Department of Biochemistry, Oxford Glycobiology Institute, University of Oxford, UK.
Glycobiology. 2012 Oct;22(10):1282-8. doi: 10.1093/glycob/cws088. Epub 2012 May 28.
Removal of α-glucose residues from nascent glycoproteins in the early secretory pathway is a requirement for further N-glycan maturation. Although deglucosylation is a stepwise process mediated by endoplasmic reticulum-associated glucosidases I and II for most glycoproteins, Golgi endo-α-mannosidase provides a backup mechanism for glycoprotein deglucosylation. Although conserved in mammals, in certain cell lines, endomannosidase activity in vitro appears to differ from its activity in cells following glucosidase inhibition. Here, we show that in bovine cells this is explained by restricted substrate specificity allowing processing of Glc(1)Man(7)GlcNAc(1/2) and Glc(1)Man(5)GlcNAc(1/2) but not fully glucosylated glycans that build up when glucosidases are inhibited. Our data further demonstrate that such specificity is determined genetically rather than post-translationally. We also demonstrate that the bovine endomannosidase is transcriptionally upregulated by comparison with glucosidase II in Madin-Darby bovine kidney cells and speculate that this is to compensate for the reduced catalytic activity as measured in the recombinant form of the enzyme.
在早期分泌途径中从新生糖蛋白上去除α-葡萄糖残基是进一步 N-糖基化成熟的要求。尽管去葡糖基化是大多数糖蛋白在内质网相关糖苷酶 I 和 II 介导的逐步过程，但高尔基内-α-甘露糖苷酶为糖蛋白去葡糖基化提供了备用机制。尽管在哺乳动物中保守，但在某些细胞系中，糖苷酶抑制后，体外的内甘露糖苷酶活性似乎与其在细胞中的活性不同。在这里，我们表明，在牛细胞中，这可以通过限制底物特异性来解释，允许加工 Glc(1)Man(7)GlcNAc(1/2) 和 Glc(1)Man(5)GlcNAc(1/2)，但不能加工当糖苷酶被抑制时积累的完全葡糖基化聚糖。我们的数据进一步表明，这种特异性是由遗传决定的，而不是翻译后决定的。我们还证明，与 Madin-Darby 牛肾细胞中的葡萄糖苷酶 II 相比，牛内甘露糖苷酶在转录水平上被上调，我们推测这是为了弥补酶的重组形式测量的降低的催化活性。
