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Vertebrate protein glycosylation: diversity, synthesis and function

Key Points

  • Contemporary studies of protein glycosylation have revealed numerous examples in which glycan structures attached to proteins and lipids have essential roles in biological recognition events. Glycans have critical functions throughout the cell, from the cytosol and secretory compartments to the cell surface and the extracellular space.

  • Conserved contributions of N-glycan structures in chaperone interactions and protein quality control are initiated co-translationally via the distinctive catalytic mechanism of the oligosaccharyltransferase, and glycan processing continues throughout the dynamic collection of secretory compartments that lead to the cell surface.

  • The complex assortment of glycosylation enzymes comprises an intricate assembly line for glycan maturation from the ER through the Golgi. The localization, dynamics, interactions, regulation and substrate competition of these enzymes within the ER and Golgi remains an active area of study.

  • Of the many roles that glycans have at the cell surface, emerging paradigms have highlighted the importance of protein domain-specific glycosylation in facilitating or modulating biological recognition events.

  • Advances in high-throughput glycan structural analysis are beginning to provide novel insights into correlations with gene expression patterns for glycosylation machinery and genome-wide associations that define global regulation of glycan diversity.

  • The diversity of glycan structures clearly provides an additional level of information content in biological systems, but the challenge for the future lies in identifying how different biological contexts determine glycan encoded functions within the bewildering array of heterogeneous glycan structures.

Abstract

Protein glycosylation is a ubiquitous post-translational modification found in all domains of life. Despite their significant complexity in animal systems, glycan structures have crucial biological and physiological roles, from contributions in protein folding and quality control to involvement in a large number of biological recognition events. As a result, they impart an additional level of 'information content' to underlying polypeptide structures. Improvements in analytical methodologies for dissecting glycan structural diversity, along with recent developments in biochemical and genetic approaches for studying glycan biosynthesis and catabolism, have provided a greater understanding of the biological contributions of these complex structures in vertebrates.

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Figure 1: Protein N-glycosylation and quality control of protein folding.
Figure 2: Control points for Golgi trafficking and cellular glycomic diversity.

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Acknowledgements

This work was supported by US National Institutes of Health (NIH) grants from the National Center for Research Resources (P41RR005351 and P41RR018502) and the National Institute of General Medical Sciences (P41GM103390 and P41GM103490) to J. Prestegard and J. M. Pierce and NIH grants R01GM047533 and R01DK075322 to K.W.M., as well as P01GM085354 and R01GM072839 to M.T.

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Essentials of Glycobiology, 2nd edn

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Glossary

Lectins

Carbohydrate binding proteins that are involved in various biological recognition phenomena.

Sialic acid

(SA; also known as neuraminic acid (Neu)). SAs are a family of nine-carbon monosaccharides with a carboxylic acid at the C1 position and a glycerol side chain at C7-C9. Two SAs predominate in vertebrates, with either an N-acetyl group (NeuAc) or an N-glycolyl group (NeuGc) at C5. Humans do not make NeuGc, but can obtain it from their diet and incorporate it into glycoconjugates.

Lipid-linked oligosaccharide

(LLO). Extended carbohydrate structure attached through a phosphodiester linkage to a polyisoprenoid lipid, usually dolichol. N-linked glycan precursor structures are commonly assembled as a lipid-linked intermediate before transfer to a polypeptide side chain.

Oligosaccharyltransferase

(OST). A multi-enzyme complex (or single subunit in bacteria) in the endoplasmic reticulum membrane that transfers glycan structures from a lipid-linked oligosaccharide precursor to acceptor sequences on nascent polypeptides.

Acceptor peptide sequons

Short amino acid sequences on glycan-acceptor polypeptide chains that are recognized by glycosyltransferases prior to glycosylation.

Calnexin

Integral membrane endoplasmic reticulum lectin that recognizes GlcMan9GlcNAc2 glycans on early glycoprotein folding intermediates and, in collaboration with an associated thiol oxidoreductase, ERp57, aids in protein folding as a part of endoplasmic reticulum quality control.

Calreticulin

A soluble lectin in the lumen of the endoplasmic reticulum that contains a K-D-E-L endoplasmic reticulum retrieval sequence and, in a similar way to calnexin, binds to GlcMan9GlcNAc2 glycoprotein folding intermediates to facilitate protein folding and quality control.

ER-associated degradation

(ERAD). A protein quality control pathway in which misfolded lumenal or integral membrane proteins are recognized (often through trimmed glycan structures) for disposal by translocation into the cytosol for proteasomal degradation.

Nocodozole

A pharmacological agent that blocks mitosis by binding to tubulin and inhibiting microtubule polymerization. Inhibition of microtubule polymerization also causes the dispersal of well-organized Golgi stacks into smaller units distributed throughout the cytoplasm.

