Generation and degradation of free asparagine-linked glycans

Abstract

Asparagine (N)-linked protein glycosylation, which takes place in the eukaryotic endoplasmic reticulum (ER), is important for protein folding, quality control and the intracellular trafficking of secretory and membrane proteins. It is known that, during N-glycosylation, considerable amounts of lipid-linked oligosaccharides (LLOs), the glycan donor substrates for N-glycosylation, are hydrolyzed to form free N-glycans (FNGs) by unidentified mechanisms. FNGs are also generated in the cytosol by the enzymatic deglycosylation of misfolded glycoproteins during ER-associated degradation. FNGs derived from LLOs and misfolded glycoproteins are eventually merged into one pool in the cytosol and the various glycan structures are processed to a near homogenous glycoform. This article summarizes the current state of our knowledge concerning the formation and catabolism of FNGs.

Lethality of mice bearing a knockout of the Ngly1-gene is partially rescued by the additional deletion of the Engase gene

“…Ngly1 cleaves N -glycans from misfolded glycoproteins during the ERAD process, and is thought to play an important role in the efficient degradation of misfolded glycoproteins [7–11]. N -glycans released by Ngly1 are known to be processed by cytoplasmic glycosidases such as ENGase and the α-mannosidase, Man2C1 (Fig 1A, upper scheme) [12–14]. Cytoplasmic ENGase is another deglycosylating enzyme that acts directly on N- glycans that are attached to glycoproteins but leaves a single N -acetylglucosamine (GlcNAc) residue attached to the protein (Fig 1A, lower scheme).…”

Non-lysosomal Degradation of Singly Phosphorylated Oligosaccharides Initiated by the Action of a Cytosolic Endo-β-N-acetylglucosaminidase

“…Dolichol-linked oligosaccharides (DLOs) 3 are glycan donor substrates for asparagine (N)-linked glycosylation in mammals (1,2). The biosynthesis of DLOs starts on the cytosolic side of the endoplasmic reticulum (ER) membrane with the assembly of Man 5 GlcNAc 2 -PP-Dol (1).…”

“…When cells are deprived of glucose, incompletely assembled DLOs are produced (4 -9), which are rapidly degraded by a currently unclarified degradation system (2,10,11). The enzyme involved in the degradation has long been believed to hydrolyze the pyrophosphate bond of DLOs, releasing phosphorylated oligosaccharides (POSs) into the cytosol (Fig.…”

“…In addition to POSs, a protein-unbound form of neutral high mannose-type N-glycans (free N-glycans; FNGs), which bare N,NЈ-diacetylchitobiose at the reducing end (Gn2-type), is produced in the cytosol via two distinct pathways as follows: the degradation of DLOs, presumably by oligosaccharyltransferase in the ER lumen (2,15,16), and their transport to the cytosol (Fig. 1, step ii); and the deglycosylation of misfolded glycoproteins by a cytosolic peptide:N-glycanase (PNGase) (Fig.…”

“…1, step ii); and the deglycosylation of misfolded glycoproteins by a cytosolic peptide:N-glycanase (PNGase) (Fig. 1, step iii) (2,17,18). In the cytosol, the Gn2-type FNGs are processed by an endo-␤-N-acetylglucosaminidase (ENGase) (19 -21) that hydrolyzes the innermost GlcNAc residue to generate Gn1-type FNGs (Fig.…”