Abstract

The endoplasmic reticulum (ER) quality control system ensures that newly synthesized proteins in the early secretory pathway are in the correct conformation. Polypeptides that have failed to fold into native conformers are subsequently retrotranslocated and degraded by the cytosolic ubiquitin–proteasome system, a process known as endoplasmic reticulum-associated degradation (ERAD). Most of the polypeptides that enter the ER are modified by the addition of N-linked oligosaccharides, and quality control of these glycoproteins is assisted by lectins that recognize specific sugar moieties and molecular chaperones that recognize unfolded proteins, resulting in proper protein folding and ERAD substrate selection. In Saccharomycescerevisiae, Yos9p, a lectin that contains a mannose 6-phosphate receptor homology (MRH) domain, was identified as an important component of ERAD. Yos9p was shown to associate with the membrane-embedded ubiquitin ligase complex, Hrd1p–Hrd3p, and provide a proofreading mechanism for ERAD. Meanwhile, the function of the mammalian homologues of Yos9p, OS-9 and XTP3-B remained elusive until recently. Recent studies have determined that both OS-9 and XTP3-B are ER resident proteins that associate with the HRD1–SEL1L ubiquitin ligase complex and are important for the regulation of ERAD. Moreover, recent studies have identified the N-glycan species with which both yeast Yos9p and mammalian OS-9 associate as M7A, a Man7GlcNAc2 isomer that lacks the α1,2-linked terminal mannose from both the B and C branches. M7A has since been demonstrated to be a degradation signal in both yeast and mammals.

Introduction

The endoplasmic reticulum (ER) is a eukaryotic intracellular organelle where most secretory and membrane proteins are synthesized. Upon entry into the ER through the Sec61 channel, nascent polypeptides can be modified by the addition of N-linked oligosaccharides on asparagines (Asn) in the consensus sequence Asn-Xaa-Ser/Thr (Xaa: any amino acid except Pro), and inter- and intramolecular disulphide bonds are introduced by oxidoreductases. Correct folding and oligomerization of the polypeptides are assisted and monitored by the ER quality control mechanism, in which only proteins that have acquired the proper conformation are sorted further through the secretory pathway (Ellgaard and Helenius 2003; van Anken and Braakman 2005; Anelli and Sitia 2008). Although cells and organisms have developed a refined network in the ER to support efficient and proper folding of diverse proteins, some portions of newly synthesized proteins still fail to obtain the correct structure. Moreover, under circumstances that evoke ER stress, misfolded proteins accumulate in the ER (Malhotra and Kaufman 2007; Ron and Walter 2007). In addition, several genetic mutations result in the generation of terminally misfolded polypeptides. Proteins that fail to acquire the correct conformation are subsequently degraded by a mechanism known as endoplasmic reticulum-associated degradation (ERAD) (Bonifacino and Weissman 1998; Vembar and Brodsky 2008). In this pathway, polypeptides destined for disposal are removed from the ER through a currently unidentified dislocon or retrotranslocation channel and ubiquitinated, followed by proteasomal degradation in the cytosol (Raasi and Wolf 2007; Hirsch et al. 2009).

During the process of protein synthesis and retention, glycoproteins in a nonnative conformation are recognized by molecular chaperones and lectins in the ER. This N-glycan-dependent quality control system was first identified as an essential step during the folding of glycoproteins in mammals (Hammond et al. 1994) and for ERAD in yeast (Knop et al. 1996). N-glycan synthesis consists of multiple steps that occur both at the cytoplasmic surface and inside the ER. Immediately after the transfer of a high-mannose oligosaccharide consisting of 14 sugar residues of Glc3Man9GlcNAc2 (G3M9) to the nascent polypeptides, glucose trimming is initiated (Trombetta and Parodi 2003; Helenius and Aebi 2004). Subsequently, glucose and mannose residues on the N-linked oligosaccharides are processed. To explain this enigmatic phenomenon, an attractive model was proposed in which N-glycans serve as tags for the quality control of various glycoproteins (Jakob et al. 1998; Helenius and Aebi 2001). The lectin chaperones calnexin and calreticulin recognize monoglucosylated N-glycans to facilitate the productive folding of glycoproteins (Schrag et al. 2003; Helenius and Aebi 2004; Caramelo and Parodi 2008), and the lectins ERGIC-53, VIP-L and VIP36 bind to high-mannose glycans and act as cargo receptors for ER–Golgi transport (Ito et al. 2005; Appenzeller-Herzog and Hauri 2006; Kato and Kamiya 2007). Mannose trimming of N-glycans plays an important role in glycoprotein ERAD (Cabral et al. 2001; Helenius and Aebi 2001). The Man8GlcNAc2 isomer B (M8B) on misfolded polypeptides has been proposed to act as the glycan signal for degradation in both budding yeast and mammals. Htm1p/Mnl1p in budding yeast and ER degradation enhancing α-mannosidase-like proteins (EDEMs) in mammals were originally cloned as potential lectins involved in ERAD, although their sugar-binding ability in vitro has not been confirmed (Herscovics et al. 2002; Kanehara et al. 2007; Olivari and Molinari 2007). In addition, the smaller M7, M6 and M5 N-glycans have also been characterized as degradation signals in the ER in mammalian cells (Lederkremer and Glickman 2005; Molinari 2007).

Yos9p was recently identified as a lectin involved in glycoprotein ERAD in Saccharomycescerevisiae (Buschhorn et al. 2004; Bhamidipati et al. 2005; Kim et al. 2005; Szathmary et al. 2005; reviewed by Kanehara et al. 2007), followed by the identification of OS-9 and XTP3-B as mammalian homologues. Recent analysis has revealed that the N-glycan recognized by mammalian OS-9 and yeast Yos9p is most probably M7A, an isomer of Man7GlcNAc2 that lacks the two terminal mannoses on both the B and C branches (Quan et al. 2008; Hosokawa et al. 2009). These findings shed light on the importance of the terminal mannose on the C branch of N-glycans. In this review, we focus on the recently identified function of mammalian OS-9 and XTP3-B in ERAD based on their N-glycan recognition specificities.

Mammalian mannose 6-phosphate receptor homology domain-containing lectins

In 2001, Munro identified six mannose 6-phosphate receptor homology (MRH) domain-containing lectins in the human genome (Munro 2001; Figure 1). Mannose 6-phosphate receptors (MPRs) are well characterized as receptors that sort lysosomal proteins through the recognition of N-glycans that have been modified with phosphomannosyl residues (Dahms et al. 1989; Ghosh et al. 2003). The crystal structure of the lectin domain of cation-dependent MPR (CD-MPR) was determined in complex with mannose 6-phosphate (Man-6-P) (Roberts et al. 1998); thus, the interaction of Man-6-P and its binding domain became clear. The MRH domain was defined by sequence homology to MPR, and proteins containing an MRH domain were proposed to act as lectins. In addition to four previously characterized proteins (CD-MPR, cation-independent MPR (CI-MPR), GlcNAc-phosphotransferase γ-subunit and ER glucosidase II β-subunit), two proteins of unknown function were also classified in this family: OS-9 and an expressed sequence tag (EST) clone, which was later named XTP3-B or Erlectin.

Fig. 1

MRH domain-containing proteins. The domain organization of six human MRH domain-containing proteins is presented. In the inset, yeast Yos9p and transcription variants of human OS-9 and XTP3-B are compared. The MRH domain is shown in light blue, the signal sequence in yellow, the transmembrane region in orange and the ER retrieval signal in green. The numbers in the inset designate the amino acid residue numbers. The segment in human OS-9 in gray is missing in variants 3 and 4. The short form of human XTP3-B lacks 54 amino acids between the two MRH domains and the six N-terminal amino acids of MRH domain 2 (indicated by dark blue) containing one of the conserved cysteines required for the tertiary structure formation.

