Wilson Cornelia M, High Stephen
Faculty of Life Sciences, University of Manchester, Michael Smith Building, Oxford Road, Manchester, M13 9PT, UK.
J Cell Sci. 2007 Feb 15;120(Pt 4):648-57. doi: 10.1242/jcs.000729. Epub 2007 Jan 30.
The mammalian oligosaccharyltransferase (OST) complex is composed of about eight subunits and mediates the N-glycosylation of nascent polypeptide chains entering the endoplasmic reticulum (ER). The conserved STT3 subunit of eukaryotic OST complexes has been identified as its catalytic centre, yet although many other subunits are equally well conserved their functions are unknown. We used RNA interference to investigate the function of ribophorin I, an ER-translocon-associated subunit of the OST complex previously shown to associate with newly synthesised membrane proteins. We show that ribophorin I dramatically enhances the N-glycosylation of selected membrane proteins and provide evidence that it is not essential for N-glycosylation per se. Parallel studies confirm that STT3 is essential for transferase activity of the complex, but reveal that the two mammalian isoforms are not functionally equivalent when modifying bona fide polypeptide substrates. We propose a new model for OST function where ribophorin I acts as a chaperone or escort to promote the N-glycosylation of selected substrates by the catalytic STT3 subunits.
哺乳动物寡糖基转移酶（OST）复合物由大约八个亚基组成，介导进入内质网（ER）的新生多肽链的N-糖基化。真核生物OST复合物中保守的STT3亚基已被确定为其催化中心，然而，尽管许多其他亚基同样高度保守，但其功能尚不清楚。我们利用RNA干扰来研究核糖体结合蛋白I的功能，它是OST复合物中与内质网转运体相关的一个亚基，此前已证明它与新合成的膜蛋白相关。我们发现核糖体结合蛋白I显著增强了某些膜蛋白的N-糖基化，并提供证据表明它本身对于N-糖基化并非必不可少。平行研究证实，STT3对于该复合物的转移酶活性至关重要，但揭示出这两种哺乳动物异构体在修饰真正的多肽底物时功能并不等同。我们提出了一种新的OST功能模型，其中核糖体结合蛋白I作为伴侣蛋白或护送蛋白，通过催化性的STT3亚基促进某些底物的N-糖基化。
