n-glycan structures: recognition and processing in the er

N-glycan structures: recognition andprocessing in the ERMarkus Aebi1, Riccardo Bernasconi2, Simone Clerc1 and Maurizio Molinari2,3

1 Institute of Microbiology, Department of Biology, Eidgenossische Technische Hochschule (ETH) Zurich, CH-8093 Zurich,

Switzerland2 Institute for Research in Biomedicine, CH-6500 Bellinzona, Switzerland3 Ecole Polytechnique Federale de Lausanne, School of Life Sciences, CH-1015 Lausanne, Switzerland

Review

The processing of N-linked glycans determines secretoryprotein homeostasis in the eukaryotic cell. Folding anddegradation of glycoproteins in the endoplasmic reticu-lum (ER) are regulated by molecular chaperones andenzymes recruited by specific oligosaccharide structures.Recent findings have identified several components ofthis protein quality control system that specificallymodify N-linked glycans, thereby generating oligosac-charide structures recognized by carbohydrate-bindingproteins, lectins. In turn, lectins direct newly synthesizedpolypeptides to the folding, secretion or degradationpathways. The ‘‘glyco-code of theER’’ displays the foldingstatus of a multitude of cargo proteins. Deciphering thiscode will be instrumental in understanding proteinhomeostasis regulation in eukaryotic cells and for inter-vention because such processes can have crucial import-ance for clinical and industrial applications.

Biosynthesis of core oligosaccharides and their transferfrom a lipid donor onto nascent chainsAsparagine (N)-linked protein glycosylation is a covalentprotein modification that occurs across the three domainsof life: Bacteria, Archaea and Eukarya. In eukaryotic cells,N-linked glycosylation is the most prominent modificationof secretory proteins. This complex biosynthetic pathwayhas been studied best in the fungus Saccharomyces cere-visiae, but it seems to be highly conserved in other fungal,plant and animal species [1]. The pathway initiates at theER membrane where a lipid carrier, dolichylpyropho-sphate, serves as a membrane anchor for the assemblyof an oligosaccharide. The bipartite biosynthesis of theoligosaccharide initiates at the cytoplasmic side of theER membrane. Nucleotide-activated sugar donors serveas substrates for a series of different glycosyl transferasesthat lead to a mannose5-N-acetylglucosamine2 oligosac-charide. This lipid-bound oligosaccharide is translocatedacross themembrane [2] where an additional fourmannoseand three glucose residues are added. In the ER lumen,dolichylphosphomannose and dolichylphosphoglucose actas substrates for the individual glycosyl transferases. Thetransfer of the N-glycan precursor onto nascent polypep-tide chains is catalysed by oligosaccharyltransferase(OST), a protein complex consisting of eight different sub-units. OST utilizes the activated glucose3-mannose9-N-acetylglucosamine2 (Glc3Man9GlcNAc2) oligosaccharide

Corresponding authors: Aebi, M. ([email protected]);Molinari, M. ([email protected]).

74 0968-0004/$ – see front matter � 2009 Elsevie

donor as a substrate to covalently modify defined aspar-agine side chains within the acceptor sequence N-X-S/T(X cannot be proline) [1]. Most of the eukaryotic speciesstudied transfer this specific oligosaccharide structure toproteins (Figure 1 and Box 1).

N-glycosylation is in many respects a unique covalentprotein modification, but in the context of this review, thespatio-temporal aspect of the transfer reaction and thephysicochemical properties of the oligosaccharide are ofcentral importance.

The N-X-S/T sequon is a short and clearly defined

acceptor sequence

TheN-X-S/T sequon can be found frequently in proteins. Incontrast to other covalent post-translational modificationssuch as O-glycosylation or phosphorylation, no additional,large protein domain is required to define an N-linkedglycosylation site. However, several studies suggest thattheN-X-S/T sequence can bemodified only if it lies within aflexible domain of the polypeptide [3], suggesting that thepeptide acceptor takes on a defined structure in the courseof the modification reaction. Accordingly, the overall num-ber of proteins and sites that are glycosylated can beincreased by executing N-glycosylation before the foldingprocess, during or immediately after the translocation ofthe nascent chain into the ER lumen [4]. Indeed, in manyeukaryotic systems, the OST is associated with the trans-locon and with the ribosome [5]. It has been proposed thatdefined subunits of the OST might act as chaperones orenzymes to modulate, or even prevent, the folding of thetarget proteins in order to facilitate N-glycosylation [6,7].

