O-mannosylation protects mutant alpha-factor precursor from endoplasmic reticulum-associated degradation
O-mannosylation protects mutant alpha-factor precursor from endoplasmic reticulum-associated degradation
复制标题
DOI：
10.1091/mbc.12.4.1093
复制
发表时间：
2001-04-01
影响因子：
3.3
通讯作者：
Römisch, K
中科院分区：
文献类型：
--
作者：
Harty, C;Strahl, S;Römisch, K
在内质网 (ER) 中无法折叠的分泌蛋白被运回细胞质并被蛋白酶体降解。目前尚不清楚细胞如何区分折叠中间体和错误折叠蛋白质。我们询问错误折叠的分泌蛋白在输出之前是否在 ER 中进行了共价修饰。我们发现，在微粒体和完整酵母细胞中，一部分突变型 α 因子前体（而非野生型）逐渐被蛋白 O-甘露糖基转移酶 2 (Pmt2p) 进行 O-甘露糖基化。在 ER 输出条件下，即存在 ATP 和胞质溶胶的情况下，O-甘露糖基化在体外显着增加，这需要 ER 膜中具有输出能力的 Sec61p。然而，PMT2 的缺失并没有消除突变α因子前体的降解，而是增强了其在完整酵母细胞中的周转。在体外，O-甘露糖基化突变 α 因子前体是稳定的并受到蛋白酶保护，并且一部分与 ER 腔中的 Sec61p 相关。因此，延长内质网停留时间可以修改错误折叠蛋白中暴露的O-甘露糖基受体位点，从而消除在靶向后阶段从内质网输出的错误折叠蛋白。我们得出的结论是，在错误折叠的蛋白质获得不适当的修饰（可能干扰通过 Sec61 通道的处理）之前，可以将其从 ER 中去除的时间是有限的。
Secretory proteins that fail to fold in the endoplasmic reticulum (ER) are transported back to the cytosol and degraded by proteasomes. It remains unclear how the cell distinguishes between folding intermediates and misfolded proteins. We asked whether misfolded secretory proteins are covalently modified in the ER before export. We found that a fraction of mutant alpha-factor precursor, but not the wild type, was progressively O-mannosylated in microsomes and in intact yeast cells by protein O-mannosyl transferase 2 (Pmt2p). O-Mannosylation increased significantly in vitro under ER export conditions, i.e., in the presence of ATP and cytosol, and this required export-proficient Sec61p in the ER membrane. Deletion of PMT2, however, did not abrogate mutant alpha-factor precursor degradation but, rather, enhanced its turnover in intact yeast cells. In vitro, O-mannosylated mutant alpha-factor precursor was stable and protease protected, and a fraction was associated with Sec61p in the ER lumen. Thus, prolonged ER residence allows modification of exposed O-mannosyl acceptor sites in misfolded proteins, which abrogates misfolded protein export from the ER at a posttargeting stage. We conclude that there is a limited window of time during which misfolded proteins can be removed from the ER before they acquire inappropriate modifications that can interfere with disposal through the Sec61 channel.