How sugars convey information on protein conformation in the endoplasmic reticulum

Abstract

The N-glycan-dependent quality control of glycoprotein folding prevents endoplasmic reticulum to Golgi exit of folding intermediates, irreparably misfolded glycoproteins and not completely 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 and a glucosidase that creates monoglucosylated epitopes in glycans transferred in protein N-glycosylation or 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 purpose of the review is to describe the most significant recent findings on the mechanism of glycoprotein folding and assembly quality control and to discuss the main still unanswered questions.

Fig. 1

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) (1). Two glucoses are removed by the sequential action of glucosidase I and GII to generate monoglucosylated species (2) that are recognized by CNX and/or CRT (only CNX is shown), that are associated with ERp57 (3). The complex between the lectins and folding intermediates/misfolded glycoproteins dissociates upon removal of the last glucose by GII, and is reformed by GT activity (4). 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 (5). These species are not recognized by GT and are transported to the Golgi (6). Glycoproteins remaining in misfolded conformations are retrotranslocated to the cytosol where they are deglycosylated and degraded by the proteasome (7). One or more mannose residues may be removed during the whole folding process.

Fig. 2

Structure of glycans. Lettering (a-n) follows the order of addition of monosaccharides in the synthesis of Glc₃Man₉GlcNAc₂-P-P-dolichol. Glucosidase I removes residue n and GII residues l and m. GT adds residue l to residue g. The Man₈GlcNAc₂ isomer A (M8A) lacks residues g and l-n.

Fig. 3

Structure of lectin-chaperones. (A) Crystal structure of the lumenal portion of CNX. (B) The arm domain of CNX embraces the globular domain of a symmetry related molecule. (C) Domain organization of CRT.