A large, hydrophilic and branched oligosaccharide

structure is transferred to proteins

The hydrophilic structure itself (Figure 1) affects the solu-bility and folding of proteins [1]. More importantly, it canbe modified by several ER-localized glycosyl hydrolasesand one glucosyl transferase in a cascade of reactions that,owing to the branched structure of the oligosaccharide, canresult in a variety of structures that serve as ligands forcarbohydrate binding proteins, lectins (Figure 2).

These characteristics of N-glycans, the possibility totransfer them to different sites of proteins without the needof an extended primary protein acceptor sequence, thesequential processing of the oligosaccharide and the bindingof specific lectins are a prerequisite for the function ofN-glycan structures as clearly defined signalling molecules

r Ltd. All rights reserved. doi:10.1016/j.tibs.2009.10.001 Available online 21 October 2009

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Figure 1. The N-linked core oligosaccharide structure. The core oligosaccharide is

composed of two N-acetylglucosamine (blue squares), nine mannose (green

circles) and three glucose residues (blue circles). A, B and C define the

oligosaccharide branch. Letters a–n are assigned to each hexose and are used

throughout the text. During biosynthesis, residues a–g are added onto the

cytosolic face of the ER membrane, and residues h–n are added after the

oligosaccharide precursor is flipped across the membrane. The linkage between

individual hexoses is shown.

Box 1. N-linked protein glycosylation occurs in all domains

of life

The Glc3Man9GlcNAc2 composition of the glycan that is transferred

to proteins is conserved in animal, plant and fungal species.

Pathways with reduced complexity have been identified in proto-

zoan species, but it is always a derivative of the above structure that

is transferred to proteins. For example, proteins expressed in

species belonging to the genera Trypanosoma, Leptomonas and

Herpetomona are modified with Man9GlcNAc2 oligosaccharides, in

Crithidia with Man7GlcNAc2, in some Leishmania species and

Blastocrithidia with Man6GlcNAc2 [78–80] and in Tetrahymena

pyroformis with Glc3Man5GlcNAc2 [81]. It has been proposed that

secondary loss of glycosyl transferases has led to the diversification

of N-glycosylation pathways within the eukaryotic domain [82]. In

an alternative model, eukaryotic N-linked protein glycosylation

originates from a prokaryotic (or perhaps archaeal?) process where

the Man5GlcNAc2 oligosaccharide is transferred to protein and

sequential addition of ER-lumen-oriented glycosyl transferases and

extension of OST complexity has led to the Glc3Man9GlcNAc2

producing pathway [1,82,83]. Regardless of the evolutionary path-

way that occurred in eukaryotes, experimental evidence indicates

that N-linked protein glycosylation in the three domains of life

shares common features:

1. Isoprenoid lipids are used as membrane anchors for the

assembly of the oligosaccharide at the plasma membrane/ER

membrane [84].

2. Assembly of the oligosaccharide takes place in the cytoplasm.

Oligosaccharide synthesis continues after translocation of the

lipid-linked oligosaccharide only in eukaryotes [85].

3. Although the oligosaccharide structures transferred to proteins

are highly diverse in the prokaryotic pathways and do not

resemble those found in eukaryotes, prokaryotic OST and the

catalytic subunit of the eukaryotic enzymes share a high level of

sequence similarity [86].

4. The consensus sequence to be N-glycosylated contains the N-X-

S/T sequon. In the one bacterial system studied so far, an

extended sequence, D/E-X1-N-X2-S/T, is required [87].

Review Trends in Biochemical Sciences Vol.35 No.2

in the process of protein folding and quality control.This signal, either alone or in combination with specificproperties of the covalently attached polypeptide, deter-mines the fate of a glycoprotein in the ER. Accordingly,not all N-linked glycans of a given protein are used as suchsignals [8,9].

