Movsichoff Federico, Castro Olga A, Parodi Armando J
Laboratory of Glycobiology, Fundación Instituto Leloir, C1405BWE Buenos Aires, Argentina.
Mol Biol Cell. 2005 Oct;16(10):4714-24. doi: 10.1091/mbc.e05-03-0246. Epub 2005 Aug 3.
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.
据推测，内质网（ER）α-甘露糖苷酶I产生的Man8GlcNAc2异构体B（M8B）构成了一种信号，可驱使无法修复的错误折叠糖蛋白进行蛋白酶体降解。与之前的报道相反，我们能够在体内（而非体外）检测到粟酒裂殖酵母中极其微弱的ER α-甘露糖苷酶活性。该酶在降解Man9GlcNAc2时产生M8B，并被基夫内新抑制。即使在错误折叠的糖蛋白在内质网中长时间停留后，活的粟酒裂殖酵母细胞对其所含Man9GlcNAc2进行去甘露糖基化的能力也极其有限。此外，未检测到带有M8B的糖蛋白有优先降解现象。然而，编码α-甘露糖苷酶的基因被破坏几乎完全阻止了错误折叠糖蛋白的降解。通过假设内质网甘露糖苷酶在糖蛋白降解中的作用与其酶活性无关，或许可以最好地解释这一现象以及其他相互矛盾的报道。该酶表现为一种凝集素，可结合多种结构的多聚甘露糖聚糖，它与其无酶活性的同源物Htm1p/Mnl1p/EDEM一起，可能属于一个负责将无法修复的错误折叠糖蛋白递送至蛋白酶体的转运链。基夫内新和1-脱氧甘露基野尻霉素作为甘露糖类似物，可能会作为内质网甘露糖苷酶或/和Htm1p/Mnl1p/EDEM假定凝集素特性的抑制剂。