Fig. 1

MRH domain-containing proteins. The domain organization of six human MRH domain-containing proteins is presented. In the inset, yeast Yos9p and transcription variants of human OS-9 and XTP3-B are compared. The MRH domain is shown in light blue, the signal sequence in yellow, the transmembrane region in orange and the ER retrieval signal in green. The numbers in the inset designate the amino acid residue numbers. The segment in human OS-9 in gray is missing in variants 3 and 4. The short form of human XTP3-B lacks 54 amino acids between the two MRH domains and the six N-terminal amino acids of MRH domain 2 (indicated by dark blue) containing one of the conserved cysteines required for the tertiary structure formation.

CI-MPR, also known as insulin-like growth factor-II (IGF-II), is a 300-kDa multifunctional protein that contains 15 MRH domain repeats (Dahms et al. 1989; Ghosh et al. 2003). In addition to transporting lysosomal proteins from the trans-Golgi network to the endosomes based on recognition of Man-6-P, CI-MPR also localizes to the plasma membrane and regulates Man-6-P-containing serum proteins and cytokines. Recently, the crystal structures of several domains of CI-MPR were determined, which revealed that they have tertiary structures similar to CD-MPR (Dahms et al. 2008; Kim et al. 2009).

GlcNAc-1-phosphotransferase catalyzes the transfer of GlcNAc-1-P to the mannose residues on N-glycans and is the first enzyme necessary for the creation of Man-6-P. It consists of six subunits: α2, β2 and γ2 (Bao et al. 1996). The α- and β-subunits have enzyme activity, whereas the γ-subunit (GNPTG), which contains the MRH domain, lacks catalytic activity and is believed to participate in substrate binding of the enzyme (Lee et al. 2007). However, the sugar-binding property of GNPTG is currently unknown.

Glucosidase II (GII) is an ER luminal protein that removes the α1,3-linked glucoses from N-glycans (Trombetta et al. 1996). Because this gene was previously cloned as a cDNA encoding a substrate for protein kinase C named protein kinase C substrate 80K-H (PRKCSH; Sakai et al. 1989), the MRH domain is also known as the PRKCSH domain. GII is an α-β heterodimer; the α chain (GIIα) is the catalytic subunit, while the β chain (GIIβ) contains the MRH domain as well as the C-terminal ER retrieval sequence (Pelham 1990). Therefore, GIIβ should retain the enzymatically active α-subunit in the ER (Trombetta et al. 1996). Recently, analysis of the sugar-binding specificity of the human GIIβ MRH domain revealed that the terminal α1,2-linked mannobiose on the B or C arm is required for binding (Hu et al. 2009). Consistent with this report, the GIIβ MRH domain in fission yeast has been reported to recognize the mannose residues on the B and C branches for glucose hydrolysis (Stigliano et al. 2009).

OS-9 was originally identified as one of the genes of unknown function amplified in osteosarcoma (Su et al. 1994, 1996; Kimura et al. 1997, 1998). OS-9 mRNA is expressed ubiquitously and is sometimes upregulated in human sarcomas. Localization studies of OS-9 indicated that the protein resides in the nucleus and cytosol or at the cytoplasmic surface of the ER (Nakayama et al. 1999; Litovchick et al. 2002; Baek et al. 2005; Wang et al. 2007). The intracellular localization of OS-9 suggests that its primary function is not likely to be in the ER. In addition, although OS-9 and XTP3-B both contain a stretch of hydrophobic residues at the N terminus, they lack the ER retrieval sequence at the C terminus (Su et al. 1996; Munro 2001).

The sequence of XTP3-B was submitted under the name XTP3-transactivated gene B (Wang et al. 2003) and was later designated Erlectin in a study of ER genes involved in Xenopus development (Cruciat et al. 2006). In this study, XTP3-B/Erlectin was shown to be an ER luminal protein that binds to a specific domain of the plasma membrane protein, Krm2, in an N-glycan-dependent manner in the ER and was suggested to regulate the glycoprotein traffic. Morpholino oligo injection revealed the significance of XTP3-B/Erlectin in Xenopus embryo development.

Yos9p is a yeast lectin that recognizes misfolded glycoproteins in ERAD

The first evidence of the contribution of MRH domain-containing lectins to ERAD came from the genome-wide screening of S. cerevisiae (Buschhorn et al. 2004). Buschhorn and coworkers demonstrated that Yos9p is a luminal, membrane-associated ER protein required for ERAD of glycoproteins but not of non-glycosylated proteins. Yos9p had been identified previously as a yeast homologue of human OS-9, sharing 12% overall identity and 20% identity in the N-terminal MRH domain (Friedmann et al. 2002). Unlike its mammalian homologue, Yos9p contains a carboxy-terminal tetrapeptide His-Asp-Glu-Leu (HDEL) ER retrieval signal (Munro 2001). Subsequent analyses confirmed that the lectin activity of Yos9p is necessary for ERAD of luminal glycoproteins (Bhamidipati et al. 2005; Kim et al. 2005; Szathmary et al. 2005). Yos9p does not bind to correctly folded carboxypeptidase Y (CPY) but does associate with misfolded glycoprotein CPY*, although it was unclear whether Yos9p interacts only with aberrant glycoproteins that bear specific N-glycans or if it also recognizes ERAD substrates that lack N-glycans.

Next, it was demonstrated that Yos9p forms a complex with the membrane-embedded ubiquitin ligase complex in the ER (Carvalho et al. 2006; Denic et al. 2006; Gauss et al. 2006). Hrd1p/Der3p is a multispanning transmembrane ubiquitin ligase (E3) with a cytosolic RING-H2 domain (Bays et al. 2001; Deak and Wolf 2001). The protein is stabilized through the formation of a complex with the type-I transmembrane protein, Hrd3p (Plemper et al. 1999; Gardner et al. 2000). Hrd3p has a relatively large luminal domain and has been confirmed to physically interact with Yos9p. The proposed mechanism whereby yeast Yos9p participates in ERAD is schematically illustrated in Figure 2A. ER quality control should be able to discriminate between nascent polypeptides and terminally misfolded conformers, both of which have exposed stretches of hydrophobic residues on their surfaces. To prevent the premature degradation of nascent polypeptides that have become mistakenly associated with Hrd3p, the polypeptides are reconfirmed by the proofreading activity of Yos9p, thereby ensuring that only unfolded polypeptides bearing specific N-glycans recognized by Yos9p are degraded. This model also explains why misfolded non-glycosylated proteins are retained undegraded in the ER. Alternatively, Yos9p may deliver the ERAD substrates directly to the membrane ubiquitin ligase complex for retrotranslocation.

Fig. 2

The function of the HRD1–SEL1L ubiquitin ligase holocomplex on the ER membrane. (A) The Hrd1–Hrd3 ubiquitin ligase holocomplex of S. cerevisiae. Hrd1/Der3, a multimembrane-spanning protein containing a ubiquitin ligase domain in the cytosol, forms a stoichiometric complex with the single transmembrane protein, Hrd3. Hrd1–Hrd3 forms a large complex with other transmembrane ERAD components and is connected to the cytosolic AAA ATPase Cdc48 complex. In the lumen of the ER, Yos9 and Kar2 associate with this ubiquitin ligase complex by binding to the large luminal domain of Hrd3. Unfolded portions of misfolded glycoproteins and non-glycosylated proteins are recognized by Hrd3. Thereafter, the sugar moiety on misfolded glycoproteins is surveyed by Yos9, and only those presenting an N-glycan degradation signal are recognized and sorted for retrotranslocation. Non-glycosylated proteins are retained in the ER. Note that not all of the components contained in the holocomplex are presented in this illustration. (B) The HRD1–SEL1L ubiquitin ligase holocomplex in mammals. Two models are proposed. Left: OS-9 and XTP3-B are stably incorporated in the complex on the ER membrane through interaction with SEL1L. The N-glycans on misfolded glycoproteins are recognized by the MRH domains of these lectins. Misfolded non-glycosylated proteins are also recognized by this membrane ubiquitin ligase holocomplex before retrotranslocation, most probably through BiP binding. Right: OS-9 and XTP3-B dynamically associate with the HRD1–SEL1L ubiquitin ligase complex on the ER membrane. Misfolded glycoproteins in the lumen are recognized by the OS9-GRP94 complex or XTP3-B and transferred to the membrane ubiquitin ligase complex. The association of SEL1L and MRH domain-containing lectins is mediated by the recognition of N-glycans on SEL1L by the MRH domains. Not all of the components of the holocomplex are depicted in this illustration. (C) A list of homologue proteins that are involved in the ubiquitin ligase holocomplex in S. cerevisiae and mammals.