Sequential processing of the oligosaccharide yieldsspecific N-glycan structures that direct protein folding,export or degradationGlycan-dependent protein folding

Processing of Glc3Man9GlcNAc2 by glucosidase I Thetriglucosylated form of the protein-bound oligosaccharidehas a half-life of a few seconds. The outermost glucoseresidue (glucose n, Figure 1) might in fact be removedimmediately after addition of the oligosaccharide onto thepolypeptide nascent chain by glucosidase I. This a1,2exoglucosidase is a type II membrane protein member

of the glycosyl hydrolase (GH) family 63 [10] that isassociated with the translocon complex in close proximityto OST [11]. This observation supports the hypothesis thatglucosidase I processing contributes to the efficiency ofglycosylation by shifting the direction of the equilibriumof the OST reaction towards the product. So far, theGlc2Man9GlcNAc2 structure generated by glucosidase Ihas attracted little interest; indeed, it was considered tobe a transient trimming intermediate rapidly processed byglucosidase II to themore long-livedmonoglucosylated formof the protein-bound oligosaccharide. However, the recentcharacterization of malectin, a well conserved ER-residenttype I membrane protein as a putative Glc2-high mannose-binding lectin [12], hints at a possible function of thisprocessing intermediate of N-linked glycans in proteinbiogenesis and quality control (Figure 2).

Processing of Glc2Man9GlcNAc2 and of Glc1Man9

GlcNAc2 by glucosidase II: association with and release

from calnexin and calreticulin The glucose residuesm andl (Figure 1) are removed by glucosidase II. This a1,3exoglucosidase is a luminal member of the GH family 31and contains a catalytic a-subunit and a regulatoryb-subunit [10]. The b-subunit contains a mannose6-phosphate receptor homology domain (MRH) [13] andcomprises the conserved C-terminal -XDEL sequence thatretains the holoenzyme in the ER lumen. The a-subunit

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Figure 2. Several different oligosaccharide structures are displayed on immature cargo polypeptides located in the ER lumen. Most of the oligosaccharide structures, as

well as the glycosyl hydrolases involved in their generation, are conserved in mammals and in S. cerevisiae. Mammalian cells are characterized by the presence of a re-

glucosylating activity (UGT1) and a more extensive de-mannosylation of non-native polypeptides can occur, resulting in the removal of up to four a1,2-bonded mannose

residues by one or more members of the GH family 47 (Box 2). The right-hand column lists the signal encoded by the combination of a given N-glycan structure and the

associated polypeptide. Mammalian-specific features are highlighted with a grey background.


alone possesses hydrolytic activity toward p-nitrophenyl a-D-glucopyranoside [14,15], but the b-subunit is required forprocessing the core oligosaccharide [14].

The regulation of glucosidase II function is not fullyunderstood. In a fast and possibly concerted action withglucosidase I, glucosidase II first removes glucose m(Figure 1) and generates Glc1Man9GlcNAc2, the ligandof the ER-resident lectin chaperones calnexin and calreti-culin (Figure 2). Calnexin is a type I transmembraneprotein and calreticulin is its soluble homolog. They arecomposed of a globular carbohydrate-binding domain,which folds into a leguminous (L)-type lectin-like b-sand-wich structure, and a proline-rich P-domain that recruitsthe lectin-associated oxidoreductase ERp57 [16,17]. ERp57catalyses the formation of native disulfide bonds in foldingpolypeptides. The different membrane topology of the twoER lectins determines their substrate specificity. Themembrane-anchored calnexin preferentially interacts withpolypeptide-bound monoglucosylated glycans close to themembrane, whereas calreticulin prefers association with

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more peripheral glycans [18–20]. Reduction of the physicalconstraints upon release from calnexin/calreticulin/ERp57results in a structural collapse that often leads to attain-ment of the native polypeptide structure [21].

