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 alpha-mannosidases in the ER, especially alpha 1,2-mannosidase I (ERManI). ERManI removes the alpha 1,2-linked mannose of the B-chain from properly folded ER glycoproteins, whereas two or more alpha 1,2-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 Man(9)GlcNAc(2) 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 alpha-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 GlcMan(9)GlcNAc(2) in model glycoproteins, such as Aspergillus oryzae beta-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.