Trimming of glucosylated N-glycans by human ER α1,2-mannosidase I

In the endoplasmic reticulum (ER), folding of proteins modified by asparagine-linked (N-linked) glycosylation is precisely monitored by quality control machinery. Upon exit from the calnexin/calreticulin cycle, glycoproteins are digested by α-mannosidases in the ER, especially α1,2-mannosidase I (ERManI). ERManI removes the α1,2-linked mannose of the B-chain from properly folded ER glycoproteins, whereas two or more α1,2-linked mannose residues are sequentially trimmed from improperly folded glycoproteins so they are recognized by a complex that mediates ER-associated degradation (ERAD). We have shown that the efficiency of Man9GlcNAc2 de-mannosylation in model glycoproteins by recombinant human ERManI (hERManI) is dependent on folding status (Aikawa et al. (In vitro mannose trimming property of human ER α-1,2 mannosidase I. Glycoconj. J 2012;29: 35-45.)). In this study, we revealed that this enzyme also accepts N-linked sugar chains with glucose moieties as substrates with nearly identical reactivity. The ability of hERManI to remove mannose residues from GlcMan9GlcNAc2 in model glycoproteins, such as Aspergillus oryzae β-galactosidase and chicken immunoglobulin Y (IgY), was markedly augmented when glycoproteins were denatured. The properties of hERManI enable rapid selection of ERAD substrates in the ER and may help maintain homeostasis of sugar metabolism in living organisms.