Fig. 2

The function of the HRD1–SEL1L ubiquitin ligase holocomplex on the ER membrane. (A) The Hrd1–Hrd3 ubiquitin ligase holocomplex of S. cerevisiae. Hrd1/Der3, a multimembrane-spanning protein containing a ubiquitin ligase domain in the cytosol, forms a stoichiometric complex with the single transmembrane protein, Hrd3. Hrd1–Hrd3 forms a large complex with other transmembrane ERAD components and is connected to the cytosolic AAA ATPase Cdc48 complex. In the lumen of the ER, Yos9 and Kar2 associate with this ubiquitin ligase complex by binding to the large luminal domain of Hrd3. Unfolded portions of misfolded glycoproteins and non-glycosylated proteins are recognized by Hrd3. Thereafter, the sugar moiety on misfolded glycoproteins is surveyed by Yos9, and only those presenting an N-glycan degradation signal are recognized and sorted for retrotranslocation. Non-glycosylated proteins are retained in the ER. Note that not all of the components contained in the holocomplex are presented in this illustration. (B) The HRD1–SEL1L ubiquitin ligase holocomplex in mammals. Two models are proposed. Left: OS-9 and XTP3-B are stably incorporated in the complex on the ER membrane through interaction with SEL1L. The N-glycans on misfolded glycoproteins are recognized by the MRH domains of these lectins. Misfolded non-glycosylated proteins are also recognized by this membrane ubiquitin ligase holocomplex before retrotranslocation, most probably through BiP binding. Right: OS-9 and XTP3-B dynamically associate with the HRD1–SEL1L ubiquitin ligase complex on the ER membrane. Misfolded glycoproteins in the lumen are recognized by the OS9-GRP94 complex or XTP3-B and transferred to the membrane ubiquitin ligase complex. The association of SEL1L and MRH domain-containing lectins is mediated by the recognition of N-glycans on SEL1L by the MRH domains. Not all of the components of the holocomplex are depicted in this illustration. (C) A list of homologue proteins that are involved in the ubiquitin ligase holocomplex in S. cerevisiae and mammals.

OS-9 and XTP3-B are involved in mammalian ERAD

As is summarized in Figure 2C, most of the components of the ERAD pathway have orthologues in S. cerevisiae and mammals. Although a relatively clear model had been proposed to explain the mechanism by which Yos9p ensures glycoprotein quality control in yeast, it remained unknown whether the mammalian homologues of Yos9p also participated in ERAD. Recent works have now clarified the involvement of OS-9 and XTP3-B in this process (Bernasconi et al. 2008; Christianson et al. 2008; Hosokawa et al. 2008, 2009; Mueller et al. 2008; Alcock and Swanton 2009). The possible mechanisms proposed in these studies are summarized schematically in Figure 2B, although many issues must still be resolved. Overall, OS-9 functions in a manner analogous to yeast Yos9p. OS-9 is characterized as an ER luminal protein that interacts with the membrane-embedded ubiquitin ligase HRD1–SEL1L complex, which is the mammalian orthologue of the yeast Hrd1p–Hrd3p complex (Figure 2C). The primary binding site of OS-9 is in the luminal domain of SEL1L. OS-9 associates with misfolded proteins, but not correctly folded proteins, and is required for the ERAD of glycoproteins. Recent studies have shown that the degradation kinetics of ERAD substrates are affected by the overexpression or RNA interference-mediated knockdown of OS-9, although the current results are not completely consistent. Some reports indicate that the degradation of misfolded glycoproteins is partially inhibited by OS-9 knockdown (Bernasconi et al. 2008; Christianson et al. 2008; Hosokawa et al. 2009), but this effect seems to depend on the ERAD substrates (Christianson et al. 2008). However, overexpression of OS-9 does not seem to accelerate the ERAD of misfolded glycoproteins; rather, it delays (Bernasconi et al. 2008; Mueller et al. 2008) or has no effect (Hosokawa et al. 2009) on the rate of intracellular degradation. This stabilization effect on ERAD substrates may be caused by the binding of the substrates to overexpressed OS-9 that is not functionally associated with the HRD1–SEL1L complex. Furthermore, the significance of the lectin activity of the MRH domain of OS-9 in vivo is also controversial and likely depends on the levels of overexpressed OS-9 and its sugar-binding defective mutants (Bernasconi et al. 2008; Mueller et al. 2008; Hosokawa et al. 2009).

Two models of the function of OS-9 are suggested at present. In the first, OS-9 stably associates with the SEL1L–HRD1 complex (Hosokawa et al. 2008, 2009; Mueller et al. 2008; Figure 2B, left). In the second, OS-9 dynamically transfers the misfolded proteins to membrane-localized SEL1L (Christianson et al. 2008; Figure 2B, right). GRP94 is an ER resident chaperone protein of the HSP90 family that has no homologue in budding yeast. Christianson and colleagues have demonstrated that OS-9 binds to SEL1L and GRP94 in a mutually exclusive manner, suggesting that OS-9 transports misfolded luminal glycoproteins to the membrane dislocation machinery. Furthermore, it has also been shown that the MRH domains of both OS-9 and XTP3-B are required for association with SEL1L through recognition of the N-glycans on SEL1L (Christianson et al. 2008), since SEL1L is an ER resident glycoprotein with five N-glycosylation sites (Mueller et al. 2006). This result supports the hypothesis that OS-9 transports terminally misfolded polypeptides to the retrotranslocation machinery but, at the same time, revises the function of the MRH domain-containing protein, Yos9p (and possibly OS-9) to designate misfolded glycoproteins for dislocation by decoding the specific oligosaccharides on the substrates. On the other hand, our results support the hypothesis that the OS-9 MRH domain is necessary for the recognition of N-glycans on ERAD substrates rather than for SEL1L binding activity (Hosokawa et al. 2009).

It has also been reported that XTP3-B is involved in ERAD (Christianson et al. 2008; Hosokawa et al. 2008), but less is known about its function. There are two splice variants of XTP3-B (Figure 1, inset), and the short isoform lacks the N-terminal cysteine that is conserved in the MRH domain. The two XTP3-B isoforms seem to have different functions, and only the long splice variant is incorporated into the HRD1–SEL1L complex. The interaction of the XTP3-B long form and SEL1L may be mediated by the region between the two MRH domains or by an overall conformational change induced by this region (Hosokawa et al. 2008).

N-glycan recognition by mammalian OS-9

Thus far, the requirement for lectin activity of the OS-9 MRH domain in glycoprotein ERAD is controversial. To fully understand the function of OS-9, it is important to identify the oligosaccharide structures recognized by the OS-9 lectin domain. We have recently identified the oligosaccharide structures that the human OS-9 (hOS-9) MRH domain binds to in vitro (Hosokawa et al. 2009). To detect the relatively weak interaction between the sugar-binding domain of mammalian lectins and specific oligosaccharides, we used frontal affinity chromatography (FAC) analysis (Kasai et al. 1986; Kamiya et al. 2005, 2008; Figure 3B). A bacterially expressed recombinant hOS-9 MRH domain was immobilized on Ni2-Sepharose and packed into a small column. Pyridylaminated oligosaccharides were applied to this column, and the elution profile was monitored. The hOS-9 MRH domain clearly showed affinity for high-mannose N-glycans lacking the C branch terminal mannose (Figure 3C, upper panel). No specific binding was observed in Glc1Man9GlcNAc2 (G1M9) and Man9GlcNAc2 (M9), which are in the calnexin/calreticulin folding cycle (Helenius and Aebi 2001; Kamiya et al. 2009). When the three M8 isoforms were compared, it was clear that the hOS-9 MRH domain has a specific affinity for M8C, which lacks the C branch terminal mannose, but not for M8A or M8B, which lacks either the A or B branch terminal mannose, respectively.

