Interactions of substrate with calreticulin, an endoplasmic reticulum chaperone

Glycan structure-based perspectives on the entry and release of glycoproteins in the calnexin/calreticulin cycle

N-glycans are attached to newly synthesised polypeptides and are involved in the folding, secretion, and degradation of N-linked glycoproteins. In particular, the calnexin/calreticulin cycle, which is the central mechanism of the entry and release of N-linked glycoproteins depending on the folding sates, has been well studied. In addition to biological studies on the calnexin/calreticulin cycle, several studies have revealed complementary roles of in vitro chemistry-based research in the structure-based understanding of the cycle. In this mini-review, we summarise chemistry-based results and highlight their importance for further understanding of the cycle.

The Role of Endoplasmic Reticulum Chaperones in Protein Folding and Quality Control

Molecular chaperones assist the folding of nascent chains in the cell. Chaperones also aid in quality control decisions as persistent chaperone binding can help to sort terminal misfolded proteins for degradation. There are two major molecular chaperone families in the endoplasmic reticulum (ER) that assist proteins in reaching their native structure and evaluating the fidelity of the maturation process. The ER Hsp70 chaperone, BiP, supports adenine nucleotide-regulated binding to non-native proteins that possess exposed hydrophobic regions. In contrast, the carbohydrate-dependent chaperone system involving the membrane protein calnexin and its soluble paralogue calreticulin recognize a specific glycoform of an exposed hydrophilic protein modification for which the composition is controlled by a series of glycosidases and transferases. Here, we compare and contrast the properties, mechanisms of action and functions of these different chaperones systems that work in parallel, as well as together, to assist a large variety of substrates that traverse the eukaryotic secretory pathway.

Folding and Quality Control of Glycoproteins

Folding of proteins is essential so that they can exert their functions. For proteins that transit the secretory pathway, folding occurs in the endoplasmic reticulum (ER) and various chaperone systems assist in acquiring their correct folding/subunit formation. N-glycosylation is one of the most conserved posttranslational modification for proteins, and in eukaryotes it occurs in the ER. Consequently, eukaryotic cells have developed various systems that utilize N-glycans to dictate and assist protein folding, or if they consistently fail to fold properly, to destroy proteins for quality control and the maintenance of homeostasis of proteins in the ER.

Calnexin cycle - structural features of the ER chaperone system

The endoplasmic reticulum (ER) is the major folding compartment for secreted and membrane proteins and is the site of a specific chaperone system, the calnexin cycle, for folding N-glycosylated proteins. Recent structures of components of the calnexin cycle have deepened our understanding of quality control mechanisms and protein folding pathways in the ER. In the calnexin cycle, proteins carrying monoglucosylated glycans bind to the lectin chaperones calnexin and calreticulin, which recruit a variety of function-specific chaperones to mediate protein disulfide formation, proline isomerization, and general protein folding. Upon trimming by glucosidase II, the glycan without an inner glucose residue is no longer able to bind to the lectin chaperones. For proteins that have not yet folded properly, the enzyme UDP-glucose:glycoprotein glucosyltransferase (UGGT) acts as a checkpoint by adding a glucose back to the N-glycan. This allows the misfolded proteins to re-associate with calnexin and calreticulin for additional rounds of chaperone-mediated refolding and prevents them from exiting the ERs. Here, we review progress in structural studies of the calnexin cycle, which reveal common features of how lectin chaperones recruit function-specific chaperones and how UGGT recognizes misfolded proteins.

Intracellular Transport and Cytotoxicity of the Protein Toxin Ricin

Ricin can be isolated from the seeds of the castor bean plant (Ricinus communis). It belongs to the ribosome-inactivating protein (RIP) family of toxins classified as a bio-threat agent due to its high toxicity, stability and availability. Ricin is a typical A-B toxin consisting of a single enzymatic A subunit (RTA) and a binding B subunit (RTB) joined by a single disulfide bond. RTA possesses an RNA N-glycosidase activity; it cleaves ribosomal RNA leading to the inhibition of protein synthesis. However, the mechanism of ricin-mediated cell death is quite complex, as a growing number of studies demonstrate that the inhibition of protein synthesis is not always correlated with long term ricin toxicity. To exert its cytotoxic effect, ricin A-chain has to be transported to the cytosol of the host cell. This translocation is preceded by endocytic uptake of the toxin and retrograde traffic through the trans-Golgi network (TGN) and the endoplasmic reticulum (ER). In this article, we describe intracellular trafficking of ricin with particular emphasis on host cell factors that facilitate this transport and contribute to ricin cytotoxicity in mammalian and yeast cells. The current understanding of the mechanisms of ricin-mediated cell death is discussed as well. We also comment on recent reports presenting medical applications for ricin and progress associated with the development of vaccines against this toxin.

Toxins Utilize the Endoplasmic Reticulum-Associated Protein Degradation Pathway in Their Intoxication Process

Several bacterial and plant AB-toxins are delivered by retrograde vesicular transport to the endoplasmic reticulum (ER), where the enzymatically active A subunit is disassembled from the holotoxin and transported to the cytosol. In this process, toxins subvert the ER-associated degradation (ERAD) pathway. ERAD is an important part of cellular regulatory mechanism that targets misfolded proteins to the ER channels, prior to their retrotranslocation to the cytosol, ubiquitination and subsequent degradation by a protein-degrading complex, the proteasome. In this article, we present an overview of current understanding of the ERAD-dependent transport of AB-toxins to the cytosol. We describe important components of ERAD and discuss their significance for toxin transport. Toxin recognition and disassembly in the ER, transport through ER translocons and finally cytosolic events that instead of overall proteasomal degradation provide proper folding and cytotoxic activity of AB-toxins are discussed as well. We also comment on recent reports presenting medical applications for toxin transport through the ER channels.

