Trends in Cell Biology
The glycan code of the endoplasmic reticulum: asparagine-linked carbohydrates as protein maturation and quality-control tags
Section snippets
Glycan addition and processing in the endoplasmic reticulum
Asn-linked glycosylation begins with the assembly of the complete dolichol-linked oligosaccharide donor (OS-PP-Dol), a 14-residue oligosaccharide (Figure 2) [4]. This process begins on the cytoplasmic face of the ER and is finished within the lumen, where the Asn-linked glycan is then transferred en bloc by the oligosaccharyltransferase (OST) to an Asn residue in the consensus sequence Asn-X-Ser/Thr (Figure 3, step 1). The transfer reaction requires that the glycosylation consensus sequence
Carbohydrate trimming to enter a lectin-based chaperone system
Following translocation into the ER lumen and glycosylation, proteins encounter chaperones, which facilitate the maturation process and detection of defective cargo. The first lectin chaperones that a nascent glycoprotein meets are calnexin and calreticulin. Calnexin is a type I membrane protein and calreticulin is its soluble paralog. Both proteins possess a single binding site for glycans that have been trimmed sequentially by glucosidase I and then glucosidase II to a mono-glucosylated state
A glycoprotein-specific oxidoreductase
Proteins that traverse the oxidizing environment of the ER can also be covalently modified by the formation of disulfide bonds. This oxidation reaction is catalyzed by proteins that act as oxidizing agents or electron acceptors. Alternatively, these oxidoreductases can reduce disulfide bonds by providing a source of electrons. This activity can support the isomerization of disulfides such that the native links can be reached or disulfide bonds can be reduced before the retrotranslocation of
A single glucose residue as a quality-control signal
Glycan signals are also used to target aberrant proteins for destruction. The hallmark for an unfolded protein and binding by traditional chaperones is exposed hydrophobic regions, yet binding with the lectin chaperones is dictated by hydrophilic interactions. Therefore, how are calnexin and calreticulin binding connected to the signatures of immature, malfolded or unassembled proteins? This link is, at least in part, provided by UDP-glucose: glycoprotein glucosyltransferase (GT), which
Anterograde transport lectin sorting receptors
Lectins of the early secretory pathway are also involved in the selective anterograde trafficking of glycoprotein cargo. Studies by the Hauri laboratory have shown that the type I membrane mannose-binding lectin ERGIC-53 assists in the transport of glycoproteins from the ER to the ER-Golgi intermediate compartment (ERGIC) (Figure 3, step 7) [37]. ERGIC-53 binds to high-mannose glycans and aids in the efficient secretion of glycoproteins, including cathepsin C and the blood-coagulation factors V
ER-associated glycoprotein degradation
Protein folding is error-prone, and amino acid mutations or deletions can further decrease folding efficiency. Non-native polypeptides generated as physiologic and pathologic byproducts of protein biogenesis need to be rapidly degraded to avoid accumulation and/or deposition in the ER lumen. The ER-associated degradation (ERAD) process is operated by the cytosolic proteasome and therefore requires recognition of the folding-incompetent polypeptides by molecular chaperones and enzymes in the ER
Mannose trimming to generate a degradation tag
Folding-competent and folding-incompetent polypeptides enter co-translationally in the calnexin/calreticulin cycle [19]. While the folding-competent proteins are eventually released as native polypeptides in the secretory pathway, the folding-incompetent ones are retained, temporarily protected from the degradation machinery, and subjected to additional folding attempts. Their Asn-linked glycans are eventually trimmed of terminal mannoses by the ER enzyme α-mannosidase I. Although removal of
EDEM decodes the N-glycan degradation tag to interrupt futile folding attempts
The slower progression of misfolded polypeptides through the calnexin cycle and their reduced affinity for calnexin/calreticulin upon mannose removal might offer an advantage to mannose-binding lectins present in the ER lumen (termed EDEM [53] and EDEM2 54, 55 for: ER degradation enhancing α-mannosidase like proteins in mammals, and Htm1p/Mnl1p for: mannosidase like protein in yeast 47, 56). EDEM and EDEM2 (and EDEM3, whose function in the ER has not yet been established 54, 55) are members of
A glycan tag to direct misfolded glycoproteins to the cytosolic proteasome
Most misfolded proteins in the cytoplasm, including those arriving from the ER, are decorated with polyubiquitin chains to promote degradation by the 26S proteasome. Polyubiquitylation occurs by the concerted action of activating E1, conjugating E2 and ligating E3 enzymes located in the cytosol [64]. There are several E3 variants, and one of the best-characterized E3 complexes, the SCF (for: Skp1, Cul1, Roc1) contains a Fbs1 (F-box sugar recognition) protein in the adult brain and testis or a
Concluding remarks
Glycans play diverse roles inside and outside of the cell. Here, we have focused on the function of Asn-linked glycans in the early secretory pathway, where the glycan processing steps are more homogenous than in downstream locations. The past decade has seen a large increase in our understanding of the role glycans play in directing the maturation and quality-control traffic in the ER. The glycan composition is used as a signal to recruit chaperones and sorting receptors to assist in the
Acknowledgements
D.N.H. is supported by U.S. Public Health Service Grant CA79864. M.M. is supported by grants from Max Cloetta Foundation, Foundation for Research on Neurodegenerative Diseases, Swiss National Center of Competence in Research on Neural Plasticity and Repair, Swiss National Science Foundation, Telethon, and Synapsis and Bangerter-Rhyner Foundation.