Fig. 3

The specificities of N-glycans recognized by Yos9p and the hOS-9 MRH domain. (A) The structure of the N-linked oligosaccharide G3M9. The name of each branch and the glycosidic linkages between mannoses and glucoses are indicated. (B) A scheme representing detection of the lectin–glycan interaction by FAC. Purified lectins are immobilized on the column, and fluorescence-labeled glycans are applied. When a glycan (glycan B) interacts with the lectin, the elution of the glycan through the column is delayed compared to that of the glycans that do not bind to this lectin (glycan A). (C) The binding affinity of yeast Yos9p (upper panel; Quan et al. 2008) and the human OS-9 MRH domain (lower panel; Hosokawa et al. 2009) for each oligosaccharide. The affinity is indicated by the Ka value. Values are the mean of three independent experiments.

Fig. 3

The specificities of N-glycans recognized by Yos9p and the hOS-9 MRH domain. (A) The structure of the N-linked oligosaccharide G3M9. The name of each branch and the glycosidic linkages between mannoses and glucoses are indicated. (B) A scheme representing detection of the lectin–glycan interaction by FAC. Purified lectins are immobilized on the column, and fluorescence-labeled glycans are applied. When a glycan (glycan B) interacts with the lectin, the elution of the glycan through the column is delayed compared to that of the glycans that do not bind to this lectin (glycan A). (C) The binding affinity of yeast Yos9p (upper panel; Quan et al. 2008) and the human OS-9 MRH domain (lower panel; Hosokawa et al. 2009) for each oligosaccharide. The affinity is indicated by the Ka value. Values are the mean of three independent experiments.

We next analyzed the significance of N-glycan structures on ERAD substrates that are degraded through the OS-9 pathway in vivo. To modify the N-linked oligosaccharides on the misfolded α1-antitrypsin null Hong Kong variant (NHK), we overexpressed ER α1,2-mannosidase I (ER ManI) or EDEM3 in cultured HEK293 cells. In combination with OS-9 knockdown, we confirmed that degradation of NHK bearing M7 and M6 N-glycans created by overexpressed EDEM3 is dependent on OS-9, whereas the degradation of NHK with M8B and G1M8B N-glycans generated by transfected ER ManI is independent of OS-9 activity. Taken together, these findings indicate that the presence of N-glycans lacking the C branch terminal mannose is a structural requirement for OS-9 sugar recognition. This probably occurs most often in the form of M7A, an isomer that lacks the α1,2-linked terminal mannose from both the B and C branches (see below).

N-glycan recognition by yeast Yos9p

Recently, Quan and colleagues characterized the oligosaccharide structures that are recognized by yeast Yos9p in vivo and in vitro by FAC analysis of the sugar-binding specificity of Yos9p (Quan et al. 2008; Figure 3C, lower panel). Importantly, the oligosaccharides that Yos9p associates with are essentially the same as those recognized by the human OS-9 MRH domain, with the exception of an extraordinarily high binding affinity of Yos9p with the M5 glycan. To create an M7A glycan that lacks the two terminal mannoses on both the B and C branches in yeast cells, a glycosyltransferase was overexpressed in genetic mutants with a defective N-glycan synthesis pathway. Analysis of the degradation kinetics of CPY*, a well-characterized ERAD-luminal substrate glycoprotein in yeast (Wolf and Fink 1975; Finger et al. 1993), showed that CPY* with the M7A glycan is recognized by Yos9p and subjected to ERAD.

Furthermore, deletion of Htm1p/Mnl1p in these yeast strains demonstrated that Htm1p acts upstream of Yos9p, possibly as a processing α-mannosidase, and that Yos9p is essential for proofreading of N-glycans containing α1,6-linked mannose residues.

Significance of N-glycan trimming in ERAD

Once the specificities of N-glycans recognized by mammalian OS-9 and yeast Yos9p were characterized, the next question to be answered was which enzyme is responsible for the trimming of the terminal mannose on the C branch. In S. cerevisiae, the overexpression of Htm1p/Mnl1p demonstrated that it was the processing α1,2-mannosidase responsible for removal of the terminal mannose on the C branch, and this process occurs downstream of ER mannosidase I (Clerc et al. 2009). In yeast, terminal α1,6-linked mannose residues on N-glycans are generated by manipulating the Asn-linked glycosylation biosynthesis pathway, which modulates the N-glycan conformers on lipid-linked oligosaccharides (Burda et al. 1999). By transferring sugars bearing terminal α1,6-linked mannose residues to the misfolded glycoproteins, the mutant yeast does not require Htm1p/Mnl1p for the efficient degradation of the ERAD substrate, although Yos9p is still required (Quan et al. 2008; Clerc et al. 2009). These results support the idea that Htm1p/Mnl1p acts as the processing α-mannosidase that exposes the α1,6-linked mannose residue on the C branch, which is then recognized by Yos9p and degraded (Figure 4A). This also explains previous observations indicating that Yos9p and Htm1p/Mnl1p are genetically in the same pathway. These findings have helped to clarify the N-glycan degradation signal, the decoding lectin and the mannosidases responsible for signal creation in S. cerevisiae (Figure 4A).

Fig. 4

N-glycan processing and recognition by the lectins in the ER. (A) N-glycan processing and recognition in S. cerevisiae. Glucose residues from the G3M9 oligosaccharide attached to nascent polypeptides are sequentially processed by glucosidases I and II. Monoglucosylated N-linked oligosaccharide is recognized by Cne1p, the yeast homologue of calnexin. The terminal mannose on the middle branch of the N-glycan is trimmed by ER α1,2-mannosidase, generating M8B. Subsequently, Htm1p/Mnl1p processes the α1,2-mannose on the C branch of N-glycan to create M7A, which is then recognized by Yos9p as a degradation signal. Isomers of M8 and M7 are indicated in the inset. (B) N-glycan processing and recognition in mammals. Calnexin and calreticulin bind to monoglucosylated N-glycans and assist in the folding of glycoproteins. These lectins dissociate from the glycoprotein upon removal of the terminal glucose on monoglucosylated N-glycans by glucosidase II. In mammals, UDP-glucose:glycoprotein glucosyltransferase (UGGT), which is absent from the yeast genome, adds a glucose back onto the N-glycan if the glycoprotein has still not acquired the native conformation, thus forming the calnexin/calreticulin or monoglucose cycle. ER ManI generates M8B, followed by further mannose trimming to produce M7A, M6 and M5 N-glycans, which are recognized by OS-9. The candidate α1,2-mannosidases responsible for this process include EDEM3, ER ManI concentrated in the ER quality control compartment (ERQC), and Golgi α1,2-mannosidases for the ERAD substrates that may recycle between the ER and Golgi prior to degradation.

Fig. 4

N-glycan processing and recognition by the lectins in the ER. (A) N-glycan processing and recognition in S. cerevisiae. Glucose residues from the G3M9 oligosaccharide attached to nascent polypeptides are sequentially processed by glucosidases I and II. Monoglucosylated N-linked oligosaccharide is recognized by Cne1p, the yeast homologue of calnexin. The terminal mannose on the middle branch of the N-glycan is trimmed by ER α1,2-mannosidase, generating M8B. Subsequently, Htm1p/Mnl1p processes the α1,2-mannose on the C branch of N-glycan to create M7A, which is then recognized by Yos9p as a degradation signal. Isomers of M8 and M7 are indicated in the inset. (B) N-glycan processing and recognition in mammals. Calnexin and calreticulin bind to monoglucosylated N-glycans and assist in the folding of glycoproteins. These lectins dissociate from the glycoprotein upon removal of the terminal glucose on monoglucosylated N-glycans by glucosidase II. In mammals, UDP-glucose:glycoprotein glucosyltransferase (UGGT), which is absent from the yeast genome, adds a glucose back onto the N-glycan if the glycoprotein has still not acquired the native conformation, thus forming the calnexin/calreticulin or monoglucose cycle. ER ManI generates M8B, followed by further mannose trimming to produce M7A, M6 and M5 N-glycans, which are recognized by OS-9. The candidate α1,2-mannosidases responsible for this process include EDEM3, ER ManI concentrated in the ER quality control compartment (ERQC), and Golgi α1,2-mannosidases for the ERAD substrates that may recycle between the ER and Golgi prior to degradation.

