Role of N-linked polymannose oligosaccharides in targeting glycoproteins for endoplasmic reticulum-associated degradation

Abstract

Misfolded or incompletely assembled multisubunit glycoproteins undergo endoplasmic reticulum-associated degradation (ERAD) regulated in large measure by their N-linked polymannose oligosaccharides. In this quality control system lectin interaction with Glc₃Man₉GlcNAc₂ glycans after trimming with endoplasmic reticulum (ER) α-glucosidases and α-mannosidases sorts out persistently unfolded glycoproteins for N-deglycosylation and proteolytic degradation. Monoglucosylated (Glc₁Man₉GlcNAc₂) glycoproteins take part in the calnexin/calreticulin glucosylation-deglucosylation cycle, while the Man₈GlcNAc₂ isomer B product of ER mannosidase I interacts with EDEM. Proteasomal degradation requires retrotranslocation into the cytosol through a Sec61 channel and deglycosylation by peptide: N-glycosidase (PNGase); in alternate models both PNGase and proteasomes may be either free in the cytosol or ER membrane-imbedded/attached. Numerous proteins appear to undergo nonproteasomal degradation in which deglycosylation and proteolysis take place in the ER lumen. The released free oligosaccharides (OS) are transported to the cytosol as OS-GlcNAc₂ along with similar components produced by the hydrolytic action of the oligosaccharyltransferase, where they together with OS from the proteasomal pathway are trimmed to Man₅GlcNAc₁ by the action of cytosolic endo-β-N-acetylglucosaminidase and α-mannosidase before entering the lysosomes. Some misfolded glycoproteins can recycle between the ER, intermediate and Golgi compartments, where they are further processed before ERAD. Moreover, properly folded glycoproteins with mannose-trimmed glycans can be deglucosylated in the Golgi by endomannosidase, thereby releasing calreticulin and permitting formation of complex OS. A number of regulatory controls have been described, including the glucosidase-glucosyltransferase shuttle, which controls the level of Glc₃Man₉GlcNAc₂-P-P-Dol, and the unfolded protein response, which enhances synthesis of components of the quality control system.