References (72)
On the frequency of protein glycosylation, as deduced from analysis of the SWISS-PROT database
Biochim. Biophys. Acta
(1999)- et al.
The oligosaccharyltransferase complex from yeast
Biochim. Biophys. Acta
(1999) Oligosaccharyltransferase isoforms that contain different catalytic STT3 subunits have distinct enzymatic properties
Mol. Cell
(2003)- et al.
Determination of the distance between oligosaccharyltransferase active site and the endoplasmic reticulum membrane
J. Biol. Chem.
(1993) Protein folding during cotranslational translocation in the endoplasmic reticulum
Mol. Cell
(2002)Intracellular folding of tissue-type plasminogen activator: Effects of disulfide bond formation and N-linked glycosylation and secretion
J. Biol. Chem.
(1995)- et al.
Unraveling the mechanism of protein N-glycosylation
J. Biol. Chem.
(2005) The structure of calnexin, an ER chaperone involved in quality control of protein folding
Mol. Cell
(2001)Interactions of substrate with calreticulin, an endoplasmic reticulum chaperone
J. Biol. Chem.
(2003)Chaperone function of calreticulin when expressed in the endoplasmic reticulum as the membrane-anchored and soluble forms
J. Biol. Chem.
(1995)
N-linked glycans direct the cotranslational maturation of influenza hemagglutinin
Mol. Cell
Protein modulators: multi-functional mediators of protein translocation across membranes
Cell
Glucose trimming and reglucosylation determines glycoprotein association with calnexin
Cell
The UDP-glucose:Glycoprotein glucosyltransferase is organized in at least two tightly bound domains from yeast to mammals
J. Biol. Chem.
The endoplasmic reticulum glucosyltransferase recognizes nearly native glycoprotein folding intermediates
J. Biol. Chem.
Lectins and traffic in the secretory pathway
FEBS Lett.
Mutations in the ER-Golgi intermediate compartment protein ERGIC-53 cause combined deficiency of coagulation factors V and VIII
Cell
pH-induced conversion of the transport lectin ERGIC-53 triggers glycoprotein release
J. Biol. Chem.
Enhancement of endoplasmic reticulum (ER) degradation of misfolded null Hong Kong alpha1-antitrypsin by human ER mannosidase I
J. Biol. Chem.
Endoplasmic reticulum-associated degradation of mammalian glycoproteins involves sugar chain trimming to Man6-5GlcNAc2
J. Biol. Chem.
Pre-Golgi degradation of yeast prepro-α-factor in a mammalian cell
J. Biol. Chem.
Definition of the lectin-like properties of the molecular chaperone, calreticulin, and demonstration of its copurification with endomannosidase from rat liver Golgi
J. Biol. Chem.
Oligosaccharide modification in the early secretory pathway directs the selection of a misfolded glycoprotein for degradation by the proteasome
J. Biol. Chem.
Dissecting glycoprotein quality control in the secretory pathway
Trends Biochem. Sci.
A novel stress-induced EDEM variant regulating endoplasmic reticulum-associated glycoprotein degradation
J. Biol. Chem.
Mnl1p, an alpha-mannosidase-like protein in yeast Saccharomyces cerevisiae, is required for endoplasmic reticulum-associated degradation of glycoproteins
J. Biol. Chem.
Mechanism of class 1 (glycosylhydrolase family 47) {alpha}-mannosidases involved in N-glycan processing and endoplasmic reticulum quality control
J. Biol. Chem.
A time-dependent phase shift in the mammalian unfolded protein response
Dev. Cell
EDEM contributes to maintenance of protein folding efficiency and secretory capacity
J. Biol. Chem.
Using a small molecule inhibitor of Peptide: N-glycanase to probe its role in glycoprotein turnover
Chem. Biol.
Congenital disorders of glycosylation: genetic model systems lead the way
Trends Cell Biol.
Statistical analysis of the protein environment of N-glycosylation sites: implications for occupancy, structure, and folding
Glycobiology
A genetic approach to mammalian glycan function
Annu. Rev. Biochem.
Roles of N-linked glycans in the endoplasmic reticulum
Annu. Rev. Biochem.
Photocross-linking of nascent chains to the STT3 subunit of the oligosaccharyltransferase complex
J. Cell Biol.
Yeast mutants deficient in protein glycosylation
Proc. Natl. Acad. Sci. U. S. A.
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