In mammals, we propose that the N-glycan degradation signal is essentially the same as in budding yeast (Figure 4B). In addition to M7A, M6 and M5 glycans are also recognized as degradation signals in mammals (Lederkremer and Glickman 2005). However, the processing α-mannosidases responsible for the exposure of α1,6-linked mannose on the C branch in mammals are still intriguing. ER ManI concentrated in the ER quality control compartment, a specialized subcompartment of the ER where ERAD machinery and substrates are enriched, has been shown to play an important role in this process (Avezov et al. 2008). We believe that EDEM3 also participates in the removal of mannoses, since the increase of M7, M6 and M5 N-glycans on misfolded NHK in ER ManI-overexpressing cells is relatively small compared to the increase seen in EDEM3-overexpressing cells (Hosokawa et al. 2003; Hirao et al. 2006). At present, the enzyme activity of EDEM3 has not been demonstrated in vitro, and the specificity of the mannose residues that are processed by EDEM3 is not clear. We have recently shown that EDEM1 is also capable of trimming mannose on the C branch of N-glycans (Hosokawa et al. 2010). Based on the sequence similarity of the glycosidase domains of EDEM1 and EDEM3, we believe that EDEM3 is responsible for trimming the α1,2-linked mannose on the C branch during ERAD. On the other hand, the ability of EDEM1 to process A branch mannose has also been reported (Olivari et al. 2006). Further analysis is required to identify the specific α-mannosidases responsible for this trimming.

Concluding remarks and remaining questions

The mammalian MRH domain-containing lectins OS-9 and XTP3-B have recently been implicated as regulators of the degradation of misfolded proteins. Both proteins are confirmed to reside in the ER, and some, but not all, physically interact with SEL1L through incorporation into a large complex centered on ubiquitin ligase HRD1–SEL1L. Thus, OS-9 and XTP3-B functionally cooperate with the degradation machinery to process misfolded proteins. Furthermore, the oligosaccharide specificities of mammalian OS-9 and yeast Yos9p have been characterized, suggesting that the primary glycan signal for glycoprotein ERAD in both yeast and mammals is M7A lacking the terminal mannoses on the B and C branches, as well as isomers of M6 and M5 (Figure 4).

Because the functional analyses of mammalian OS-9 and XTP3-B in ERAD are very recent, many questions remain unanswered. First, do OS-9 and XTP3-B bind to the N-glycans on ERAD substrates or to those on SEL1L? Second, for both OS-9 and XTP3-B, substrate specificity seems to exist. How are misfolded glycoproteins that are not recognized by OS-9/XTP3-B degraded? Furthermore, free oligosaccharides exist in the cytosol, and most of them are believed to originate from the N-glycans of ERAD substrates (Suzuki and Funakoshi 2006; Chantret and Moore 2008). In wild-type S. cerevisiae, the free oligosaccharide that is predominantly detected in the cytosol is Man8GlcNAc2 (Chantret et al. 2003). The major free oligosaccharides detected in the cytosol of human HepG2 cells treated with swainsonine, an inhibitor of cytosolic α-mannosidase, are Man9GlcNAc and Man8BGlcNAc. This suggests that ERAD substrates bearing M9 and M8 glycans are removed from the ER (Yanagida et al. 2006). If these are true, it must also be determined whether these N-glycans are removed via a Yos9p/OS-9 independent pathway. Third, in S. cerevisiae, ERAD substrates that lack N-glycans are usually stabilized, whereas in mammals, the NHK-QQQ mutant that is deficient of N-glycosylation sites is more rapidly degraded than N-glycosylated NHK (Hirao et al. 2006). However, misfolded glycoproteins and aberrant non-glycosylated proteins are surveyed by MRH domain-containing lectin(s) and the Hrd1p–Hrd3p/HRD1–SEL1L ubiquitin ligase complex in both yeast and mammals. How is this difference in the degradation of glycosylated and non-glycosylated proteins between yeast and mammals explained? Can it be attributed to unidentified components or proteins of undetermined function included in the complex? Furthermore, substrate binding and dislocation enhancement seem to be separate steps. How are these steps regulated at the molecular level?

The oligosaccharide structures that are recognized by XTP3-B have not been clarified yet. It remains to be determined whether the two MRH domains of XTP3-B have a repertoire similar to that of the lectin domain of OS-9. The functional difference between OS-9 and XTP3-B also remains unclear. Are they redundant, or do they have discrete functions in ERAD? Are there functional differences among the four OS-9 transcription variants? It has been reported that Yos9p binds to non-glycosylated proteins to recognize the bipartite signal on misfolded glycoproteins. Similarly, the binding of OS-9 and XTP3-B to non-glycosylated ERAD substrates has been reported in mammals (Bernasconi et al. 2008; Christianson et al. 2008; Hosokawa et al. 2008, 2009). Based on the recent report showing that recognition of unfolded polypeptides is mediated by Kar2 in yeast (Xie et al. 2009), it will be interesting to determine whether mammalian OS-9 and XTP3-B recognize unfolded polypeptide directly or in cooperation with BiP. Finally, which enzymes are responsible for the processing of the C branch mannose in ERAD in mammals? N-glycan profiles from total glycoproteins in cells overexpressing each of the three EDEM proteins (EDEM 1, 2 and 3) are different, and EDEM2 appears to lack enzyme activity (Mast et al. 2005). The discovery that Yos9p and OS-9/XTP3-B recognize specific N-glycan signals has advanced our understanding of the ER quality control process, but many questions remain to be answered.

Funding

This work was supported by Hayashi Memorial Foundation for Female Natural Scientists (N.H.) and by grants from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (N.H. [19GS0314], Y.K. [21870052], and K.K. [21370050, 20107004]).

Abbreviations

  • CD-MPR

    cation-dependent MPR

  • CI-MPR

    cation-independent MPR

  • CPY

    carboxypeptidase Y

  • EDEM

    ER degradation enhancing α-mannosidase-like protein

  • ER

    endoplasmic reticulum

  • ER ManI

    ER α1,2-mannosidase I

  • ERAD

    ER-associated (protein) degradation

  • FAC

    frontal affinity chromatography

  • G3M9

    Glc3Man9GlcNAc2

  • GII

    Glucosidase II

  • GNPTG

    GlcNAc-1-phosphotransferaseγ-subunit

  • hOS-9

    human OS-9

  • M9

    Man9GlcNAc2

  • M7A

    Man7GlcNAc2 isomer A

  • M7C

    Man7GlcNAc2 isomer C

  • M8A

    Man8GlcNAc2 isomer A

  • M8B

    Man8GlcNAc2 isomer B

  • M8C

    Man8GlcNAc2 isomer C

  • Man-6-P

    mannose 6-phosphate

  • MPR

    mannose-6-phosphate receptor

  • MRH domain

    mannose 6-phosphate receptor homology domain

  • NHK

    α1-antitrypsin null (Hong Kong)