Uponrelease of the foldingpolypeptide fromcalnexinandcalreticulin, glucosidase II removes the innermost glucose l(Figure 1), thereby generating the Man9GlcNAc2 structure.Although both reactions catalysed by glucosidase II removean a1,3-linked glucose, the second cleavage event requiresthe transient separation and repositioning of the glucosi-dase II active site [22]. This time window is possiblyexploited by calnexin and calreticulin to associate withthe folding polypeptide [11]. When tested in isolated micro-someswith chains arrested in the translocon, glucosidase IIactivity progresses with high efficiency only when a secondN-linked glycan is available on the same polypeptide chain.It has been suggested that association of the b-subunit withthe 6’-tetramannosyl branch of an oligosaccharide allowsproper positioning of thea-subunit to cleave glucose l fromadistinct oligosaccharide in spatial proximity [11]. However,

Figure 3. Oligosaccharide preferences of ER-resident glycosyl transferase, glycosyl hydrolases, glucosyl transferase and lectins. The figure summarizes the transfer of the

core oligosaccharide on newly synthesized cargo polypeptides (OST), the progressive modification of the N-linked glycan by glycosyl hydrolases (glucosidases and

mannosidases) and a glucosyl transferase (UGT1). Different oligosaccharides act as ligands for a series of ER-localized lectins (malectin, CNX, CRT, cargo receptors, OS-9

and XTP3-B splice variants). A colour code shows arbitrary strength of affinities/activities of the given ER-resident sugar binding/modifying protein for a given

oligosaccharide structure (from white (no affinity/activity) to red (higher affinity/activity)). For cargo receptors and ERAD lectins see also Ref. [52].


both in vitro [14,23,24] and in vivo [25], the presence of asingleN-linked glycan is sufficient for glucosidase II activityand for substrate associationwith calnexin and calreticulin.The rapid nature of the concerted action of glucosidase I andglucosidase II in living cells allows immediate association ofpolypeptide chains emerging in theER lumenwith calnexin,calreticulin and ERp57, thereby resulting in co-transla-tional formation of native disulfide bonds [26,27].

Processing of Man9GlcNAc2 by UGT1: the calnexin/

calreticulin cycle Glycoproteins can fold properly after asingle association with a lectin chaperone, as observed inmammalian cells [28] and in Schizosaccharomyces pombe[29]. However, propermaturation of the glycoproteinmightrequire more than one association [30,31]. The ER ofmulticellular eukaryotes, of certain fungi (e.g. S. pombeand Mucor rouxii) and of unicellular eukaryotes such asTrypanosoma cruzi [32] contains a folding sensor, theUDP-glucose:glycoprotein glucosyltransferase (UGT1 orUGGT), a member of the glycosyl transferase family 24[10]. UGT1 consists of a large N-terminal domain thatbinds non-native protein structures and a C-terminalcarbohydrate transferase domain [33]. Thisbifunctionality allows UGT1 to ‘‘inspect’’ polypeptidesthat display Man9GlcNAc2 oligosaccharides and toexclusively re-glucosylate the terminal mannose g(Figure 1) in polypeptides that have a pseudo-nativeprotein structure [34,35]. It remains controversialwhether UGT1 activity requires the proximity of theoligosaccharide to be re-glucosylated with a structuraldefect in the immature polypeptide chain [36,37].Regeneration of the Glc1Man9GlcNAc2 oligosaccharide

results in substrate re-association with calnexin/calreticulin/ERp57 for another round of folding-attempts(Figure 2). The repeated removal and re-addition of glucosel by the counteracting actions of glucosidase II and UGT1drives cycles of substrate release and re-association withthe lectin chaperones, the so-called calnexin/calreticulincycle [38]. During the off-phase, N-glycans are exposed toER-resident a1,2-mannosidases belonging to the GHfamily 47 [39] that can remove, one-by-one, the terminala1,2-bonded mannose residues. Progressive N-glycan de-mannosylation renders the associated polypeptide aweaker ligand for calnexin/calreticulin [40], a bettersubstrate for glucosidase II [23] and a suboptimalsubstrate for UGT1 (Figure 3) [41]. Altogether, itfacilitates the interruption of folding attempts anddirects folding-defective polypeptides into the ER-associated degradation (ERAD) pathway [42,43].

