UDP-GlC:glycoprotein glucosyltransferase-glucosidase II, the ying-yang of the ER quality control

Abstract

The N-glycan-dependent quality control of glycoprotein folding prevents endoplasmic to Golgi exit of folding intermediates, irreparably misfolded glycoproteins and incompletely assembled multimeric complexes. It also enhances folding efficiency by preventing aggregation and facilitating formation of proper disulfide bonds. The control mechanism essentially involves four components, resident lectin-chaperones that recognize monoglucosylated polymannose glycans, a lectin-associated oxidoreductase acting on monoglucosylated glycoproteins, a glucosyltransferase that creates monoglucosytlated epitopes in protein-linked glycans and a glucosidase that removes the glucose units added by the glucosyltransferase. This last enzyme is the only mechanism component sensing glycoprotein conformations as it creates monoglucosylated glycans exclusively in not properly folded species or in not completely assembled complexes. The glucosidase is a dimeric heterodimer composed of a catalytic subunit and an additional one that is partially responsible for the ER localization of the enzyme and for the enhancement of the deglucosylation rate as its mannose 6-phosphate receptor homologous domain presents the substrate to the catalytic site. This review deals with our present knowledge on the glucosyltransferase and the glucosidase.

Fig. 1

Structure of glycans. Lettering (a-n) follows the order of addition of monosaccharides in the synthesis of Glc₃Man₉GlcNAc₂-P-P-dolichol. GI removes residue n and GII residues l and m. UGGT adds residue l to residue g.

Fig. 2

Model proposed for the quality control of glycoprotein folding. Proteins entering the ER are N-glycosylated by the oligosaccharyltransferase (OST) as they emerge from the translocon (Sec61). Two glucoses are removed by the sequential action of GI and GII to generate monoglucosylated species that are recognized by CNX and/or CRT (only CNX is shown), that are associated with ERp57. The complex formed by the lectins and folding intermediates/misfolded glycoproteins dissociates upon removal of the last glucose by GII and is reformed by UGGT activity. Once glycoproteins have acquired their native conformations, either free or complexed with the lectins, GII hydrolyses the remaining glucose residue and releases the glycoproteins from the lectin anchors. These species are nor recognized by UGGT and are transported to the Golgi. Glycoproteins unable to properly fold are retrotranslocated to the cytosol where they are deglycosylated by the proteasome. One or more mannose residues may be removed by the ER α–mannosidase I (α-Man) during the whole folding process.

Fig. 3

Models proposed for GIIβ-mediated enhancement of N-glycan deglucosylation. (A) Upon binding mannose units in the B and/or C arms of the glycan, the GIIβ MRH domain presents bonds to be cleaved to the GIIα catalytic site (star). Please note that in this model a rotation of A arm is required for both cleavages to proceed. (B) Binding of a glycan to the GIIβ MRH domain would result in a conformational change in their GIIβ subunit, which in turn would modify GIIα structure thus activating the catalytic site and allowing hydrolysis of the middle Glc residue in another glycan. Cleavage of the inner Glc in this last glycan would be enhanced by the interaction of Man units in its own B and/or C arms with the MRH domain as the bond to be cleaved and the arms lie on the same face.