  • PRKCSH

    protein kinase C substrate 80K-H

References

Alcock
F
Swanton
E
Mammalian OS-9 is upregulated in response to endoplasmic reticulum stress and facilitates ubiquitination of misfolded glycoproteins
J Mol Biol
 , 
2009
, vol. 
385
 (pg. 
1032
-
1042
)
Anelli
T
Sitia
R
Protein quality control in the early secretory pathway
Embo J
 , 
2008
, vol. 
27
 (pg. 
315
-
327
)
Appenzeller-Herzog
C
Hauri
HP
The ER-Golgi intermediate compartment (ERGIC): in search of its identity and function
J Cell Sci
 , 
2006
, vol. 
119
 (pg. 
2173
-
2183
)
Avezov
E
Frenkel
Z
Ehrlich
M
Herscovics
A
Lederkremer
GZ
Endoplasmic reticulum (ER) mannosidase I is compartmentalized and required for N-glycan trimming to Man5-6GlcNAc2 in glycoprotein ER-associated degradation
Mol Biol Cell
 , 
2008
, vol. 
19
 (pg. 
216
-
225
)
Baek
JH
Mahon
PC
Oh
J
Kelly
B
Krishnamachary
B
Pearson
M
Chan
DA
Giaccia
AJ
Semenza
GL
OS-9 interacts with hypoxia-inducible factor 1alpha and prolyl hydroxylases to promote oxygen-dependent degradation of HIF-1alpha
Mol Cell
 , 
2005
, vol. 
17
 (pg. 
503
-
512
)
Bao
M
Booth
JL
Elmendorf
BJ
Canfield
WM
Bovine UDP-N-acetylglucosamine:lysosomal-enzyme N-acetylglucosamine-1-phosphotransferase. I. Purification and subunit structure
J Biol Chem
 , 
1996
, vol. 
271
 (pg. 
31437
-
31445
)
Bays
NW
Gardner
RG
Seelig
LP
Joazeiro
CA
Hampton
RY
Hrd1p/Der3p is a membrane-anchored ubiquitin ligase required for ER-associated degradation
Nat Cell Biol
 , 
2001
, vol. 
3
 (pg. 
24
-
29
)
Bernasconi
R
Pertel
T
Luban
J
Molinari
M
A dual task for the Xbp1-responsive OS-9 variants in the mammalian endoplasmic reticulum: inhibiting secretion of misfolded protein conformers and enhancing their disposal
J Biol Chem
 , 
2008
, vol. 
283
 (pg. 
16446
-
16454
)
Bhamidipati
A
Denic
V
Quan
EM
Weissman
JS
Exploration of the topological requirements of ERAD identifies Yos9p as a lectin sensor of misfolded glycoproteins in the ER lumen
Mol Cell
 , 
2005
, vol. 
19
 (pg. 
741
-
751
)
Bonifacino
JS
Weissman
AM
Ubiquitin and the control of protein fate in the secretory and endocytic pathways
Annu Rev Cell Dev Biol
 , 
1998
, vol. 
14
 (pg. 
19
-
57
)
Burda
P
Jakob
CA
Beinhauer
J
Hegemann
JH
Aebi
M
Ordered assembly of the asymmetrically branched lipid-linked oligosaccharide in the endoplasmic reticulum is ensured by the substrate specificity of the individual glycosyltransferases
Glycobiology
 , 
1999
, vol. 
9
 (pg. 
617
-
625
)
Buschhorn
BA
Kostova
Z
Medicherla
B
Wolf
DH
A genome-wide screen identifies Yos9p as essential for ER-associated degradation of glycoproteins
FEBS Lett
 , 
2004
, vol. 
577
 (pg. 
422
-
426
)
Cabral
CM
Liu
Y
Sifers
RN
Dissecting glycoprotein quality control in the secretory pathway
Trends Biochem Sci
 , 
2001
, vol. 
26
 (pg. 
619
-
624
)
Caramelo
JJ
Parodi
AJ
Getting in and out from calnexin/calreticulin cycles
J Biol Chem
 , 
2008
, vol. 
283
 (pg. 
10221
-
10225
)
Carvalho
P
Goder
V
Rapoport
TA
Distinct ubiquitin–ligase complexes define convergent pathways for the degradation of ER proteins
Cell
 , 
2006
, vol. 
126
 (pg. 
361
-
373
)
Chantret
I
Frenoy
JP
Moore
SE
Free-oligosaccharide control in the yeast Saccharomyces cerevisiae: roles for peptide:N-glycanase (Png1p) and vacuolar mannosidase (Ams1p)
Biochem J
 , 
2003
, vol. 
373
 (pg. 
901
-
908
)
Chantret
I
Moore
SE
Free oligosaccharide regulation during mammalian protein N-glycosylation
Glycobiology
 , 
2008
, vol. 
18
 (pg. 
210
-
224
)
Christianson
JC
Shaler
TA
Tyler
RE
Kopito
RR
OS-9 and GRP94 deliver mutant alpha1-antitrypsin to the Hrd1-SEL1L ubiquitin ligase complex for ERAD
Nat Cell Biol
 , 
2008
, vol. 
10
 (pg. 
272
-
282
)
Clerc
S
Hirsch
C
Oggier
DM
Deprez
P
Jakob
C
Sommer
T
Aebi
M
Htm1 protein generates the N-glycan signal for glycoprotein degradation in the endoplasmic reticulum
J Cell Biol
 , 
2009
, vol. 
184
 (pg. 
159
-
172
)
Cruciat
CM
Hassler
C
Niehrs
C
The MRH protein Erlectin is a member of the endoplasmic reticulum synexpression group and functions in N-glycan recognition
J Biol Chem
 , 
2006
, vol. 
281
 (pg. 
12986
-
12993
)
Dahms
NM
Lobel
P
Kornfeld
S
Mannose 6-phosphate receptors and lysosomal enzyme targeting
J Biol Chem
 , 
1989
, vol. 
264
 (pg. 
12115
-
12118
)
Dahms
NM
Olson
LJ
Kim
JJ
Strategies for carbohydrate recognition by the mannose 6-phosphate receptors
Glycobiology
 , 
2008
, vol. 
18
 (pg. 
664
-
678
)
Deak
PM
Wolf
DH
Membrane topology and function of Der3/Hrd1p as a ubiquitin-protein ligase (E3) involved in endoplasmic reticulum degradation
J Biol Chem
 , 
2001
, vol. 
276
 (pg. 
10663
-
10669
)
Denic
V
Quan
EM
Weissman
JS
A luminal surveillance complex that selects misfolded glycoproteins for ER-associated degradation
Cell
 , 
2006
, vol. 
126
 (pg. 
349
-
359
)
Ellgaard
L
Helenius
A
Quality control in the endoplasmic reticulum
Nat Rev Mol Cell Biol
 , 
2003
, vol. 
4
 (pg. 
181
-
191
)
Finger
A
Knop
M
Wolf
DH
Analysis of two mutated vacuolar proteins reveals a degradation pathway in the endoplasmic reticulum or a related compartment of yeast
Eur J Biochem
 , 
1993
, vol. 
218
 (pg. 
565
-
574
)
Friedmann
E
Salzberg
Y
Weinberger
A
Shaltiel
S
Gerst
JE
YOS9, the putative yeast homolog of a gene amplified in osteosarcomas, is involved in the endoplasmic reticulum (ER)-Golgi transport of GPI-anchored proteins
J Biol Chem
 , 
2002
, vol. 
277
 (pg. 
35274
-
35281
)
Gardner
RG
Swarbrick
GM
Bays
NW
Cronin
SR
Wilhovsky
S
Seelig
L
Kim
C
Hampton
RY
Endoplasmic reticulum degradation requires lumen to cytosol signaling. Transmembrane control of Hrd1p by Hrd3p
J Cell Biol
 , 
2000
, vol. 
151
 (pg. 
69
-
82
)
Gauss
R
Jarosch
E
Sommer
T
Hirsch
C
A complex of Yos9p and the HRD ligase integrates endoplasmic reticulum quality control into the degradation machinery
Nat Cell Biol
 , 
2006
, vol. 
8
 (pg. 
849
-
854
)
Ghosh
P
Dahms
NM
Kornfeld
S
Mannose 6-phosphate receptors: new twists in the tale
Nat Rev Mol Cell Biol
 , 
2003
, vol. 
4
 (pg. 
202
-
212
)
Hammond
C
Braakman
I
Helenius
A
Role of N-linked oligosaccharide recognition, glucose trimming, and calnexin in glycoprotein folding and quality control
Proc Natl Acad Sci U S A
 , 
1994
, vol. 
91
 (pg. 
913
-
917
)
Helenius
A
Aebi
M
Intracellular functions of N-linked glycans
Science
 , 
2001
, vol. 
291
 (pg. 
2364
-
2369
)
Helenius
A
Aebi
M
Roles of N-linked glycans in the endoplasmic reticulum
Annu Rev Biochem
 , 
2004
, vol. 
73
 (pg. 
1019
-
1049
)
Herscovics
A
Romero
PA
Tremblay
LO
The specificity of the yeast and human class I ER alpha 1, 2-mannosidases involved in ER quality control is not as strict previously reported
Glycobiology
 , 
2002
, vol. 
12
 (pg. 
14G
-
15G
)
Hirao
K
Natsuka
Y
Tamura
T
Wada
I
Morito
D
Natsuka
S
Romero
P
Sleno
B
Tremblay
LO
Herscovics
A
Nagata
K
Hosokawa
N
EDEM3, a soluble EDEM homolog, enhances glycoprotein endoplasmic reticulum-associated degradation and mannose trimming
J Biol Chem
 , 
2006
, vol. 
281
 (pg. 
9650
-
9658
)
Hirsch
C
Gauss
R
Horn
SC
Neuber
O
Sommer
T
The ubiquitylation machinery of the endoplasmic reticulum
Nature
 , 
2009
, vol. 
458
 (pg. 
453
-
460
)
Hosokawa
N
Kamiya
Y
Kamiya
D
Kato
K
Nagata
K
Human OS-9, a lectin required for glycoprotein endoplasmic reticulum-associated degradation, recognizes mannose-trimmed N-glycans
J Biol Chem
 , 
2009
, vol. 
284
 (pg. 
17061
-
17068
)
Hosokawa
N
Tremblay
LO
Sleno
B
Kamiya
Y
Wada
I
Nagata
K
Kato
K
Herscovics
A
EDEM1 accelerates the trimming of α1,2-linked mannose on the C branch of N-glycans
Glycobiology
 , 
2010
 