Novel molecular mechanism of cellular transformation by a mutant molecular chaperone in myeloproliferative neoplasms

Deregulation of the cytokine‐receptor signaling pathway plays a significant role in tumorigenesis. Such deregulation is frequently caused by alterations in the genes involved in the signaling pathway. At the end of 2013, recurrent somatic mutations in the calreticulin (CALR) gene that encodes a molecular chaperone were identified in a subset of patients with Philadelphia‐chromosome negative myeloproliferative neoplasms (MPN). The present review focuses on the role of CALR mutations in the oncogenic transformations observed in MPN. All the CALR mutations were found to generate a + 1 frameshift in the reading frame on exon 9, which encodes the carboxy (C)‐terminus end of CALR, and thus conferred a common mutant‐specific sequence in all the CALR mutants. The mutant CALR (but not the wild‐type) constitutively activates the thrombopoietin (TPO) receptor, myeloproliferative leukemia protein (MPL), even in the absence of TPO to induce cellular transformation. Preferential interaction between the mutant CALR and MPL is achieved by a presumptive conformational change induced by the mutant‐specific C‐terminus domain, which allows N‐domain binding to MPL. Even though mutant CALR is expressed on the cell surface and is secreted out of cells, it only presents autocrine capacity for MPL activation. These findings define a novel molecular mechanism by which the mutant molecular chaperone constitutively activates the cytokine receptor to induce cellular transformation.

Substrate specificity in the context of molecular chaperones

Molecular chaperones are one of the key players in protein biology and as such their structure and mechanism of action have been extensively studied. However the substrate specificity of molecular chaperones has not been well investigated. This review aims to summarize what is known about the substrate specificity and substrate recognition motifs of chaperones so as to better understand what substrate specificity means in the context of molecular chaperones. Available literature shows that the majority of chaperones have broad substrate range and recognize non-native conformations of proteins depending on recognition of hydrophobic and/or charged patches. Based on these recognition motifs chaperones can select for early, mid or late folding intermediates. Another major contributor to chaperone specificity are the co-chaperones they interact with as well as the sub-cellular location they are expressed in and the inducability of their expression. Some chaperones which have only one or a few known substrates are reported. In their case the mode of recognition seems to be specific structural complementarity between chaperone and substrate. It can be concluded that the vast majority of chaperones do not show a high degree of specificity but recognize elements that signal non-native protein conformation and their substrate range is modulated by the context they function in. However a few chaperones are known that display exquisite specificity of their substrate e.g. mammalian heat shock protein 47 collagen interaction. © 2017 IUBMB Life, 69(9):647-659, 2017.

Structures of parasite calreticulins provide insights into their flexibility and dual carbohydrate/peptide-binding properties

Calreticulin (CRT) is a multifaceted protein, initially discovered as an endoplasmic reticulum (ER) chaperone protein, that is essential in calcium metabolism. Various implications in cancer, early development and immunology have been discovered more recently for CRT, as well as its role as a dominant 'eat-me' prophagocytic signal. Intriguingly, cell-surface exposure/secretion of CRT is among the infective strategies used by parasites such as Trypanosoma cruzi, Entamoeba histolytica, Taenia solium, Leishmania donovani and Schistosoma mansoni. Because of the inherent flexibility of CRTs, their analysis by X-ray crystallography requires the design of recombinant constructs suitable for crystallization, and thus only the structures of two very similar mammalian CRT lectin domains are known. With the X-ray structures of two distant parasite CRTs, insights into species structural determinants that might be harnessed to fight against the parasites without affecting the functions of the host CRT are now provided. Moreover, although the hypothesis that CRT can exhibit both open and closed conformations has been proposed in relation to its chaperone function, only the open conformation has so far been observed in crystal structures. The first evidence is now provided of a complex conformational transition with the junction reoriented towards P-domain closure. SAXS experiments also provided additional information about the flexibility of T. cruzi CRT in solution, thus complementing crystallographic data on the open conformation. Finally, regarding the conserved lectin-domain structure and chaperone function, evidence is provided of its dual carbohydrate/protein specificity and a new scheme is proposed to interpret such unusual substrate-binding properties. These fascinating features are fully consistent with previous experimental observations, as discussed considering the broad spectrum of CRT sequence conservations and differences.

N-Glycan-based ER Molecular Chaperone and Protein Quality Control System: The Calnexin Binding Cycle

Helenius and colleagues proposed over 20-years ago a paradigm-shifting model for how chaperone binding in the endoplasmic reticulum was mediated and controlled for a new type of molecular chaperone- the carbohydrate-binding chaperones, calnexin and calreticulin. While the originally established basics for this lectin chaperone binding cycle holds true today, there has been a number of important advances that have expanded our understanding of its mechanisms of action, role in protein homeostasis, and its connection to disease states that are highlighted in this review.

Regulation of calreticulin-major histocompatibility complex (MHC) class I interactions by ATP

The MHC class I peptide loading complex (PLC) facilitates the assembly of MHC class I molecules with peptides, but factors that regulate the stability and dynamics of the assembly complex are largely uncharacterized. Based on initial findings that ATP, in addition to MHC class I-specific peptide, is able to induce MHC class I dissociation from the PLC, we investigated the interaction of ATP with the chaperone calreticulin, an endoplasmic reticulum (ER) luminal, calcium-binding component of the PLC that is known to bind ATP. We combined computational and experimental measurements to identify residues within the globular domain of calreticulin, in proximity to the high-affinity calcium-binding site, that are important for high-affinity ATP binding and for ATPase activity. High-affinity calcium binding by calreticulin is required for optimal nucleotide binding, but both ATP and ADP destabilize enthalpy-driven high-affinity calcium binding to calreticulin. ATP also selectively destabilizes the interaction of calreticulin with cellular substrates, including MHC class I molecules. Calreticulin mutants that affect ATP or high-affinity calcium binding display prolonged associations with monoglucosylated forms of cellular MHC class I, delaying MHC class I dissociation from the PLC and their transit through the secretory pathway. These studies reveal central roles for ATP and calcium binding as regulators of calreticulin-substrate interactions and as key determinants of PLC dynamics.

A sweet code for glycoprotein folding

Glycoprotein synthesis is initiated in the endoplasmic reticulum (ER) lumen upon transfer of a glycan (Glc3Man9GlcNAc2) from a lipid derivative to Asn residues (N-glycosylation). N-Glycan-dependent quality control of glycoprotein folding in the ER prevents exit to Golgi of folding intermediates, irreparably misfolded glycoproteins and incompletely assembled multimeric complexes. It also enhances folding efficiency by preventing aggregation and facilitating formation of proper disulfide bonds. The control mechanism essentially involves four components, resident lectin-chaperones (calnexin and calreticulin) that recognize monoglucosylated polymannose protein-linked glycans, lectin-associated oxidoreductase acting on monoglucosylated glycoproteins (ERp57), a glucosyltransferase that creates monoglucosylated epitopes in protein-linked glycans (UGGT) and a glucosidase (GII) that removes the glucose units added by UGGT. This last enzyme is the only mechanism component sensing glycoprotein conformations as it creates monoglucosylated glycans exclusively in not properly folded glycoproteins or in not completely assembled multimeric glycoprotein complexes. Glycoproteins that fail to properly fold are eventually driven to proteasomal degradation in the cytosol following the ER-associated degradation pathway, in which the extent of N-glycan demannosylation by ER mannosidases play a relevant role in the identification of irreparably misfolded glycoproteins.

