Zhao Dan, Wu Xudong, Rapoport Tom A
Howard Hughes Medical Institute and Department of Cell Biology, Harvard Medical School, Boston, MA, USA.
Westlake Laboratory of Life Sciences and Biomedicine, Hangzhou, China.
Nat Struct Mol Biol. 2025 Feb 10. doi: 10.1038/s41594-025-01491-y.
Misfolded glycoproteins in the endoplasmic reticulum (ER) lumen are translocated into the cytosol and degraded by the proteasome, a conserved process called ER-associated protein degradation (ERAD). In Saccharomyces cerevisiae, the glycan of these proteins is trimmed by the luminal mannosidase Mnl1 (Htm1) to generate a degradation signal. Interestingly, Mnl1 is associated with protein disulfide isomerase (Pdi1). Here we used cryo-electron microscopy, biochemical and in vivo experiments to elucidate how this complex initiates ERAD. The Mnl1-Pdi1 complex first demannosylates misfolded, globular proteins that are recognized through the C-terminal domain (CTD) of Mnl1; Pdi1 causes the CTD to ignore completely unfolded polypeptides. The disulfides of these globular proteins are then reduced by the Pdi1 component of the complex. Mnl1 blocks the canonical oxidative function of Pdi1, allowing it to function as a disulfide reductase in ERAD. The generated unfolded polypeptides can then be translocated across the membrane into the cytosol.
内质网（ER）腔中错误折叠的糖蛋白会被转运到细胞质中，并由蛋白酶体降解，这一保守过程称为内质网相关蛋白降解（ERAD）。在酿酒酵母中，这些蛋白的聚糖会被内质网腔甘露糖苷酶Mnl1（Htm1）修剪，以产生降解信号。有趣的是，Mnl1与蛋白质二硫键异构酶（Pdi1）相关。在这里，我们使用冷冻电子显微镜、生化和体内实验来阐明该复合物如何启动ERAD。Mnl1-Pdi1复合物首先去除通过Mnl1的C末端结构域（CTD）识别的错误折叠的球状蛋白的甘露糖；Pdi1使CTD忽略完全未折叠的多肽。然后，这些球状蛋白的二硫键被复合物中的Pdi1成分还原。Mnl1阻断Pdi1的典型氧化功能，使其在ERAD中作为二硫键还原酶发挥作用。然后，生成的未折叠多肽可以跨膜转运到细胞质中。