Brefeldin A

A fungally derived antibiotic that inhibits the guanine nucleotide exchange factor (GEF) responsible for activating ARF1 GTPase. Activation of ARF1 recruits coatomer protein complex I (COPI) to Golgi membranes to form vesicles. In the absence of Golgi vesicle formation, cis and medial cisternae fuse with the endoplasmic reticulum and the trans and trans-Golgi network components disperse into a drug-induced entity called the brefeldin compartment.

Cisternal maturation

An early model of Golgi trafficking that proposed that Golgi cisternae formed from the fusion of vesicles leaving the endoplasmic reticulum and were subsequently matured by the import of appropriate processing enzymes as the cisternae were pushed forward towards the trans face. The original cisternal maturation model had its groundings in extensive electron microscopic observations of Golgi morphology.

Vesicular transport

A model for Golgi trafficking, which proposed that cisternae are stable structures and that cargo proteins move between cisternae in transport vesicles that are targeted to specific Golgi domains.

Rapid partitioning

A model for Golgi trafficking, which proposes that cargo proteins can exit the Golgi in vesicles arising from all cisternae and that new protein arriving at the Golgi rapidly gains access to the entire apparatus. Distinct transport and processing domains within each Golgi cistern are defined by their lipid content, which varies systematically from cis-to-trans and may provide favourable environments for cisternae-specific subsets of glycan processing enzymes.

Glycosphingolipids

Glycoconjugates comprised of a ceramide lipid (Cer) carrying a glycan headgroup. The glycan is usually initiated by Glc, although GalCer is an important component of some cells. GlcCer is elongated to generate four major types of neutral cores in mammalian tissues (ganglio-, globo-, lacto- and neolacto-series), each of which can be capped and branched to produce additional structural diversity.

Type 2 transmembrane proteins

Single pass transmembrane proteins with their amino terminus oriented towards the cytosol and their carboxyl terminus facing the lumen of the secretory pathway or cell exterior.

Vacuolar protein sorting 74

(Vps74). A yeast protein identified as having a vacuolar protein sorting function. Yeast vps74 mutants are deficient in intra-Golgi transport and exhibit mis-localized glycosyltransferases. Golgi phosphoprotein 3 (GOLPH3) is the mammalian homologue of Vps74.

Coatomer protein complex I

(COPI). A protein complex that is recruited to membranes by ARF (ADP-ribosylation factor) GTPases and that mediates intra-Golgi and Golgi-to-endoplasmic reticulum retrograde transport.

Mucin-type O-linked glycosylation

A form of protein glycosylation initiated by the addition of a GalNAc residue to protein Ser or Thr side chains and extended and branched with other complex termini. It is commonly found on highly glycosylated mucin glycoproteins, but also on many non-mucin polypeptides.

Golgins

A family of Golgi tethering factors characterized by their extended coiled-coil domains. Individual family members exhibit specific sub-Golgi localizations, GTPase interactions and effector activities. The golgins form long filaments that emanate from the cytoplasmic face of Golgi cisternae, providing access to transport vesicles in the near environment. Some golgins possess multiple binding sites for RAB proteins, suggesting that they capture transport vesicles carrying specific RAB proteins.

Conserved oligomeric Golgi complex

(COG complex). A multiprotein tethering factor comprised of eight protein subunits. The complex facilitates Golgi retrograde transport and glycosyltransferase localization.

Congenital disorders of glycosylation

(CDGs). A growing group of recessive human disorders characterized by altered protein glycosylation. In type 1 CDGs, the number of N-linked glycans added to a protein is affected. In type 2 CDGs, the nature of the glycan on a glycoprotein is changed. Most CDGs arise from partial loss-of-function mutations and are diagnosed clinically by altered serum protein glycosylation.

Retrograde transport

The movement of vesicles containing Golgi-resident proteins (glycosyltransferases and other processing enzymes) from late to early cisternae. Retrograde transport provides a mechanism for retrieving enzymes that are displaced forward by anterograde flow or cisternal maturation.

Anterograde flow

The bulk movement of cisternal contents and cargo proteins from early to late Golgi compartments.

EGF domains

Protein motifs comprised of 30–40 amino acids, including six Cys residues forming three characteristic disulphide bonds, and a mainly β-sheet structure, found in all ERBB-binding growth factors and many other cell surface and extracellular matrix proteins.

Thrombospondin type 1 repeats

(TSRs). Protein domains of 50 amino acids. They are rich in Cys residues and are comprised of three antiparallel β-strands along with regions without a secondary structure. TSRs are found in matrix and transmembrane proteins with functions in matrix organization, cell–cell interactions and cell guidance.

Dystroglycanopathies

A clinically heterogeneous collection of muscular dystrophies that are associated with aberrant glycosylation of α-dystroglycan, which is a component of the multiprotein dystrophin glycoprotein complex that bridges the extracellular matrix, plasma membrane and cytoskeleton.

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Moremen, K., Tiemeyer, M. & Nairn, A. Vertebrate protein glycosylation: diversity, synthesis and function. Nat Rev Mol Cell Biol 13, 448–462 (2012). https://doi.org/10.1038/nrm3383

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