Vertebrate protein glycosylation: diversity, synthesis and function

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.

Figure 1. Protein N-glycosylation and quality control of protein folding

a | During glycoprotein biosynthesis, the translation of nascent polypeptides is followed by their translocation through the SEC61 pore and the simultaneous transfer of a glycan from a lipid-linked intermediate to peptide acceptor sequons by the oligosaccharyltransferase (OST). One cleft in the STT3 subunit of OST scans for Asn-X-Ser/Thr acceptor sequons, while an adjacent cleft binds the glycan donor. b | Glycan trimming through Glc removal occurs immediately after transfer by α-glucosidase I (GIsI) and the α-glucosidase II α–β heterodimer (GIsIIα/β). Folding intermediates containing Glc₁Man₉GlcNAc₂ structures are ligands for the lectins calnexin or calreticulin, which function in complex with ERp57. Dissociation from the lectins is followed by further Glc cleavage. Additional chaperone assistance is provided by the ATP-driven chaperone BiP (also known as GRP78). Correctly folded glycoproteins are packaged for transport to the Golgi. c | Incompletely folded glycoproteins are recognized by the folding sensor UDP-Glc:glycoprotein glucosyltransferase (UGGT1). They are then re-glucosylated through the addition of a Glc residue back to the glycan structure and are reintegrated into the calnexin cycle. d | Terminally misfolded glycoproteins are subjected to endoplasmic reticulum (ER) disposal by Man trimming (through the activity of ER α-mannosidase I (ERManI) or GolgiManIA, GolgiManIB and GolgiManIC (not shown), followed by the activity of ER degradation-enhancing α-mannosidase-like 1 (EDEM1), EDEM2 and EDEM3 (which are homologues of Htm1 (also known as Mnl1) in yeast)). The trimmed glycans bind the ER lectins OS9 or XTP3B (not shown) and are translocated into the cytosol via a complex of derlin 1 (DER1), DER2 and DER3 (the SEL1L complex; Hrd3 in yeast) using the driving force of the cytosolic ubiquitin binding protein and ATPase functions of valocin-containing protein (VCP; also known as TER ATPase; known as Cdc48, Ufd1 or Npl4 in yeast). The peptide is deglycosylated by a cytosolic PNGase and degraded by the proteasome.

Figure 2. Control points for Golgi trafficking and cellular glycomic diversity

Intra-Golgi transport mechanisms provide regulatory points for glycome modulation. Two idealized Golgi compartments are depicted. Guanine nucleotide exchange cycles acting on small GTPases regulate trafficking steps important for glycosylation. a | ADP ribosylation factor 1 (ARF1) activation recruits coatomer protein complex I (COPI) coats to nascent transport vesicles. b,c | Golgin interactions with cisternal and vesicular GTPases influence vesicle capture^,. At least 15 small GTPases of the RAB family and three of the ARF and ARF-like (ARL) family are associated with specific cisternae, vesicles and tethering factors^,. Golgin tethering factors exist as transmembrane or peripheral membrane proteins that are retained at Golgi cisternae through binding interactions with myristoylated Golgi reassembly-stacking protein of 55 kDa (GRASP55; also known as GRS2) or GRASP65 (also known as GRS1) or small GTPases (prenylated RABs, acylated ARF1 or ARL1). d | The conserved oligomeric Golgi (COG) complex, a multiprotein-tethering complex, interacts with a subset of proteins called GEARs (including golgins), facilitating the fusion of appropriately tethered transport vesicles and modulating the stability of processing enzymes. e,f,g | Sphingolipid and cholesterol content, as well as glycosphingolipid complexity (that is, glycan size and charge) increase from early to late Golgi^,. Glycosyltransferases catalysing sequential reaction sets may congregate into selective Golgi processing domains on the basis of preferences for specific lipid content imposed by the biophysical characteristics of their transmembrane domains. Therefore, altered lipid biosynthesis may affect glycosylation by contracting or expanding facilitative lipid domains within Golgi cisternae. t SNARE, target soluble N-ethylmaleimide-sensitive factor attachment protein receptor; v-SNARE, vesicle SNARE.