi et al. proposed the notion of the unfolded protein response (UPR) [1]. According to them, accumulation of unfolded or misfolded proteins in the endoplasmic reticulum (ER) per se transcriptionally induces ER chaperones BiP and GRP94. The UPR was soon accepted as a substantial and unique cellular response, since yeast genetic studies identified cellular factors that are actually involved in the UPR [2, 3, 4]. An ER-located transmembrane protein Ire1 is activated as an endoribonuclease, which causes unconventional cytoplasmic splicing of the HAC1 mRNA (Figure 1a). The spliced version of this mRNA is translated into a transcription factor that induces various genes to alleviate the stress condition.
Eukaryotic cells evoke cellular responses including the UPR, when exposed to ER stress, which roughly speaking, has been considered to be a synonym for ‘accumulation of unfolded proteins in the ER’. Ire1 is conserved among eukaryotes and activated upon ER stress. Mammals carry two Ire1 paralogues, of which IRE1α is ubiquitously expressed and shows the more severe knockout phenotypes [5] (see Figure 1b for IRE1α’s functions). In addition, metazoan PERK is an ER-located transmembrane protein that phosphorylates and inactivates a translation initiation factor eIF2α [3, 4] (see Figure 1c for PERK's functions). Moreover, it is widely known that in mammals, ATF6 is anchored on the ER membrane under non-stress conditions, and upon ER stress, transported to the Golgi apparatus where it is cleaved to yield a soluble transcription factor fragment [3, 4] (Figure 1d). As shown in Figure 1b–d, these ER-stress sensors are activated to produce different transcription factors, which do not always target the same genes, resulting in complexity in the mammalian ER-stress responses.
Because the type-I transmembrane proteins Ire1 and PERK have almost the same-sized ER-luminal domains with partly similar sequences, it may be reasonable to postulate that they share a common mechanism for sensing ER stress. These two proteins have no structural similarity to the type-II transmembrane protein ATF6. This review focuses on our current understanding about how ER stress activates these ER-stress sensors.
Section snippets
Negative regulation of the ER-stress sensors by BiP
Overexpression of BiP attenuates the UPR [6], implying that a factor promoting the UPR is negatively regulated by BiP. Indeed, Bertolotti et al. and Okamura et al. independently reported physical association of BiP with Ire1 and PERK, together with its ER stress-dependent dissociation [7, 8]. A negative regulatory role of this BiP association is supported by the finding that some yeast BiP-mutant strains exhibit impairment both of Ire1 activation and of BiP dissociation from Ire1 [9, 10]. Shen
Direct interaction of unfolded proteins as another important event for upregulation of yeast Ire1
Based on the results from a serial deletion scanning approach, Kimata et al. proposed that the luminal domain of yeast Ire1 is divided into five subregions [10] (Figure 2a). Notably, consistent observations were obtained from partial-proteolysis and X-ray crystallographic analyses of recombinant fragments of the luminal domain [15, 16••]. As presented in Figure 2b, it is thus highly likely that Subregions I and V are loosely folded, while Subregions II and IV form one tightly folded domain,
Similarities and differences between the luminal domains of IRE1α, PREK and yeast Ire1
Mammalian IRE1α and PERK, as well as yeast Ire1, carry the BiP-binding sites on the juxtamembrane regions, within which deletions compromise association with BiP [21, 22] (Figure 4). Oikawa et al. proposed that the interaction with BiP is the principal determinant of IRE1α’s activity, since unlike the case of yeast Ire1, an IRE1α mutant with which BiP only weakly associates was considerably activated even under non-stress conditions [21]. According to the X-ray crystal structure reported by
Other factors activating the ER-stress sensors
The transmembrane or cytosolic domains of the ER-stress sensors may also contribute to stress sensing and modulation of these molecules. Yeast Ire1 is still upregulated by ER stress, even when its luminal domain is replaced by a non-related peptide domain forming a leucine zipper-based dimer [24]. Interestingly, the cytosolic domain of yeast Ire1 has a ligand-binding pocket into which the flavonol quercetin is specifically incorporated for activation of this sensor [25•]. Although the
Fidelity and complexity on activation of the ER-stress sensors may be explained by the multiplicity of regulatory mechanisms
Although the direct interaction of unfolded proteins with the CSSR seems to be the principal determinant for activation of yeast Ire1 [16••, 17••], the regulation by BiP is actually important for fidelity of Ire1's response, which means that Ire1 is activated only by ER stress. ΔV Ire1, to which BiP does not bind, was activated by stress stimuli other than ER stress [10]. Moreover, Pincus et al. presented experimental data and a mathematical model which claim that BiP association prevents
Conclusions
In a context where unfolded proteins directly interact with and upregulate at least yeast Ire1, the classical notion that unfolded or misfolded proteins per se trigger the UPR is likely correct. Meanwhile, the negative regulation of the ER-stress sensors by the ER chaperone BiP may not support this notion in a straightforward manner, since the molecular mechanism facilitating BiP dissociation from the ER-stress sensors is still unclear. Moreover, some stress stimuli are likely to activate the
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
We thank Dr. Takao Iwawaki and Dr. Daisuke Oikawa, RIKEN Japan, for valuable discussions. This work is supported by KAKENHI (grant numbers 22657030 and 21112516 to YK, and 19058010 and 20380062 to KK) from MEXT or JSPS.
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