Characterization of Schizosaccharomyces pombe ER alpha-mannosidase: a reevaluation of the role of the enzyme on ER-associated degradation.
Characterization of Schizosaccharomyces pombe ER alpha-mannosidase: a reevaluation of the role of the enzyme on ER-associated degradation.
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DOI：
10.1091/mbc.e05-03-0246
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发表时间：
2005-10
影响因子：
3.3
通讯作者：
Federico Movsichoff;O. Castro;A. Parodi
中科院分区：
文献类型：
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作者：
Federico Movsichoff;O. Castro;A. Parodi
据推测，通过内质网（ER）α-甘露糖苷酶I产生Man 8 GlcNAc 2异构体B（M8 B）构成了驱动不可修复的错误折叠糖蛋白进行蛋白酶体降解的信号。与以前的报告相反，我们能够在体内（但不是在体外）检测到一个非常微弱的ER α-甘露糖苷酶活性粟酒裂殖酵母。该酶降解Man 9 GlcNAc 2产生M8 B，并被kifunensin抑制。利弗湾粟酒裂殖酵母细胞显示出对错误折叠的糖蛋白中存在的Man 9 GlcNAc 2进行demannosylate的能力极其有限，即使在ER中长时间停留后也是如此。此外，未检测到含M8 B物质的优先降解。然而，α-甘露糖苷酶编码基因的破坏几乎完全阻止了错误折叠的糖蛋白的降解。这和其他相互矛盾的报告可能是最好的解释，假设ER甘露糖苷酶对糖蛋白降解的作用是独立的酶活性。该酶作为凝集素结合不同结构的聚甘露糖聚糖，与其无酶活性的同源物Htm 1 p/Mnl 1 p/EDEM一起属于负责将不可修复的错误折叠的糖蛋白递送至蛋白酶体的转运链。Kifunensin和1-deoxymannojirimycin是甘露糖同系物，可作为ER甘露糖苷酶或/和Htm 1 p/Mnl 1 p/EDEM推定凝集素特性的抑制剂。
It has been postulated that creation of Man8GlcNAc2 isomer B (M8B) by endoplasmic reticulum (ER) alpha-mannosidase I constitutes a signal for driving irreparably misfolded glycoproteins to proteasomal degradation. Contrary to a previous report, we were able to detect in vivo (but not in vitro) an extremely feeble ER alpha-mannosidase activity in Schizosaccharomyces pombe. The enzyme yielded M8B on degradation of Man9GlcNAc2 and was inhibited by kifunensin. Live S. pombe cells showed an extremely limited capacity to demannosylate Man9GlcNAc2 present in misfolded glycoproteins even after a long residence in the ER. In addition, no preferential degradation of M8B-bearing species was detected. Nevertheless, disruption of the alpha-mannosidase encoding gene almost totally prevented degradation of a misfolded glycoprotein. This and other conflicting reports may be best explained by assuming that the role of ER mannosidase on glycoprotein degradation is independent of its enzymatic activity. The enzyme, behaving as a lectin binding polymannose glycans of varied structures, would belong together with its enzymatically inactive homologue Htm1p/Mnl1p/EDEM, to a transport chain responsible for delivering irreparably misfolded glycoproteins to proteasomes. Kifunensin and 1-deoxymannojirimycin, being mannose homologues, would behave as inhibitors of the ER mannosidase or/and Htm1p/Mnl1p/EDEM putative lectin properties.