The cellular response to protein misfolding in the endoplasmic reticulum.
The cellular response to protein misfolding in the endoplasmic reticulum.
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发表时间：
1999
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通讯作者：
A. Welihinda;W. Tirasophon;R. Kaufman
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作者：
A. Welihinda;W. Tirasophon;R. Kaufman
在真核细胞中，未折叠蛋白质在内质网（ER）腔中的积累导致应激反应。细胞通过上调ER驻留蛋白伴侣的合成来响应ER应激，从而增加该细胞器中的折叠能力。此外，这种反应还激活诱导程序性细胞死亡的途径。应激诱导的伴侣蛋白合成在转录水平上受到调控。在酿酒酵母中，具有丝氨酸/苏氨酸激酶和位点特异性核糖核酸内切酶活性的跨膜蛋白Ire 1 p被认为是ER中未折叠蛋白的传感器，其传递来自ER的信号以激活核中的转录。未折叠蛋白反应（UPR）途径的激活也需要bZIP转录因子Haclp。虽然HACl是组成型转录的，但mRNA的翻译很差。在未折叠蛋白积累后，Ire 1 p产生新的加工形式的HACl mRNA，其通过去除252个碱基序列而有效翻译。使用酵母相互作用陷阱系统，我们确定了额外的组件的普遍定期审议。酵母转录辅激活因子复合物Gcn 5 p/Ada由Gcn 5 p、Ada 2 p、Ada 3 p和Ada 5 p组成，与Ire 1 p和Hac 1 p相互作用。GCN 5、ADA 2和/或ADA 3的缺失减少了响应于ER中错误折叠蛋白的转录诱导，并且ADA 5的缺失完全消除了响应于ER中错误折叠蛋白的转录诱导。蛋白磷酸酶Ptc 2 p也被鉴定为UPR的负调节剂，其直接与活化的Ire 1 p相互作用并使其去磷酸化。最近，Ire 1 p的两个哺乳动物同源物IRE 1和IRE 2被鉴定。hIre 1 p优先定位于核膜，并需要功能性核酸酶活性来传递UPR。这些结果表明，UPR的一些特征从酵母到人类都是保守的，并且可能由一种多组分复合物组成，该复合物受磷酸化状态调节，并与核被相关，以调节包括转录诱导和mRNA加工在内的过程。我们认为Ire 1 p的激活诱导了HAC 1 mRNA的剪接，并将Gcn 5/Ada/Hac 1蛋白复合物结合并靶向于响应于ER中未折叠蛋白而转录激活的基因。通过靶向酵母中的组蛋白乙酰化酶Gcn 5 p促进转录激活，以促进编码ER应激反应基因的染色质处的组蛋白乙酰化。此外，Ire 1 p的激活导致脂质生物合成增加，从而允许ER扩张以适应增加的内腔成分。在更严重的应激条件下，细胞激活通过诱导GADD 153/CHOP介导的Ire 1 p依赖性死亡途径。
In eukaryotic cells, accumulation of unfolded proteins in the lumen of the endoplasmic reticulum (ER) leads to a stress response. Cells respond to ER stress by upregulating the synthesis of ER resident protein chaperones, thus increasing the folding capacity in this organelle. In addition, this response also activates pathways to induce programmed cell death. The stress-induced chaperone synthesis is regulated at the level of transcription. In Saccharomyces cerevisiae, the transmembrane protein, Ire1p, with both serine/threonine kinase and site-specific endoribonuclease activities is implicated as the sensor of unfolded proteins in the ER that transmits the signal from the ER to activate transcription in the nucleus. Activation of the unfolded protein response (UPR) pathway also requires the bZIP transcription factor, Haclp. Although HACl is transcribed constitutively, the mRNA is poorly translated. Upon accumulation of unfolded proteins, Ire1p generates a new processed form of HACl mRNA that is efficiently translated by removal of a 252 base sequence. Using the yeast-interaction trap system we identified additional components of the UPR. A yeast transcriptional coactivator complex, Gcn5p/Ada, which is composed of Gcn5p, Ada2p, Ada3p, and Ada5p, was identified that interacts with Ire1p and Hac1p. Deletion of GCN5, ADA2, and/or ADA3 reduces, and deletion of ADA5 completely abrogates, the transcriptional induction in response to misfolded protein in the ER. A protein phosphatase, Ptc2p, was also identified as a negative regulator of the UPR that directly interacts with and dephosphorylates activated Ire1p. Recently, two mammalian homologues of Ire1p, IRE1 and IRE2, were identified. hIre1p, is preferentially localized to the nuclear envelope and requires a functional nuclease activity to transmit the UPR. These results indicate that some features of the UPR are conserved from yeast to humans and may be composed of a multicomponent complex that is regulated by phosphorylation status and is associated with the nuclear envelope to regulate processes including transcriptional induction and mRNA processing. We propose that activation of Ire1p induces splicing of HACl mRNA as well as engages and targets the Gcn5/Ada/Hac1 protein complex to genes that are transcriptionally activated in response to unfolded protein in the ER. The transcriptional activation is facilitated by targeting the histone acetylase, Gcn5p in yeast, to promote histone acetylation at chromatin encoding ER stress-responsive genes. In addition, activation of Ire1p leads to increased lipid biosynthesis, thereby allowing ER expansion to accommodate increasing lumenal constituents. Under conditions of more severe stress, cells activate an Ire1p-dependent death pathway that is mediated through induction of GADD153/CHOP.