in press
Hosokawa
N
Tremblay
LO
You
Z
Herscovics
A
Wada
I
Nagata
K
Enhancement of endoplasmic reticulum (ER) degradation of misfolded Null Hong Kong alpha1-antitrypsin by human ER mannosidase I
J Biol Chem
 , 
2003
, vol. 
278
 (pg. 
26287
-
26294
)
Hosokawa
N
Wada
I
Nagasawa
K
Moriyama
T
Okawa
K
Nagata
K
Human XTP3-B forms an endoplasmic reticulum quality control scaffold with the HRD1-SEL1L ubiquitin ligase complex and BiP
J Biol Chem
 , 
2008
, vol. 
283
 (pg. 
20914
-
20924
)
Hu
D
Kamiya
Y
Totani
K
Kamiya
D
Kawasaki
N
Yamaguchi
D
Matsuo
I
Matsumoto
N
Ito
Y
Kato
K
Yamamoto
K
Sugar-binding activity of the MRH domain in the ER alpha-glucosidase II beta subunit is important for efficient glucose trimming
Glycobiology
 , 
2009
, vol. 
19
 (pg. 
1127
-
1135
)
Ito
Y
Hagihara
S
Matsuo
I
Totani
K
Structural approaches to the study of oligosaccharides in glycoprotein quality control
Curr Opin Struct Biol
 , 
2005
, vol. 
15
 (pg. 
481
-
489
)
Jakob
CA
Burda
P
Roth
J
Aebi
M
Degradation of misfolded endoplasmic reticulum glycoproteins in Saccharomyces cerevisiae is determined by a specific oligosaccharide structure
J Cell Biol
 , 
1998
, vol. 
142
 (pg. 
1223
-
1233
)
Kamiya
Y
Kamiya
D
Urade
R
Suzuki
T
Kato
K
Powell
G
McCabe
O
Sophisticated modes of sugar recognition by intracellular lectins involved in quality control of glycoporteins
Glycobiology Research Trends
 , 
2009
New York
Nova Science Publishers, Inc
(pg. 
27
-
40
)
Kamiya
Y
Kamiya
D
Yamamoto
K
Nyfeler
B
Hauri
HP
Kato
K
Molecular basis of sugar recognition by the human L-type lectins ERGIC-53, VIPL, and VIP36
J Biol Chem
 , 
2008
, vol. 
283
 (pg. 
1857
-
1861
)
Kamiya
Y
Yamaguchi
Y
Takahashi
N
Arata
Y
Kasai
K
Ihara
Y
Matsuo
I
Ito
Y
Yamamoto
K
Kato
K
Sugar-binding properties of VIP36, an intracellular animal lectin operating as a cargo receptor
J Biol Chem
 , 
2005
, vol. 
280
 (pg. 
37178
-
37182
)
Kanehara
K
Kawaguchi
S
Ng
DT
The EDEM and Yos9p families of lectin-like ERAD factors
Semin Cell Dev Biol
 , 
2007
, vol. 
18
 (pg. 
743
-
750
)
Kasai
K
Oda
Y
Nishikawa
M
Ishii
S
Frontal affinity chromatoraphy: theory for its application to studies on specofoc interactions of biomolecules
J Chromatogr
 , 
1986
, vol. 
376
 (pg. 
33
-
47
)
Kato
K
Kamiya
Y
Structural views of glycoprotein-fate determination in cells
Glycobiology
 , 
2007
, vol. 
17
 (pg. 
1031
-
1044
)
Kim
JJ
Olson
LJ
Dahms
NM
Carbohydrate recognition by the mannose-6-phosphate receptors
Curr Opin Struct Biol
 , 
2009
, vol. 
19
 (pg. 
534
-
542
)
Kim
W
Spear
ED
Ng
DT
Yos9p detects and targets misfolded glycoproteins for ER-associated degradation
Mol Cell
 , 
2005
, vol. 
19
 (pg. 
753
-
764
)
Kimura
Y
Nakazawa
M
Tsuchiya
N
Asakawa
S
Shimizu
N
Yamada
M
Genomic organization of the OS-9 gene amplified in human sarcomas
J Biochem
 , 
1997
, vol. 
122
 (pg. 
1190
-
1195
)
Kimura
Y
Nakazawa
M
Yamada
M
Cloning and characterization of three isoforms of OS-9 cDNA and expression of the OS-9 gene in various human tumor cell lines
J Biochem (Tokyo)
 , 
1998
, vol. 
123
 (pg. 
876
-
882
)
Knop
M
Hauser
N
Wolf
DH
N-Glycosylation affects endoplasmic reticulum degradation of a mutated derivative of carboxypeptidase yscY in yeast
Yeast
 , 
1996
, vol. 
12
 (pg. 
1229
-
1238
)
Lederkremer
GZ
Glickman
MH
A window of opportunity: timing protein degradation by trimming of sugars and ubiquitins
Trends Biochem Sci
 , 
2005
, vol. 
30
 (pg. 
297
-
303
)
Lee
WS
Payne
BJ
Gelfman
CM
Vogel
P
Kornfeld
S
Murine UDP-GlcNAc:lysosomal enzyme N-acetylglucosamine-1-phosphotransferase lacking the gamma-subunit retains substantial activity toward acid hydrolases
J Biol Chem
 , 
2007
, vol. 
282
 (pg. 
27198
-
27203
)
Litovchick
L
Friedmann
E
Shaltiel
S
A selective interaction between OS-9 and the carboxyl-terminal tail of meprin beta
J Biol Chem
 , 
2002
, vol. 
277
 (pg. 
34413
-
34423
)
Malhotra
JD
Kaufman
RJ
The endoplasmic reticulum and the unfolded protein response
Semin Cell Dev Biol
 , 
2007
, vol. 
18
 (pg. 
716
-
731
)
Mast
SW
Diekman
K
Karaveg
K
Davis
A
Sifers
RN
Moremen
KW
Human EDEM2, a novel homolog of family 47 glycosidases, is involved in ER-associated degradation of glycoproteins
Glycobiology
 , 
2005
, vol. 
15
 (pg. 
421
-
436
)
Molinari
M
N-glycan structure dictates extension of protein folding or onset of disposal
Nat Chem Biol
 , 
2007
, vol. 
3
 (pg. 
313
-
320
)
Mueller
B
Klemm
EJ
Spooner
E
Claessen
JH
Ploegh
HL
SEL1L nucleates a protein complex required for dislocation of misfolded glycoproteins
Proc Natl Acad Sci U S A
 , 
2008
, vol. 
105
 (pg. 
12325
-
12330
)
Mueller
B
Lilley
BN
Ploegh
HL
SEL1L, the homologue of yeast Hrd3p, is involved in protein dislocation from the mammalian ER
J Cell Biol
 , 
2006
, vol. 
175
 (pg. 
261
-
270
)
Munro
S
The MRH domain suggests a shared ancestry for the mannose 6-phosphate receptors and other N-glycan-recognising proteins
Curr Biol
 , 
2001
, vol. 
11
 (pg. 
R499
-
501
)
Nakayama
T
Yaoi
T
Kuwajima
G
Yoshie
O
Sakata
T
Ca2()-dependent interaction of N-copine, a member of the two C2 domain protein family, with OS-9, the product of a gene frequently amplified in osteosarcoma
FEBS Lett
 , 
1999
, vol. 
453
 (pg. 
77
-
80
)
Olivari
S
Cali
T
Salo
KE
Paganetti
P
Ruddock
LW
Molinari
M
EDEM1 regulates ER-associated degradation by accelerating de-mannosylation of folding-defective polypeptides and by inhibiting their covalent aggregation
Biochem Biophys Res Commun
 , 
2006
, vol. 
349
 (pg. 
1278
-
1284
)
Olivari
S
Molinari
M
Glycoprotein folding and the role of EDEM1, EDEM2 and EDEM3 in degradation of folding-defective glycoproteins
FEBS Lett
 , 
2007
, vol. 
581
 (pg. 
3658
-
3664
)
Pelham
HR
The retention signal for soluble proteins of the endoplasmic reticulum
Trends Biochem Sci
 , 
1990
, vol. 
15
 (pg. 
483
-
486
)
Plemper
RK
Bordallo
J
Deak
PM
Taxis
C
Hitt
R
Wolf
DH
Genetic interactions of Hrd3p and Der3p/Hrd1p with Sec61p suggest a retro-translocation complex mediating protein transport for ER degradation
J Cell Sci
 , 
1999
, vol. 
112
 
