Fig. 2

The glycoprotein folding cycle within the endoplasmic reticulum lumen. Protein glycosylation is a highly conserved process and plays crucial biological and physiological roles. Polypeptides translated on ribosomes from mRNA are escorted to an ER translocon via the signal recognition particle (SRP) and receptor. As the polypeptide enters the ER, an en bloc transfer N-glycans (Glc(3)Man(9)GlcNAc(2)) where glucose is represented as green circles and mannose as red, and N-acetylglucosamine (GlcNAc) is Y shaped green structure attached to the nascent polypeptide chain. FZD4 and MuSK have two N-glycosylation sites in their extracellular domains. α-glucosidase I and II (GI /GII) remove two of the three glucoses forming a monoglucosylated glycoprotein. This monoglucosylated protein is a signal for interacting with CNX and CRT, both lectins bound to protein disulfide isomerase family A member 3 (PDIA3). CRT is the soluble form of CNX and they form interchain disulfide bonds (S-S) with the bound glycoproteins. Removal of the last glucose by GII allows the glycoprotein to be released from the chaperones and leave the ER through ER exit sites to the golgi apparatus. Lipidation is a co or post-translational modification where lipid moieties are covalently attached to the polypeptide to increase hydrophobicity, conformation, and stability. Misfolded proteins trigger UDP-glucose-glucosyltransferase to re-add a single glucose on to the glycan and the cycle of protein folding is repeated. If the glycoprotein is permanently misfolded, the terminal mannose α1–2Man from the central arm of Man(9)GlcNAc(2), shown as a blue triangle, from the b branch of the oligosaccharide is removed by α-1,2-mannosidase I yielding a Man(8)GlcNAc(2) b-isomer. A second ER resident α-mannosidase I–like protein which lacks enzyme activity known as ER degradation-enhancing α-mannosidase I–like protein (EDEM), recognizes misfolded glycoproteins and targets them for ERAD machinery (Milhem 2015)