Synthesis, Processing, and Function of N-glycans in N-glycoproteins

Abstract

Many membrane-resident and secrected proteins, including growth factors and their receptors are N-glycosylated. The initial N-glycan structure consists of 14 sugar residues (Glc3Man9GlcNAc2) that are first synthesized in the endoplasmic reticulum (ER) as a branched structure on a lipid anchor (dolicholpyrophosphate) and then co-translationally, “en bloc” transferred and linked via N-acetylglucosamine (GlcNAc) to asparagine within a specific N-glycosylation acceptor sequence (Asn-X-Ser/Thr) of the nascent recipient protein. In the ER and then the Golgi apparatus, the N-linked glycan structure is modified by hydrolytic removal of sugar residues (“trimming”) followed by re-glycosylation with additional sugar residues (“processing”) such as galactose, fucose or sialic acid in complex N-glycoproteins. While the sequence of the reactions leading to biosynthesis, “en bloc” transfer and processing of N-glycans is well investigated, it is still not completely understood how N-glycans affect the biological fate and function of N-glycoproteins. Initially, N-glycans have been found to be critical for proper protein folding and quality control by chaperones in the ER, a process now known as the “calnexin/calreticulin cycle” in the unfolded protein response and ER-assisted degradation (ERAD). More recently, N-glycans have been shown to modulate the function of many cell surface proteins involved in migration and adhesion, including those regulating myelination. Currently, the Golgi has emerged as an organelle that is intimately linked to editing the function of N-glycans in N-glycoprotein transport and sorting. For one, it has been shown that mutations in Golgi glycosyltransferases and transport proteins lead to defects in N-glycan processing that cause severe congenital disorders of glycosylation (CDG). On the other hand, it has been found that N-glycans affect transport of glycosylated proteins in the Golgi, including sorting of secreted proteins such as prions and amyloid. Our group has shown that N-glycan-dependent enzyme complex formation may entangle processing of N-glycosylated glycosyltransferases with glycosphingolipid metabolism, which appears to be important for ganglioside class switches during embryonic brain development. This review will discuss the biology of N-glycoprotein synthesis, processing and function with specific reference to the physiology and pathophysiology of the nervous system.

Eight Kinetically Stable but Thermodynamically Activated Molecules that Power Cell Metabolism

“…In a different set of metabolic intersections the assembly of the tetradecasaccharyl-dolchol as the starting point for all N -glycoprotein glycosylation events uses equivalent logic via glycosyl transferase action on NDP-hexose building blocks attached to the long chain isoprenyl dolichol-P. 129 The universal reliance on NDP-hexoses as glycosyl group donors to a wide range of cosubstrate nucleophiles ensures that essentially every glycosyl residue in biology is regiospecifically connected to its neighboring O, N, S, or C atom through C 1 .…”

Controlled tetra-Fc sialylation of IVIg results in a drug candidate with consistent enhanced anti-inflammatory activity

“…The core of the N297 N-linked glycan is composed of two GlcNAc and three mannose residues. Typically, this core can be further extended with fucose, galactose, sialic acid, and bisecting GlcNAc monosaccharides through selective enzymatic glycosylation reactions (12,13). Alterations to the N297 glycan composition have been shown to have a significant impact in modulating the interaction between the IgG Fc fragment and the Fc receptors.…”

New Insights on Leucine-Rich Repeats Receptor-Like Kinase Orthologous Relationships in Angiosperms

“…Newly synthetised membrane-resident proteins translocate first into the endoplasmic reticulum where they are subjected to folding and modifications like formation of disulfide bridges. It is also the place where nascent polypeptides are glycosylated – the most common post-traductional modification which is a crucial event during protein folding and quality control processes (Bieberich, 2014). The LRR-RLKs are part of the large family of plant proteins which are N-glycosylated and many N-glycosylation acceptor sequences are present in all ECDs.…”