Glycan-dependent export of native proteins from the ER

Cargo receptors: VIPL, ERGIC-53 and VIP36

Glycopolypeptides are offered a time window to completetheir folding program in the calnexin chaperone system[42]. Upon attainment of the native structure, the majorityof cargo proteins are exported from the ER in vesiclescoated with cytosolic coatamer protein II (COPII) thatbud at ER exit sites. In mammalian cells, thesetransport vesicles undergo homotypic fusion to generatea stationary ER–Golgi intermediate compartment(ERGIC) from which cargo proteins reach the cis-Golgiin COPI-coated vesicles [44]. By contrast, COPII-coatedcargo vesicles in yeast are delivered directly to the Golgicompartment [45]. Transmembrane proteins can interact

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Box 2. Oligosaccharide de-mannosylation in vitro and in vivo

Yeast Mns1p efficiently removes mannose i from Man9 oligosacchar-

ides to generate Man8B in vitro [39,88]. Although prolonged incuba-

tion (up to 24 h) with high concentrations of Mns1p results in the

generation of Man7 and Man6 sugars in vitro [88], this does not occur

in living cells where Mns1p generates only Man8B. In fact, further

trimming of protein-bound oligosaccharides in S. cerevisiae requires

Htm1p, the yeast ortholog of EDEM [56].

Mammalian ERManI also efficiently removes mannose i from Man9

oligosaccharides to generate Man8B in vitro [39,88]. In contrast to the

yeast protein, even after 24 hours incubation at non-physiologic

concentrations of enzyme, further oligosaccharide processing to

Man6-7 species by ERManI is negligible [88]. Consistently, over-

expression of ERManI in mammalian cells results in accumulation of

Man8 species [89]. We therefore propose that, like yeast, extensive

substrate de-mannosylation in mammalian cells requires interven-

tion by several members of the GH family 47. However, the identity of

the mannosidases supporting ERManI in extensive substrate de-

mannosylation remains a matter of controversy. Part of the

uncertainty stems from the lack of inhibitors that specifically

inactivate individual subclasses of the GH family 47 and from the

presence of multiple isoforms of EDEM and Golgi mannosidases that

could have redundant activities in the ER lumen, especially when

ectopically expressed. Based on the conservation of the yeast

machineries, EDEM proteins seem good candidates; however, no

in vitro glycanase activity has been detected so far. EDEM proteins

were initially described as mannosidase-like lectins devoid of

enzymatic activity because they lack two cysteines thought to be

conserved in all active a1,2-mannosidases [90–92] and because

substrate de-mannosylation is abolished in S. cerevisiae strains that

lack Mns1p [91]. However, more recent data show that: (i) the two

missing cysteines in EDEM proteins are dispensable for mannosi-

dase activity and are not conserved among a1,2-mannosidases [93];

(ii) the removal of mannose i by Mns1p is a prerequisite for further

oligosaccharide processing by Htm1 [56]; (iii) overexpression of

EDEM1 [65,94] or EDEM3 [89] enhances de-mannosylation of

folding-defective polypeptides in mammalian cells; (iv) EDEM

proteins conserve all catalytic residues required for enzymatic

activity, as well as the architecture of the active site and the residues

required to bind kifunensine, a specific inhibitor of a1,2-mannosi-

dases [39,95].


directlywith the cytosolicCOPII coat,whereas soluble cargoproteins can require specific receptors for recruitment intoCOPII-coatedvesicles [46].ERexportof certainglycosylatedproteins is facilitated by several leguminous L-type lectinslocated in the ER (VIPL), cycling between ER and ERGIC(ERGIC-53) or between ERGIC and cis-Golgi (VIP36)[47–49]. Emp47p and Emp46p, yeast orthologs of ERGIC-53, have beenproposed to act as cargo receptors between theER and the Golgi in S. cerevisiae [50,51].

It is significant to note that VIP36 and VIPL preferen-tially associate with native proteins displaying high-man-nose glycans with three mannose residues, but not glucose,on the oligosaccharide branch A (i.e. Man9 oligosacchar-ides, Man8 structures lacking the terminal mannose resi-due on branch B or the one on branch C and Man7

structures lacking both terminal mannoses on branchesB and C) (Figure 3) [49]. These cargo export lectins havemuch lower affinity for extensively de-mannosylated gly-cans that, consistently, serve to tag the associated poly-peptide for disposal rather than for export [52]. A thirdcargo receptor, ERGIC-53, binds with lower affinity to abroader range of oligosaccharides, even if they are cappedwith a terminal glucose residue (Figure 3) [49].

