Yang Xiaochen, Srivastava Renu, Howell Stephen H, Bassham Diane C
Department of Genetics, Development and Cell Biology, Iowa State University, Ames, IA, 50011, USA.
Interdepartmental Plant Biology Program, Iowa State University, Ames, IA, 50011, USA.
Plant J. 2016 Jan;85(1):83-95. doi: 10.1111/tpj.13091.
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.
内质网应激被定义为未折叠蛋白在内质网中的积累，它由诸如热或引起内质网应激的因子（包括衣霉素和二硫苏糖醇）等条件所引发。自噬是液泡中大分子降解的主要途径，它被这些应激因子以依赖肌醇需求酶1b（IRE1b）的方式激活，并将内质网片段输送到液泡进行降解。在本研究中，我们研究了拟南芥内质网应激期间自噬激活的机制。发现化学伴侣4-苯基丁酸钠和牛磺熊去氧胆酸可减少衣霉素或二硫苏糖醇诱导的自噬，但不能减少由无关应激引起的自噬。同样，编码热休克蛋白70（HSP70）分子伴侣的结合免疫球蛋白蛋白（BIP）的过表达也减少了自噬。还发现热应激激活的自噬部分依赖于IRE1b，并被4-苯基丁酸钠抑制，这表明热诱导的自噬是由于未折叠蛋白在内质网中的积累。在内质网中错误折叠的蛋白模拟物zeolin或羧肽酶Y（CPY*）的突变形式在拟南芥中的表达也以IRE1b依赖的方式诱导自噬。此外，zeolin和CPY*与自噬体标记物GFP-ATG8e部分共定位，表明通过自噬输送到液泡中。我们得出结论，在内质网应激条件下，未折叠蛋白在内质网中的积累是自噬的触发因素。
