On the mechanism of sensing unfolded protein in the endoplasmic reticulum
On the mechanism of sensing unfolded protein in the endoplasmic reticulum
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DOI：
10.1073/pnas.0509487102
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发表时间：
2005-12-27
影响因子：
11.1
通讯作者：
Walter, P
中科院分区：
文献类型：
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作者：
Credle, JJ;Finer-Moore, JS;Walter, P
内质网（ER）中未折叠的蛋白质激活内质网跨膜传感器Ire1，从而触发未折叠蛋白反应（UPR），这是一种根据需要调节内质网蛋白质折叠能力的内稳态信号通路。Ire1是一种双功能酶，包含细胞质激酶结构域和核糖核酸酶结构域，人们对它们在Ire1下游信号转导中的作用有了一定的了解。相比之下，其内质网腔结构域（LD）如何感知未折叠蛋白质的问题一直是个谜。LD保守核心区域（cLD）的3.0埃晶体结构以及随后基于结构的功能分析使我们对其反应机制提出了一种设想。cLD呈现出一种独特的蛋白质折叠结构，并且足以控制未折叠蛋白质对Ire1的激活。cLD单体在一个大的界面上二聚化，形成一个由位于β-折叠层上的α-螺旋构成的共享中央凹槽。这个凹槽在整体结构上让人联想到主要组织相容性复合体（MHCs）的肽结合结构域。Ire1中面向凹槽的保守氨基酸侧链被证明对UPR激活很重要，因为它们的突变会降低反应。突变分析表明，cLD二聚体之间需要进一步相互作用以形成UPR激活所必需的高阶寡聚体。我们提出，cLD直接结合未折叠的蛋白质，这改变了膜平面内单体的四级缔合。内质网腔中的这些变化反过来又将细胞质中的Ire1激酶结构域置于最佳位置，以便进行自磷酸化来启动UPR。
Unfolded proteins in the endoplasmic reticulum (ER) activate the ER transmembrane sensor Ire1 to trigger the unfolded protein response (UPR), a homeostatic signaling pathway that adjusts ER protein folding capacity according to need. Ire1 is a bifunctional enzyme, containing cytoplasmic kinase and RNase domains whose roles in signal transduction downstream of Ire1 are understood in some detail. By contrast, the question of how its ER-luminal domain (LD) senses unfolded proteins has remained an enigma. The 3.0-angstrom crystal structure and consequent structure-guided functional analyses of the conserved core region of the LID (cLD) leads us to a proposal for the mechanism of response. cLD exhibits a unique protein fold and is sufficient to control Ire1 activation by unfolded proteins. Dimerization of cLD monomers across a large interface creates a shared central groove formed by alpha-helices that are situated on a beta-sheet floor. This groove is reminiscent of the peptide binding domains of major histocompatibility complexes (MHCs) in its gross architecture. Conserved amino acid side chains in Ire1 that face into the groove are shown to be important for UPR activation in that their mutation reduces the response. Mutational analyses suggest that further interaction between cLD dinners is required to form higher-order oligomers necessary for UPR activation. We propose that cLD directly binds unfolded proteins, which changes the quaternary association of the monomers in the membrane plane. The changes in the ER lumen in turn position Ire1 kinase domains in the cytoplasm optimally for autophosphorylation to initiate the UPR.