Glycan-dependent quality control and ERAD

Folding-defective polypeptides or components that do notincorporate into protein complexes must eventually becleared from the ER folding environment. In both S. cer-evisiae and higher eukaryotes, the importance of mannosetrimming for ERAD of misfolded glycoproteins has beenreported [9,53,54], supporting the suggestion of a mannosetimer mechanism [55]. The slowly acting ER a1,2-manno-sidase I (ERManI in mammals; Mns1p in yeast) wasproposed to operate as such a molecular timer, with theMan8GlcNAc2 isomer B generated upon removal of thea1,2-bonded mannose of the central oligosaccharidebranch acting as the N-glycan degradation signal if dis-played on terminallymisfolded polypeptides (Figure 2, Box2). More recent results suggest that more extensive de-mannosylation of misfolded polypeptides occurs [52] andthat, at least in yeast, different members of the GH family

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47 of a1,2-mannosidases intervene sequentially to gener-ate the ERAD signal [56]. In mammalian cells, the GHfamily 47 of a1,2-mannosidases comprises ERManI andthe ER degradation enhancer, mannosidase a-like proteinsEDEM1, EDEM2, EDEM3 (Htm1p in yeast) as well as thethree Golgi-resident MAN1A, MAN1B and MAN1C (Box 2and Box 3) [10].

Generating the degradation signal: processing of

Man8GlcNAc2 by members of the GH family 47 Recentanalyses in S. cerevisiae revealed that removal of at leasttwo a1,2-linked mannose residues from the protein-bound oligosaccharide is required for degradation ofglycoproteins. The N-glycan degradation signal is aterminal a1,6-linked mannose, generated by processingthe C branch (removal of mannose k, Figure 1) of aMan8GlcNAc2 glycan [56,57]. This reaction is catalysedby Htm1p (EDEM), which functionally depends on priorprocessing of theN-glycan by glucosidase I and II as well asMns1p (ERManI) [56]. Htm1p was found in a complex withthe oxidoreductase Pdi1p. Similar to the function proposedfor the EDEM1-associated mammalian reductase ERdj5[58], Pdi1p could be recruited to identify non-properlyfolded proteins as substrates for Htm1p or to promoteunfolding of ERAD substrates to facilitate theirdislocation across the ER membrane.

In higher eukaryotes, further processing of Man8-

GlcNAc2 was observed as well, and generation of Man7-

GlcNAc2, Man6GlcNAc2 and Man5GlcNAc2 glycanstructures was reported to precede or elicit degradation(Figure 2) [59–64]. Interestingly, in cell lines characterizedby N-glycosylation with aberrant oligosaccharides lackingthe cleavable mannose residues on branches B and C (e.g.the Chinese hamster ovary mutant lines B3F7 andMadIA214), processing of a1,2-linked mannose residuesis still required for glycoprotein disposal [64,65]. In thesecells, the only cleavable mannose residues are those on thebranch A that, whenmodified with a terminal a1,3-bondedglucose, determine polypeptide retention by the calnexinchaperone system. Extraction from calnexin/calreticulinis possible only by endomannosidase cleavage of branch

Box 3. De-mannosylation as a signal for disposal

In the model system S. cerevisiae, a terminal a1,6-linked mannose

generated by sequential removal of mannose i (by Mns1p) and k (by

Htm1p) (Figure 1) recruits the ERAD lectin Yos9p. Association of

misfolded polypeptides with Yos9p directs them to the site of

dislocation across the ER membrane, thus facilitating polypeptide

disposal [56,57]. Due to the presence of the calnexin/calreticulin cycle

and the existence of a multitude of ER-localized mannosidases, the

role of de-mannosylation and the subsequent recognition of ERAD

substrates in mammalian cells is less clear. Three different models

have been proposed:

Model 1. Removal of mannose i by ERManI generates the ERAD

signal

It was initially proposed that removal of mannose i (Figure 1) by

ERManI was required and sufficient to elicit disposal of folding-

defective polypeptides from the ER lumen. In this model, EDEM

proteins act as Man8-lectins to direct folding-defective polypeptides to

the ERAD pathway (Box 2) [91,92]

Model 2. Extensive substrate de-mannosylation by ERManI generates

the ERAD signal

Several reports show that misfolded polypeptides are subjected to

extensive de-mannosylation in the ER lumen [52]. ERManI shows

exquisite specificity for removing mannose i from oligosaccharides.

