Promiscuous activity of ER glucosidase II discovered through donor specificity analysis of UGGT

Research highlights: → UGGT has a narrow donor specificity. → UGGT gave several non-natural high-mannose-type glycans. → G-II has a promiscuous activity as broad specificity hexosidase. -- Abstract: In glycoprotein quality control system in the endoplasmic reticulum (ER), UGGT (UDP-glucose:glycoprotein glucosyltransferase) and glucosidase II (G-II) play key roles. UGGT serves as a glycoprotein folding sensor by virtue of its unique specificity to glucosylate glycoproteins at incompletely folded stage. By using various UDP-Glc analogues, we first analyzed donor specificity of UGGT, which was proven to be rather narrow. However, marginal activity was observed with UDP-galactose and UDP-glucuronic acid as well as with 3-, 4- and 6-deoxy glucose analogues to give corresponding transfer products. Intriguingly, G-II smoothly converted all of them back to Man9GlcNAc2, providing an indication that G-II has a promiscuous activity as a broad specificity hexosidase.