Roles of O-mannosylation of aberrant proteins in reduction of the load for endoplasmic reticulum chaperones in yeast
Roles of O-mannosylation of aberrant proteins in reduction of the load for endoplasmic reticulum chaperones in yeast
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DOI：
10.1074/jbc.m403234200
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发表时间：
2004-11-26
影响因子：
4.8
通讯作者：
Endo, T
中科院分区：
文献类型：
--
作者：
Nakatsukasa, K;Okada, S;Endo, T
内质网(ER)中的蛋白质质量控制系统确保只有正确折叠的蛋白质才能在细胞中部署。当非本原蛋白在内质网中积累时，未折叠的蛋白质反应被触发，以限制非本原蛋白的进一步积累，并通过内质网相关降解(ERAD)清除积累的非本原蛋白。在酵母ER中，异常的非固有蛋白主要针对ERAD，但其中明显的一部分转而接受O-甘露糖化。为了验证O-甘露糖化作用是否也是内质网中异常蛋白的加工机制，我们在体内外分析了O-甘露糖化作用对两种模型异常蛋白的影响，这两种蛋白是一系列前α因子糖基化位点突变体和一个前区域缺失的尼氏根霉天冬氨酸蛋白酶-I衍生物(Delapro)。O-甘露糖基化增加了异常蛋白质的溶解度，使它们不那么依赖内质网伴侣蛋白Bip来溶解。从内质网伴侣蛋白的释放使异常蛋白退出内质网，进行正常的分泌途径运输。当负责这些异常蛋白O-甘露糖化的Pmt2p基因和ERAD基因同时缺失时，细胞表现出增强的未折叠蛋白反应。因此，O-甘露糖化可能通过溶解ERAD途径溢出的异常蛋白和减少ER伴侣的负荷来发挥ERAD的故障保护机制。
The protein quality control system in the endoplasmic reticulum (ER) ensures that only properly folded proteins are deployed throughout the cells. When nonnative proteins accumulate in the ER, the unfolded protein response is triggered to limit further accumulation of nonnative proteins and the ER is cleared of accumulated nonnative proteins by the ER-associated degradation (ERAD). In the yeast ER, aberrant nonnative proteins are mainly directed for the ERAD, but a distinct fraction of them instead receive O-mannosylation. In order to test whether O-mannosylation might also be a mechanism to process aberrant proteins in the ER, here we analyzed the effect of O-mannosylation on two kinds of model aberrant proteins, a series of N-glycosylation site mutants of prepro-alpha-factor and a pro-region-deleted derivative of Rhizopus niveus aspartic proteinase-I (Deltapro) both in vitro and in vivo. O-Mannosylation increases solubilities of the aberrant proteins and renders them less dependent on the ER chaperone, BiP, for being soluble. The release from ER chaperones allows the aberrant proteins to exit out of the ER for the normal secretory pathway transport. When the gene for Pmt2p, responsible for the O-mannosylation of these aberrant proteins, and that for the ERAD were simultaneously deleted, the cell exhibited enhanced unfolded protein response. O-Mannosylation may therefore function as a fail-safe mechanism for the ERAD by solubilizing the aberrant proteins that overflowed from the ERAD pathway and reducing the load for ER chaperones.