However, prolonged incubation with high concentrations of the

purified enzyme might result in more extensive de-mannosylation

(but refer to Box 2) [88]. It has been postulated that such conditions are

achieved in living cells upon enrichment of ERManI in the ER quality

control (ERQC) compartment [96]. In this model, EDEM proteins might

act as lectins and deliver ERAD substrates to the ERQC [52].

Model 3. Extensive substrate de-mannosylation by members of the

GH family 47 generates the ERAD signal

Extensive de-mannosylation of ERAD candidates requires the con-

certed and combined action of several members of the GH family 47;

namely, ERManI [60], and/or EDEM1-3 [65,89], and/or Golgi manno-

sidases MAN1A–C [97]. This scenario is closer to the model proposed

for S. cerevisiae, where the concerted intervention of Mns1p and

Htm1p eventually exposes the a1,6-bonded mannose j, which is a

ligand for the ERAD lectin Yos9p [56,57]. In mammalian cells, an

additional strong signal for disposal is represented by removal of

mannose g from the A branch of the protein-bound oligosaccharide

(Figure 1) [64,65]. As this is the only re-glucosylatable residue of

protein-bound oligosaccharides, its removal results in irreversible

polypeptide extraction from the calnexin cycle, thus interrupting futile

folding attempts.


A or by preventing entry into the cycle due to exomannosi-dase removal of the re-glucosylatable mannose g (Figure 1).Thesedata highlight the importance of extraction of folding-defective glycopolypeptides from the calnexin/calreticulinchaperone system, which becomes irreversible, only uponde-mannosylation of branch A [66]. Based on the temporalorder of N-glycan processing that is determined by thesubstrate specificity of the different glycosyl hydrolases(Figures 2 and 3), we propose that removal of the terminala1,2-linked mannose of the A branch ensures the exit fromthe calnexin/calreticulin cycle and is necessary, but notsufficient, for glycoprotein degradation. Additional trim-ming of the C branch generates the terminal misfoldingsignal, the a1,6-linked mannose, recognized by the humanER-degradation lectins OS-9 (Yos9p) and XTP3-B [67–69],which target the N-glycoprotein for ERAD (Figure 2).

Interpreting the degradation signal: ER lectins Yos9p,

OS-9 and XTP3-B The finding that the chemical structureof the N-glycan is important in the degradation ofmisfolded glycoproteins led to the proposal that a lectinreceptor is involved in the recognition of ERAD substrates

Box 4. Key unanswered questions

The generation of a glycan signal that displays the folding status of a

protein requires the recognition of unfolded or partially folded protein

domains. It appears that specific sensors (UGT1) or components of

the ER protein folding machinery are instrumental for this purpose.

How this recognition is translated into a covalent modification of a

defined oligosaccharide, however, is not fully understood.

With respect to the identification of central activities required for

the generation of the glycan signals, the latest results from the

Aebi and Weissman laboratories have established the role of yeast

EDEM as an a1,2-mannosidase that specifically removes mannose

k from oligosaccharides [56,57]. By contrast, the role of EDEM

proteins in mammalian ERAD remains a matter of some debate.