The intrinsic and extrinsic effects of N-linked glycans on glycoproteostasis

Proteins that traffic through the eukaryotic secretory pathway are commonly modified with N-linked carbohydrates. These bulky amphipathic modifications at asparagines intrinsically enhance solubility and folding energetics through carbohydrate-protein interactions. N-linked glycans can also extrinsically enhance glycoprotein folding by using the glycoprotein homeostasis or 'glycoproteostasis' network, which comprises numerous glycan binding and/or modification enzymes or proteins that synthesize, transfer, sculpt and use N-linked glycans to direct folding and trafficking versus degradation and trafficking of nascent N-glycoproteins through the cellular secretory pathway. If protein maturation is perturbed by misfolding, aggregation or both, stress pathways are often activated that result in transcriptional remodeling of the secretory pathway in an attempt to alleviate the insult (or insults). The inability to achieve glycoproteostasis is linked to several pathologies, including amyloidoses, cystic fibrosis and lysosomal storage diseases. Recent progress on genetic and pharmacologic adaptation of the glycoproteostasis network provides hope that drugs of this mechanistic class can be developed for these maladies in the near future.

Biophysical properties of UDP-glucose:glycoprotein glucosyltransferase, a folding sensor enzyme in the ER, delineated by synthetic probes

UDP-glucose:glycoprotein glucosyltransferase plays a key role in glycoprotein quality control in the endoplasmic reticulum, by virtue of its ability to discriminate folding states. Although lines of evidence have clarified the ability of UGGT to recognize a partially unfolded protein, its mechanistic rationale has been obscure. In this study, the substrate recognition mechanism of UGGT was studied using synthetic substrate of UGGT. Although UGGT has high extent of surface hydrophobicity, it clearly lacks property of typical molecular chaperones. Furthermore, it was revealed that the addition of the substrate caused secondary structure change of UGGT in a dose-dependent manner, resulting that the K(d) value of the UGGT-substrate interaction was estimated from theoretical formula based on 1:1 complexation between UGGT and the acceptor substrate. Moreover, the kinetic analysis of glucosyltransferase activity of UGGT elucidated Michaelis constant K(m) correctly.

Sialyl Lewis(x): a "pre-organized water oligomer"?

Organized and released: Sialyl Lewis(x) (sLe(x)) represents a "pre-organized water oligomer", that is, a surrogate for clustered water molecules attached to a scaffold. The impetus for sLe(x) binding to E-selectin is shown to be the high degree of pre-organization allowing an array of directed hydrogen bonds, and the entropic benefit of the release of water molecules from the large binding interface to bulk water (see picture).

Molecular and structural basis for N-glycan-dependent determination of glycoprotein fates in cells

BACKGROUND

N-linked oligosaccharides operate as tags for protein quality control, consigning glycoproteins to different fates, i.e. folding in the endoplasmic reticulum (ER), vesicular transport between the ER and the Golgi complex, and ER-associated degradation of glycoproteins, by interacting with a panel of intracellular lectins in the early secretory pathway.

SCOPE OF REVIEW

This review summarizes the current state of knowledge regarding the molecular and structural basis for glycoprotein-fate determination in cells that is achieved through the actions of the intracellular lectins and its partner proteins.

MAJOR CONCLUSIONS

Cumulative frontal affinity chromatography (FAC) data demonstrated that the intracellular lectins exhibit distinct sugar-binding specificity profiles. The glycotopes recognized by these lectins as fate determinants are embedded in the triantennary structures of the high-mannose-type oligosaccharides and are exposed upon trimming of the outer glucose and mannose residues during the N-glycan processing pathway. Furthermore, recently emerged 3D structural data offer mechanistic insights into functional interplay between an intracellular lectin and its binding partner in the early secretory pathway.

GENERAL SIGNIFICANCE

Structural biology approaches in conjunction with FAC methods provide atomic pictures of the mechanisms behind the glycoprotein-fate determination in cells. This article is a part of a Special issue entitled: Glycoproteomics.

Convergent synthesis of homogeneous Glc1Man9GlcNAc2-protein and derivatives as ligands of molecular chaperones in protein quality control

A detailed understanding of the molecular mechanism of chaperone-assisted protein quality control is often hampered by the lack of well-defined homogeneous glycoprotein probes. We describe here a highly convergent chemoenzymatic synthesis of the monoglucosylated glycoforms of bovine ribonuclease (RNase) as specific ligands of lectin-like chaperones calnexin (CNX) and calreticulin (CRT) that are known to recognize the monoglucosylated high-mannose oligosaccharide component of glycoproteins in protein folding. The synthesis of a selectively modified glycoform Gal(1)Glc(1)Man(9)GlcNAc(2)-RNase was accomplished by chemical synthesis of a large N-glycan oxazoline and its subsequent enzymatic ligation to GlcNAc-RNase under the catalysis of a glycosynthase. Selective removal of the terminal galactose by a β-galactosidase gave the Glc(1)Man(9)GlcNAc(2)-RNase glycoform in excellent yield. CD spectroscopic analysis and RNA-hydrolyzing assay indicated that the synthetic RNase glycoforms maintained essentially the same global conformations and were fully active as the natural bovine ribonuclease B. SPR binding studies revealed that the Glc(1)Man(9)GlcNAc(2)-RNase had high affinity to lectin CRT, while the synthetic Man(9)GlcNAc(2)-RNase glycoform and natural RNase B did not show CRT-binding activity. These results confirmed the essential role of the glucose moiety in the chaperone molecular recognition. Interestingly, the galactose-masked glycoform Gal(1)Glc(1)Man(9)GlcNAc(2)-RNase also showed significant affinity to lectin CRT, suggesting that a galactose β-1,4-linked to the key glucose moiety does not significantly block the lectin binding. These synthetic homogeneous glycoprotein probes should be valuable for a detailed mechanistic study on how molecular chaperones work in concert to distinguish between misfolded and folded glycoproteins in the protein quality control cycle.