Pt 22
(pg. 
4123
-
4134
)
Quan
EM
Kamiya
Y
Kamiya
D
Denic
V
Weibezahn
J
Kato
K
Weissman
JS
Defining the glycan destruction signal for endoplasmic reticulum-associated degradation
Mol Cell
 , 
2008
, vol. 
32
 (pg. 
870
-
877
)
Raasi
S
Wolf
DH
Ubiquitin receptors and ERAD: a network of pathways to the proteasome
Semin Cell Dev Biol
 , 
2007
, vol. 
18
 (pg. 
780
-
791
)
Roberts
DL
Weix
DJ
Dahms
NM
Kim
JJ
Molecular basis of lysosomal enzyme recognition: three-dimensional structure of the cation-dependent mannose 6-phosphate receptor
Cell
 , 
1998
, vol. 
93
 (pg. 
639
-
648
)
Ron
D
Walter
P
Signal integration in the endoplasmic reticulum unfolded protein response
Nat Rev Mol Cell Biol
 , 
2007
, vol. 
8
 (pg. 
519
-
529
)
Sakai
K
Hirai
M
Minoshima
S
Kudoh
J
Fukuyama
R
Shimizu
N
Isolation of cDNAs encoding a substrate for protein kinase C: nucleotide sequence and chromosomal mapping of the gene for a human 80K protein
Genomics
 , 
1989
, vol. 
5
 (pg. 
309
-
315
)
Schrag
JD
Procopio
DO
Cygler
M
Thomas
DY
Bergeron
JJ
Lectin control of protein folding and sorting in the secretory pathway
Trends Biochem Sci
 , 
2003
, vol. 
28
 (pg. 
49
-
57
)
Stigliano
ID
Caramelo
JJ
Labriola
CA
Parodi
AJ
D'Alessio
C
Glucosidase II beta subunit modulates N-glycan trimming in fission yeasts and mammals
Mol Biol Cell
 , 
2009
, vol. 
20
 (pg. 
3974
-
3984
)
Su
YA
Hutter
CM
Trent
JM
Meltzer
PS
Complete sequence analysis of a gene (OS-9) ubiquitously expressed in human tissues and amplified in sarcomas
Mol Carcinog
 , 
1996
, vol. 
15
 (pg. 
270
-
275
)
Su
YA
Trent
JM
Guan
XY
Meltzer
PS
Direct isolation of genes encoded within a homogeneously staining region by chromosome microdissection
Proc Natl Acad Sci U S A
 , 
1994
, vol. 
91
 (pg. 
9121
-
9125
)
Suzuki
T
Funakoshi
Y
Free N-linked oligosaccharide chains: formation and degradation
Glycoconj J
 , 
2006
, vol. 
23
 (pg. 
291
-
302
)
Szathmary
R
Bielmann
R
Nita-Lazar
M
Burda
P
Jakob
CA
Yos9 protein is essential for degradation of misfolded glycoproteins and may function as lectin in ERAD
Mol Cell
 , 
2005
, vol. 
19
 (pg. 
765
-
775
)
Trombetta
ES
Parodi
AJ
Quality control and protein folding in the secretory pathway
Annu Rev Cell Dev Biol
 , 
2003
, vol. 
19
 (pg. 
649
-
676
)
Trombetta
ES
Simons
JF
Helenius
A
Endoplasmic reticulum glucosidase II is composed of a catalytic subunit, conserved from yeast to mammals, and a tightly bound noncatalytic HDEL-containing subunit
J Biol Chem
 , 
1996
, vol. 
271
 (pg. 
27509
-
27516
)
van Anken
E
Braakman
I
Versatility of the endoplasmic reticulum protein folding factory
Crit Rev Biochem Mol Biol
 , 
2005
, vol. 
40
 (pg. 
191
-
228
)
Vembar
SS
Brodsky
JL
One step at a time: endoplasmic reticulum-associated degradation
Nat Rev Mol Cell Biol
 , 
2008
, vol. 
9
 (pg. 
944
-
957
)
Wang
Y
Fu
X
Gaiser
S
Kottgen
M
Kramer-Zucker
A
Walz
G
Wegierski
T
OS-9 regulates the transit and polyubiquitination of TRPV4 in the endoplasmic reticulum
J Biol Chem
 , 
2007
, vol. 
282
 (pg. 
36561
-
36570
)
Wolf
DH
Fink
GR
Proteinase C (carboxypeptidase Y) mutant of yeast
J Bacteriol
 , 
1975
, vol. 
123
 (pg. 
1150
-
1156
)
Xie
W
Kanehara
K
Sayeed
A
Ng
DT
Intrinsic conformational determinants signal protein misfolding to the Hrd1/Htm1 endoplasmic reticulum-associated degradation system
Mol Biol Cell
 , 
2009
, vol. 
20
 (pg. 
3317
-
3329
)
Yanagida
K
Natsuka
S
Hase
S
Structural diversity of cytosolic free oligosaccharides in the human hepatoma cell line, HepG2
Glycobiology
 , 
2006
, vol. 
16
 (pg. 
294
-
304
)