Their intervention as active mannosidases is questioned and a

demonstration of their activity in vitro is still awaited. Reports

showing that EDEM1 forms functional complexes with calnexin to

sequester misfolded polypeptides released from this ER lectin [98]

[54,70,71]. Yos9p and mammalian OS-9.1/OS-9.2 andXTP3-B1/XTP3-B2 constitute a group of proteins thathave lectin-like domains with homology to the mannose6-phosphate receptor family and are implicated in therecognition of misfolded glyoproteins for degradation[68,69]. S. cerevisiae Yos9p is part of the Hrd1p complex(named for HMG-CoA reductase degradation) to which it isbound via Hrd3p, a transmembrane protein with a largeluminal domain, where it performs a proof-reading orgating function [72,73]. Hrd3p recruits the misfoldedproteins while Yos9p scans the substrate for the correctN-glycan structure, signalling terminal misfolding.Determination of the N-glycan substrate specificity ofYos9p revealed that it binds N-glycans that have aterminal a1,6-linked mannose [57]. This findingcorroborates the working model in which this N-glycanstructure, generated by Htm1p (EDEM) [56], is the N-glycan signal for glycoprotein degradation in yeast.Mammalian OS-9 and XTP3-B splice variants were foundalso in large complexes containing the HRD1 E3 ubiquitinligase and SEL1L (orthologs of Hrd1p and Hrd3p,respectively) [67–69,74,75]. The components of this

and/or with ERdj5 to unfold terminally misfolded polypeptides [58]

and/or with derlins [99] and/or SEL1L [94] to deliver misfolded

polypeptides to dislocons in the ER membrane and/or with ERManI

[100], thereby stabilizing and activating this short-lived glycosyl

hydrolase are certainly of interest but need to be confirmed by

studying the involvement in such complexes of the endogenous

proteins. The reports showing that putative ERAD lectins, such as

EDEM1 [94] and OS-9 [69], use their lectin sites to form complexes

with components of the dislocation machinery (i.e. SEL1L) rather

than with misfolded proteins are of particular interest because they

open new questions about the actual role of protein-bound

oligosaccharides in ERAD.

Finally, it remains to be shown that the ERAD system is required for

the degradation of misfolded proteins (as used for the experimental

visualisation of the process) and for the maintenance of ER protein

homeostasis in the secretory network of eukaryotic cells.

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mammalian complex are similar to those in S. cerevisiae,suggesting evolutionary conservation. The lectin activity ofOS-9 is required for enhancement of glycoprotein ERADand, likeYos9p,OS-9 canspecifically bindN-glycans lackingthe terminal a1,2-linked mannose from the C branch(Figure 3) [67]. However, a study using lectin mutantssuggested that the MRH domains of OS-9 and XTP3-Bmight not be required for binding to ERAD substrates,but rather for interaction with SEL1L [69], which carriesseveral N-glycans (Box 4) [76,77].

Concluding remarksThe unique properties of the N-glycosylation process,namely the transfer of a large oligosaccharide structureto a well defined but short acceptor sequence, are essentialprerequisites for the use of the N-linked glycan as a uni-versal signal. The signal displays the folding status of aprotein and is interpreted by the quality control machineryof protein homeostasis in the ER. In the course of eukar-yotic evolution, a fine-tuned, universal system, based ondifferential processing and recognition of N-linked glycanshas evolved that, in combination with the folding factors inthe ER, ensures proper folding of a multitude of differentproteins and homeostasis of secretory proteins.

Interestingly, we observe a correlation between thephylogenetic origin of the glycan domains and their rolein the glycan-dependent protein-processing in the ER: TheA-branch of the N-linked glycan, phylogenetically old andmade in the cytoplasm, is used as a signal in the primaryprocess of glycan-assisted protein folding. This requiresthat N-glycosylation occurs before folding. The B- and theC-branch of theN-linked glycan (phylogenetically younger)are used for the secondary process of protein degradationand they are the basis for the generation of the ‘‘degra-dation signal’’. In this combination, the quality controlprocess in the ER represents a prime example of a codethat is embedded in the seemingly complex structures of N-linked glycans. Years after the original proposal by AriHelenius that N-linked glycans serve as signals in thequality control of protein folding [55], we are still in theprocess of deciphering this code, how the signals are gener-ated and how they are interpreted. We can expect novelfascinating discoveries along this way.

AcknowledgementsM.M. is supported by grants from the Foundation for Research onNeurodegenerative Diseases, the Fondazione San Salvatore, the SwissNational Center of Competence in Research on Neural Plasticity andRepair, the Swiss National Science Foundation, the Synapsis Foundationand the Bangerter-Rhyner Foundation. Work in the laboratory of M.A. issupported by the Swiss National Science Foundation and the ETHZurich.

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