Structural and functional relationships between the lectin and arm domains of calreticulin

Calreticulin and calnexin are key components in maintaining the quality control of glycoprotein folding within the endoplasmic reticulum. Although their lectin function of binding monoglucosylated sugar moieties of glycoproteins is well documented, their chaperone activity in suppressing protein aggregation is less well understood. Here, we use a series of deletion mutants of calreticulin to demonstrate that its aggregation suppression function resides primarily within its lectin domain. Using hydrophobic peptides as substrate mimetics, we show that aggregation suppression is mediated through a single polypeptide binding site that exhibits a K(d) for peptides of 0.5-1 μM. This site is distinct from the oligosaccharide binding site and differs from previously identified sites of binding to thrombospondin and GABARAP (4-aminobutyrate type A receptor-associated protein). Although the arm domain of calreticulin was incapable of suppressing aggregation or binding hydrophobic peptides on its own, it did contribute to aggregation suppression in the context of the whole molecule. The high resolution x-ray crystal structure of calreticulin with a partially truncated arm domain reveals a marked difference in the relative orientations of the arm and lectin domains when compared with calnexin. Furthermore, a hydrophobic patch was detected on the arm domain that mediates crystal packing and may contribute to calreticulin chaperone function.

The interactions of calreticulin with immunoglobulin G and immunoglobulin Y

Calreticulin is a chaperone of the endoplasmic reticulum (ER) assisting proteins in achieving the correctly folded structure. Details of the binding specificity of calreticulin are still a matter of debate. Calreticulin has been described as an oligosaccharide-binding chaperone but data are also accumulating in support of calreticulin as a polypeptide binding chaperone. In contrast to mammalian immunoglobulin G (IgG), which has complex type N-glycans, chicken immunoglobulin Y (IgY) possesses a monoglucosylated high mannose N-linked glycan, which is a ligand for calreticulin. Here, we have used solid and solution-phase assays to analyze the in vitro binding of calreticulin, purified from human placenta, to human IgG and chicken IgY in order to compare the interactions. In addition, peptides from the respective immunoglobulins were included to further probe the binding specificity of calreticulin. The experiments demonstrate the ability of calreticulin to bind to denatured forms of both IgG and IgY regardless of the glycosylation state of the proteins. Furthermore, calreticulin exhibits binding to peptides (glycosylated and non-glycosylated) derived from trypsin digestion of both immunoglobulins. Additionally, calreticulin peptide binding was examined with synthetic peptides covering the IgG Cγ2 domain demonstrating interaction with approximately half the peptides. Our results show that the dominant binding activity of calreticulin in vitro is toward the polypeptide moieties of IgG and IgY even in the presence of the monoglucosylated high mannose N-linked oligosaccharide on IgY.

Structural basis of carbohydrate recognition by calreticulin

The calnexin cycle is a process by which glycosylated proteins are subjected to folding cycles in the endoplasmic reticulum lumen via binding to the membrane protein calnexin (CNX) or to its soluble homolog calreticulin (CRT). CNX and CRT specifically recognize monoglucosylated Glc(1)Man(9)GlcNAc(2) glycans, but the structural determinants underlying this specificity are unknown. Here, we report a 1.95-Å crystal structure of the CRT lectin domain in complex with the tetrasaccharide α-Glc-(1→3)-α-Man-(1→2)-α-Man-(1→2)-Man. The tetrasaccharide binds to a long channel on CRT formed by a concave β-sheet. All four sugar moieties are engaged in the protein binding via an extensive network of hydrogen bonds and hydrophobic contacts. The structure explains the requirement for glucose at the nonreducing end of the carbohydrate; the oxygen O(2) of glucose perfectly fits to a pocket formed by CRT side chains while forming direct hydrogen bonds with the carbonyl of Gly(124) and the side chain of Lys(111). The structure also explains a requirement for the Cys(105)-Cys(137) disulfide bond in CRT/CNX for efficient carbohydrate binding. The Cys(105)-Cys(137) disulfide bond is involved in intimate contacts with the third and fourth sugar moieties of the Glc(1)Man(3) tetrasaccharide. Finally, the structure rationalizes previous mutagenesis of CRT and lays a structural groundwork for future studies of the role of CNX/CRT in diverse biological pathways.

The role of UDP-Glc:glycoprotein glucosyltransferase 1 in the maturation of an obligate substrate prosaposin

An endoplasmic reticulum (ER) quality control system assists in efficient folding and disposal of misfolded proteins. N-linked glycans are critical in these events because their composition dictates interactions with molecular chaperones. UDP-glucose:glycoprotein glucosyltransferase 1 (UGT1) is a key quality control factor of the ER. It adds glucoses to N-linked glycans of nonglucosylated substrates that fail a quality control test, supporting additional rounds of chaperone binding and ER retention. How UGT1 functions in its native environment is poorly understood. The role of UGT1 in the maturation of glycoproteins at basal expression levels was analyzed. Prosaposin was identified as a prominent endogenous UGT1 substrate. A dramatic decrease in the secretion of prosaposin was observed in ugt1(-/-) cells with prosaposin localized to large juxtanuclear aggresome-like inclusions, which is indicative of its misfolding and the essential role that UGT1 plays in its proper maturation. A model is proposed that explains how UGT1 may aid in the folding of sequential domain-containing proteins such as prosaposin.

Calnexin regulates apoptosis induced by inositol starvation in fission yeast

Inositol is a precursor of numerous phospholipids and signalling molecules essential for the cell. Schizosaccharomyces pombe is naturally auxotroph for inositol as its genome does not have a homologue of the INO1 gene encoding inositol-1-phosphate synthase, the enzyme responsible for inositol biosynthesis. In this work, we demonstrate that inositol starvation in S. pombe causes cell death with apoptotic features. This apoptotic death is dependent on the metacaspase Pca1p and is affected by the UPR transducer Ire1p. Previously, we demonstrated that calnexin is involved in apoptosis induced by ER stress. Here, we show that cells expressing a lumenal version of calnexin exhibit a 2-fold increase in the levels of apoptosis provoked by inositol starvation. This increase is reversed by co-expression of a calnexin mutant spanning the transmembrane domain and C-terminal cytosolic tail. Coherently, calnexin is physiologically cleaved at the end of its lumenal domain, under normal growth conditions when cells approach stationary phase. This cleavage suggests that the two naturally produced calnexin fragments are needed to continue growth into stationary phase and to prevent cell death. Collectively, our observations indicate that calnexin takes part in at least two apoptotic pathways in S. pombe, and suggest that the cleavage of calnexin has regulatory roles in apoptotic processes involving calnexin.

Mechanisms of pre-apoptotic calreticulin exposure in immunogenic cell death

Dying tumour cells can elicit a potent anticancer immune response by exposing the calreticulin (CRT)/ERp57 complex on the cell surface before the cells manifest any signs of apoptosis. Here, we enumerate elements of the pathway that mediates pre-apoptotic CRT/ERp57 exposure in response to several immunogenic anticancer agents. Early activation of the endoplasmic reticulum (ER)-sessile kinase PERK leads to phosphorylation of the translation initiation factor eIF2alpha, followed by partial activation of caspase-8 (but not caspase-3), caspase-8-mediated cleavage of the ER protein BAP31 and conformational activation of Bax and Bak. Finally, a pool of CRT that has transited the Golgi apparatus is secreted by SNARE-dependent exocytosis. Knock-in mutation of eIF2alpha (to make it non-phosphorylatable) or BAP31 (to render it uncleavable), depletion of PERK, caspase-8, BAP31, Bax, Bak or SNAREs abolished CRT/ERp57 exposure induced by anthracyclines, oxaliplatin and ultraviolet C light. Depletion of PERK, caspase-8 or SNAREs had no effect on cell death induced by anthracyclines, yet abolished the immunogenicity of cell death, which could be restored by absorbing recombinant CRT to the cell surface.

Endoplasmic reticulum glucosidase II is inhibited by its end products

The calnexin/calreticulin cycle is a quality control system responsible for promoting the folding of newly synthesized glycoproteins entering the endoplasmic reticulum (ER). The association of calnexin and calreticulin with the glycoproteins is regulated by ER glucosidase II, which hydrolyzes Glc 2Man X GlcNAc 2 glycans to Glc 1Man X GlcNAc 2 and further to Glc 0Man X GlcNAc 2 ( X represents any number between 5 and 9). To gain new insights into the reaction mechanism of glucosidase II, we developed a kinetic model that describes the interactions between glucosidase II, calnexin/calreticulin, and the glycans. Our model accurately reconstructed the hydrolysis of glycans with nine mannose residues and glycans with seven mannose residues, as measured by Totani et al. [Totani, K., Ihara, Y., Matsuo, I., and Ito, Y. (2006) J. Biol. Chem. 281, 31502-31508]. Intriguingly, our model predicted that glucosidase II was inhibited by its nonglucosylated end products, where the inhibitory effect of Glc 0Man 7GlcNAc 2 was much stronger than that of Glc 0Man 9GlcNAc 2. These predictions were confirmed experimentally. Moreover, our model suggested that glycans with a different number of mannose residues can be equivalent substrates of glucosidase II, in contrast to what had been previously thought. We discuss the possibility that nonglucosylated glycans, existing in the ER, might regulate the entry of newly synthesized glycoproteins into the calnexin/calreticulin cycle. Our model also shows that glucosidase II does not interact with monoglucosylated glycans while they are bound to calnexin or calreticulin.

Consequences of individual N-glycan deletions and of proteasomal inhibition on secretion of active BACE

BACE is an aspartic protease involved in the production of a toxic peptide accumulating in the brain of Alzheimer's disease patients. After attainment of the native structure in the endoplasmic reticulum (ER), BACE is released into the secretory pathway. To better understand the mechanisms regulating protein biogenesis in the mammalian ER, we determined the fate of five variants of soluble BACE with 4, 3, 2, 1, or 0 N-linked glycans. The number of N-glycans displayed on BACE correlated directly with folding and secretion rates and with the yield of active BACE harvested from the cell culture media. Addition of a single N-glycan was sufficient to recruit the calnexin chaperone system and/or for oligosaccharide de-glucosylation by the ER-resident alpha-glucosidase II. Addition of 1-4 N-glycans progressively enhanced the dissociation rate from BiP and reduced the propensity of newly synthesized BACE to enter aberrant soluble and insoluble aggregates. Finally, inhibition of the proteasome increased the yield of active BACE. This shows that active protein normally targeted for destruction can be diverted for secretion, as if for BACE the quality control system would be acting too stringently in the ER lumen, thus causing loss of functional polypeptides.

A cell-based reglucosylation assay demonstrates the role of GT1 in the quality control of a maturing glycoprotein

The endoplasmic reticulum (ER) protein GT1 (UDP-glucose: glycoprotein glucosyltransferase) is the central enzyme that modifies N-linked carbohydrates based upon the properties of the polypeptide backbone of the maturing substrate. GT1 adds glucose residues to nonglucosylated proteins that fail the quality control test, supporting ER retention through persistent binding to the lectin chaperones calnexin and calreticulin. How GT1 functions in its native environment on a maturing substrate is poorly understood. We analyzed the reglucosylation of a maturing model glycoprotein, influenza hemagglutinin (HA), in the intact mammalian ER. GT1 reglucosylated N-linked glycans in the slow-folding stem domain of HA once the nascent chain was released from the ribosome. Maturation mutants that disrupted the oxidation or oligomerization of HA also supported region-specific reglucosylation by GT1. Therefore, GT1 acts as an ER quality control sensor by posttranslationally reglucosylating glycans on slow-folding or nonnative domains to recruit chaperones specifically to critical aberrant regions.

The sugar-binding ability of ERGIC-53 is enhanced by its interaction with MCFD2

Combined deficiency of factors V and VIII (F5F8D) is a bleeding disorder caused by mutations in LMAN1 or MCFD2. LMAN1 encodes ERGIC-53, a cargo receptor with an L-type lectin domain, and MCFD2 is a EF-hand-containing protein. We prepared a biotinylated, soluble form of ERGIC-53, which we labeled with R-phycoerythrin conjugated streptavidin. By flow cytometry, sERGIC-53-SA bound to HeLaS3 cells in the presence of calcium but only after preincubation with MCFD2. Treating the cells with endo H or incubating them with high mannose-type oligosaccharides, especially M(8B), abrogated sERGIC-53-SA binding. Surface plasmon resonance experiments demonstrated that MCFD2 specifically bound to sERGIC-53 and 2 MCFD2 mutants found in F5F8D patients had a K(a) that was 3 or 4 orders of magnitude lower for sERGIC-53 than for wild-type MCFD2. The K(a) of sERGIC-53 and MCFD2 was measured at several pH values and calcium concentrations, and we found that at a calcium concentration less than 0.2 mM, this interaction became significantly weaker. These results demonstrate that the binding of ERGIC-53 to sugar is enhanced by its interaction with MCFD2, and defects in this interaction in F5F8D patients may be the cause for reduced secretion of factors V and VIII.

In and out of the ER: protein folding, quality control, degradation, and related human diseases

A substantial fraction of eukaryotic gene products are synthesized by ribosomes attached at the cytosolic face of the endoplasmic reticulum (ER) membrane. These polypeptides enter cotranslationally in the ER lumen, which contains resident molecular chaperones and folding factors that assist their maturation. Native proteins are released from the ER lumen and are transported through the secretory pathway to their final intra- or extracellular destination. Folding-defective polypeptides are exported across the ER membrane into the cytosol and destroyed. Cellular and organismal homeostasis relies on a balanced activity of the ER folding, quality control, and degradation machineries as shown by the dozens of human diseases related to defective maturation or disposal of individual polypeptides generated in the ER.

Identification of calreticulin as a ligand of GABARAP by phage display screening of a peptide library

4-Aminobutyrate type A (GABA(A)) receptor-associated protein (GABARAP) is a ubiquitin-like modifier implicated in the intracellular trafficking of GABA(A) receptors, and belongs to a family of proteins involved in intracellular vesicular transport processes, such as autophagy and intra-Golgi transport. In this article, it is demonstrated that calreticulin is a high affinity ligand of GABARAP. Calreticulin, although best known for its functions as a Ca(2+) -dependent chaperone and a Ca(2+) -buffering protein in the endoplasmic reticulum, is also localized to the cytosol and exerts a variety of extra-endoplasmic reticulum functions. By phage display screening of a randomized peptide library, peptides that specifically bind GABARAP were identified. Their amino acid sequences allowed us to identify calreticulin as a potential GABARAP binding protein. GABARAP binding to calreticulin was confirmed by pull-down experiments with brain lysate and colocalization studies in N2a cells. Calreticulin and GABARAP interact with a dissociation constant K(d) = 64 nm and a mean lifetime of the complex of 20 min. Thus, the interaction between GABARAP and calreticulin is the strongest so far reported for each protein.

Substrate recognition by the protein disulfide isomerases

Protein folding in the endoplasmic reticulum is often associated with the formation of native disulfide bonds. Their primary function is to stabilize the folded structure of the protein, although disulfide bond formation can also play a regulatory role. Native disulfide bond formation is not trivial, so it is often the rate-limiting step of protein folding both in vivo and in vitro. Complex coordinated systems of molecular chaperones and protein folding catalysts have evolved to help proteins attain their correct folded conformation. This includes a family of enzymes involved in catalyzing thiol-disulfide exchange in the endoplasmic reticulum, the protein disulfide isomerase (PDI) family. There are now 17 reported PDI family members in the endoplasmic reticulum of human cells, but the functional differentiation of these is far from complete. Despite PDI being the first catalyst of protein folding reported, there is much that is still not known about its mechanisms of action. This review will focus on the interactions of the human PDI family members with substrates, including recent research on identifying and characterizing their substrate-binding sites and on determining their natural substrates in vivo.

How sugars convey information on protein conformation in the endoplasmic reticulum

The N-glycan-dependent quality control of glycoprotein folding prevents endoplasmic reticulum to Golgi exit of folding intermediates, irreparably misfolded glycoproteins and not completely assembled multimeric complexes. It also enhances folding efficiency by preventing aggregation and facilitating formation of proper disulfide bonds. The control mechanism essentially involves four components, resident lectin-chaperones that recognize monoglucosylated polymannose glycans, a lectin-associated oxidoreductase acting on monoglucosylated glycoproteins, a glucosyltransferase and a glucosidase that creates monoglucosylated epitopes in glycans transferred in protein N-glycosylation or removes the glucose units added by the glucosyltransferase. This last enzyme is the only mechanism component sensing glycoprotein conformations as it creates monoglucosylated glycans exclusively in not properly folded species or in not completely assembled complexes. The purpose of the review is to describe the most significant recent findings on the mechanism of glycoprotein folding and assembly quality control and to discuss the main still unanswered questions.

Peptide binding specificity of the chaperone calreticulin

Calreticulin is a molecular chaperone with specificity for polypeptides and N-linked monoglucosylated glycans. In order to determine the specificity of polypeptide binding, the interaction of calreticulin with polypeptides was investigated using synthetic peptides of different length and composition. A large set of available synthetic peptides (n=127) was tested for binding to calreticulin and the results analysed by multivariate data analysis. The parameter that correlated best with binding was hydrophobicity while beta-turn potential disfavoured binding. Only hydrophobic peptides longer than 5 amino acids showed binding and a clear correlation with hydrophobicity was demonstrated for oligomers of different hydrophobic amino acids. Insertion of hydrophilic amino acids in a hydrophobic sequence diminished or abolished binding. In conclusion our results show that calreticulin has a peptide-binding specificity for hydrophobic sequences and delineate the fine specificity of calreticulin for hydrophobic amino acid residues.

Exploration of oligosaccharide-protein interactions in glycoprotein quality control by synthetic approaches

High-mannose-type oligosaccharides, which are cotranslationally introduced to nascent polypeptides, play important roles in glycoprotein quality control. This process is highly complex, involving a number of lectins, chaperones, and glycan-processing enzymes. For example, calnexin and calreticulin (CRT) are molecular chaperones that recognize monoglucosylated forms of high-mannose-type glycans. UDP-glucose : glycoprotein glucosyltransferase (UGGT) only glucosylates high-mannose-type glycans attached to partially folded proteins. Fbs1 is a component of ubiquitin ligase that recognizes sugar chains. Although recent studies have clarified the properties of these proteins, most of them used oligosaccharides derived from natural sources, which contain structural heterogeneity. In order to gain a more precise understanding, we started our program to comprehensively synthesize high-mannose-type glycans associated with a protein quality control system. Additionally, investigation of artificial glycoproteins led us to the discovery of the first nonpeptidic substrate of UGGT. These synthetic oligosaccharide probes have allowed us to conduct quantitative evaluations of the activity and specificity of CRT, Fbs1, and UGGT.

Isothermal titration calorimetric study defines the substrate binding residues of calreticulin

Earlier we established using modeling studies the residues in calreticulin (CRT) important for sugar-binding (M. Kapoor, H. Srinivas, K. Eaazhisai, E. Gemma, L. Ellgaard, S. Oscarson, A. Helenius, A. Surolia, Interactions of substrate with calreticulin, an endoplasmic reticulum chaperone, J. Biol. Chem. 278 (8) (2003) 6194-6200). Here, we discuss the relative roles of Trp-319, Asp-317, and Asp-160 for sugar-binding by using site-directed mutagenesis and isothermal titration calorimetry (ITC). Residues corresponding to Asp-160 and Asp-317 in CNX play important role towards sugar-binding. From the present study we demonstrate that the residue Asp-160 is not involved in sugar-binding, while Asp-317 plays a crucial role. Further, it is also validated that cation-pi interactions of the sugar with Trp-319 dictate sugar-binding in CRT. This study not only defines further the binding site of CRT but also highlights its subtle differences with that of calnexin.

Potent Lectin-Independent Chaperone Function of Calnexin under Conditions Prevalent within the Lumen of the Endoplasmic Reticulum†

Calnexin is a membrane-bound chaperone of the endoplasmic reticulum (ER) that participates in the folding and quality control of newly synthesized glycoproteins. Binding to glycoproteins occurs through a lectin site with specificity for Glc1Man9GlcNAc2 oligosaccharides as well as through a polypeptide binding site that recognizes non-native protein conformations. The latter interaction is somewhat controversial because it is based on observations that calnexin can suppress the aggregation of non-glycosylated substrates at elevated temperature or at low calcium concentrations, conditions that may affect the structural integrity of calnexin. Here, we examine the ability of calnexin to interact with a non-glycosylated substrate under physiological conditions of the ER lumen. We show that the soluble ER luminal domain of calnexin can indeed suppress the aggregation of non-glycosylated firefly luciferase at 37 degrees C and at the normal resting ER calcium concentration of 0.4 mM. However, gradual reduction of calcium below the resting level was accompanied by a progressive loss of native calnexin structure as assessed by thermal stability, protease sensitivity, intrinsic fluorescence, and bis-ANS binding. These assays permitted the characterization of a single calcium binding site on calnexin with a Kd = 0.15 +/- 0.05 mM. We also show that the suppression of firefly luciferase aggregation by calnexin is strongly enhanced in the presence of millimolar concentrations of ATP and that the Kd for ATP binding to calnexin in the presence of 0.4 mM calcium is 0.7 mM. ATP did not alter the overall stability of calnexin but instead triggered the localized exposure of a hydrophobic site on the chaperone. These findings demonstrate that calnexin is a potent molecular chaperone that is capable of suppressing the aggregation of substrates through polypeptide-based interactions under conditions that exist within the ER lumen.

Synthesis of monodeoxy analogues of the trisaccharide α-d-Glcp-(1→3)-α-d-Manp-(1→2)-α-d-ManpOMe recognised by Calreticulin/Calnexin

Six (3,4,4',6',3'' or 6'')-monodeoxy analogues of the title trisaccharide (1-6) have been prepared utilising monodeoxy monosaccharide precursors. The reducing end deoxy derivatives were synthesised by N-iodosuccinimide/silver trifluoromethanesulfonate (NIS/AgOTf)-promoted couplings of a common disaccharide thioglycoside donor 10 to suitably protected monodeoxy acceptors 9 and 12, affording trisaccharides, which after deprotection yielded target structures 1 and 2. The non-reducing end deoxy derivatives could similarly be produced by halide-assisted glycosylations of a common disaccharide acceptor 17 with monodeoxy glycosyl bromide donors (obtained from thioglycosides 18 and 20) to yield, after removal of protecting groups, target trisaccharides 3 and 4. The analogues with the deoxy function in the middle mannose residue, were obtained through orthogonal halide-assisted coupling of tetrabenzyl-glucopyranosyl bromide to monodeoxy thioglycoside acceptors to give thioglycoside disaccharides, which subsequently were used as donors in NIS/AgOTf-promoted couplings to a common 2-hydroxy-mannose acceptor 15 to afford trisaccharides; deprotection yielded the final target compounds 5 and 6.

Mitochondria maintain maturation and secretion of lipoprotein lipase in the endoplasmic reticulum

Considering the physiological Ca2+ dynamics within the ER (endoplasmic reticulum), it remains unclear how efficient protein folding is maintained in living cells. Thus, utilizing the strictly folding-dependent activity and secretion of LPL (lipoprotein lipase), we evaluated the impact of ER Ca2+ content and mitochondrial contribution to Ca2+-dependent protein folding. Exhaustive ER Ca2+ depletion by inhibition of sarcoplasmic/endoplasmic reticulum Ca2+-ATPases caused strong, but reversible, reduction of cell-associated and released activity of constitutive and adenovirus-encoded human LPL in CHO-K1 (Chinese-hamster ovary K1) and endothelial cells respectively, which was not due to decline of mRNA or intracellular protein levels. In contrast, stimulation with the IP3 (inositol 1,4,5-trisphosphate)-generating agonist histamine only moderately and transiently affected LPL maturation in endothelial cells that paralleled a basically preserved ER Ca2+ content. However, in the absence of extracellular Ca2+ or upon prevention of transmitochondrial Ca2+ flux, LPL maturation discontinued upon histamine stimulation. Collectively, these data indicate that Ca2+-dependent protein folding in the ER is predominantly controlled by intraluminal Ca2+ and is largely maintained during physiological cell stimulation owing to efficient ER Ca2+ refilling. Since Ca2+ entry and mitochondrial Ca2+ homoeostasis are crucial for continuous Ca2+-dependent protein maturation in the ER, their pathological alterations may result in dysfunctional protein folding.

Monitoring chaperone engagement of substrates in the endoplasmic reticulum of live cells

The folding environment in the endoplasmic reticulum (ER) depends on multiple abundant chaperones that function together to accommodate a range of substrates. The ways in which substrate engagement shapes either specific chaperone dynamics or general ER attributes in vivo remain unknown. In this study, we have evaluated how changes in substrate flux through the ER influence the diffusion of both the lectin chaperone calreticulin and an inert reporter of ER crowdedness. During acute changes in substrate load, the inert probe revealed no changes in ER organization, despite significant changes in calreticulin dynamics. By contrast, inhibition of the lectin chaperone system caused rapid changes in the ER environment that could be reversed over time by easing new substrate burden. Our findings provide insight into the normal organization and dynamics of an ER chaperone and characterize the capacity of the ER to maintain homeostasis during acute changes in chaperone activity and availability.

Design and Synthesis of Oligosaccharides that Interfere with Glycoprotein Quality-control systems

Calnexin (CNX) and its soluble homologue calreticulin (CRT) are lectin-like molecular chaperones that help newly synthesized glycoproteins to fold correctly in the rough endoplasmic reticulum (ER). To investigate the mechanism of glycoprotein-quality control, we have synthesized structurally defined high-mannose-type oligosaccharides related to this system. This paper describes the synthesis of the non-natural undecasaccharide 2 and heptasaccharide 16, designed as potential inhibitors of the ER quality-control system. Each possesses the key tetrasaccharide element (Glc1Man3) critical for the CNX/CRT binding, while lacking the pentamannosyl branch required for glucosidase II recognition. These oligosaccharides were evaluated for their ability to bind CRT by isothermal titration calorimetry (ITC). As expected, each of them had a significant affinity towards CRT. In addition, these compounds were shown to be resistant to glucosidase II digestion. Their activities in blocking the chaperone function of CRT were next measured by using malate dehydrogenase (MDH) as a substrate. Their inhibitory effects were shown to correlate well with their CRT-binding affinities, both being critically dependent upon the presence of the terminal glucose (Glc) residue.

A Novel Cysteine-rich Domain of Sep15 Mediates the Interaction with UDP-glucose:Glycoprotein Glucosyltransferase

Selenium is an essential trace element with potent cancer prevention activity in mammals. The 15-kDa selenoprotein (Sep15) has been implicated in the chemopreventive effect of dietary selenium. Although the precise function of Sep15 remains elusive, Sep15 co-purifies with UDP-glucose:glycoprotein glucosyltransferase (GT), an essential regulator of quality control mechanisms within the endoplasmic reticulum. Recent studies identified two GT and two Sep15 homologues in mammals. We characterize interactions between these protein families in this report. Sep15 and GT form a tight 1:1 complex, and these interactions are conserved between mammals and fruit flies. In mammalian cells, Sep15 co-immunoprecipitates with both GT isozymes. In contrast, a Sep15 homologue, designated selenoprotein M (SelM), does not form a complex with GT. Sequence analysis of members of the Sep15 family identified a novel N-terminal cysteine-rich domain in Sep15 that is absent in SelM. This domain contains six conserved cysteine residues that form two CxxC motifs that do not coordinate metal ions. If this domain is deleted or the cysteines are mutated, Sep15 no longer forms a complex with GT. Conversely, if the cysteine-rich domain of Sep15 is fused to the N-terminus of SelM, the resulting chimera is capable of binding GT. These data indicate that the cysteine-rich domain of Sep15 exclusively mediates protein-protein interactions with GT.

Delineation of the lectin site of the molecular chaperone calreticulin

Calreticulin (CRT) is a soluble molecular chaperone of the endoplasmic reticulum that functions to promote protein folding as well as to retain misfolded proteins. Similar to its membrane-bound paralog calnexin (CNX), CRT is a lectin that preferentially interacts with glycoproteins bearing Glc1Man5-9GlcNAc2 oligosaccharides. Although the lectin site of CNX has been delineated through X-ray crystallographic and mutagenic studies, the corresponding site for CRT has not been as well characterized. To address this issue, we attempted to construct lectin-deficient CRT mutants, using the structure of CNX as a guide to identify potential oligosaccharide-binding residues. Mutation of 4 such CRT residues (Y109, K111, Y128, D317) completely abrogated oligosaccharide binding. In contrast, mutation of CRT residues M131 and D160, which correspond to important residues in the lectin site of CNX, had no effect on oligosaccharide binding. These findings suggest that the organization of the lectin site in CRT largely resembles that of CNX but is not identical. The deficiency in oligosaccharide binding by the mutants was not due to misfolding because they exhibited wild-type protease digestion patterns, were capable of binding the thiol oxidoreductase ERp57, and functioned just as efficiently as wild-type CRT in suppressing the aggregation of the nonglycosylated substrate citrate synthase. However, they were impaired in their ability to suppress the aggregation of the glycosylated substrate jack bean alpha-mannosidase. This provides the first direct demonstration of the importance of CRT's lectin site in suppressing the aggregation of nonnative glycoproteins.

Structural approaches to the study of oligosaccharides in glycoprotein quality control

High-mannose-type oligosaccharides have been shown to play important roles in protein quality control. Several intracellular proteins, such as lectins, chaperones and glycan-processing enzymes, are involved in this process. These include calnexin/calreticulin, UDP-glucose:glycoprotein glucosyltransferase (UGGT), cargo receptors (such as VIP36 and ERGIC-53), mannosidase-like proteins (e.g. EDEM and Htm1p) and ubiquitin ligase (Fbs). They are thought to recognize high-mannose-type glycans with subtly different structures, although the precise specificities are yet to be clarified. In order to gain a clear understanding of these protein-carbohydrate interactions, comprehensive synthesis of high-mannose-type glycans was conducted. In addition, two approaches to the synthesis of artificial glycoproteins with homogeneous oligosaccharides were investigated. Furthermore, a novel substrate of UGGT was discovered.

Synthesis of the tetrasaccharide alpha-D-Glcp-(1-->3)-alpha-D-Manp-(1-->2)-alpha-D-Manp-(1-->2)-alpha-D-Manp recognized by calreticulin/calnexin

The title compound as its methyl glycoside was efficiently synthesized using a block synthesis approach. Halide-assisted glycosidations between 6-O-acetyl-2,3,4-tri-O-benzyl-alpha-D-glucopyranosyl iodide and ethyl 2-O-acetyl-4,6-di-O-benzyl-1-thio-alpha-D-mannopyranoside using triphenylphosphine oxide as promoter yielded, with complete alpha-selectivity, a disaccharide building block in high yield. The perbenzylated derivative of this proved to be an excellent donor affording 88% of the protected target tetrasaccharide in an NIS/AgOTf-promoted coupling to a known methyl dimannoside acceptor. Deprotection through catalytic hydrogenolysis then gave the target compound in 47% overall yield.