Participation of a novel 88-kD protein in the biogenesis of murine class I histocompatibility molecules

The Essential Functions of Molecular Chaperones and Folding Enzymes in Maintaining Endoplasmic Reticulum Homeostasis

It has been estimated that up to one-third of the proteins encoded by the human genome enter the endoplasmic reticulum (ER) as extended polypeptide chains where they undergo covalent modifications, fold into their native structures, and assemble into oligomeric protein complexes. The fidelity of these processes is critical to support organellar, cellular, and organismal health, and is perhaps best underscored by the growing number of disease-causing mutations that reduce the fidelity of protein biogenesis in the ER. To meet demands encountered by the diverse protein clientele that mature in the ER, this organelle is populated with a cadre of molecular chaperones that prevent protein aggregation, facilitate protein disulfide isomerization, and lower the activation energy barrier of cis-trans prolyl isomerization. Components of the lectin (glycan-binding) chaperone system also reside within the ER and play numerous roles during protein biogenesis. In addition, the ER houses multiple homologs of select chaperones that can recognize and act upon diverse peptide signatures. Moreover, redundancy helps ensure that folding-compromised substrates are unable to overwhelm essential ER-resident chaperones and enzymes. In contrast, the ER in higher eukaryotic cells possesses a single member of the Hsp70, Hsp90, and Hsp110 chaperone families, even though several homologs of these molecules reside in the cytoplasm. In this review, we discuss specific functions of the many factors that maintain ER quality control, highlight some of their interactions, and describe the vulnerabilities that arise from the absence of multiple members of some chaperone families.

Autophagy-dependent regulation of MHC-I molecule presentation

The major histocompatibility complex (MHC) class I molecules present peptide antigens to MHC class I-restricted CD8+ T lymphocytes to elicit an effective immune response. The conventional antigen-processing pathway for MHC-I presentation depends on proteasome-mediated peptide generation and peptide loading in the endoplasmic reticulum by members of the peptide loading complex. Recent discoveries in this field highlight the role of alternative MHC-I peptide loading and presentation pathways, one of them being autophagy. Autophagy is a cell-intrinsic degradative pathway that ensures cellular homoeostasis and plays critical roles in cellular immunity. In this review article, we discuss the role of autophagy in MHC class I-restricted antigen presentation, elucidating new findings on the crosstalk of autophagy and ER-mediated MHC-I peptide presentation, dendritic cell-mediated cross-presentation and also mechanisms governing immune evasion. A detailed molecular understanding of the key drivers of autophagy-mediated MHC-I modulation holds promising targets to devise effective measures to improve T cell immunotherapies.

Calnexin, More Than Just a Molecular Chaperone

Calnexin is a type I integral endoplasmic reticulum (ER) membrane protein with an N-terminal domain that resides in the lumen of the ER and a C-terminal domain that extends into the cytosol. Calnexin is commonly referred to as a molecular chaperone involved in the folding and quality control of membrane-associated and secreted proteins, a function that is attributed to its ER- localized domain with a structure that bears a strong resemblance to another luminal ER chaperone and Ca²⁺-binding protein known as calreticulin. Studies have discovered that the cytosolic C-terminal domain of calnexin undergoes distinct post-translational modifications and interacts with a variety of proteins. Here, we discuss recent findings and hypothesize that the post-translational modifications of the calnexin C-terminal domain and its interaction with specific cytosolic proteins play a role in coordinating ER functions with events taking place in the cytosol and other cellular compartments.

Co-chaperones of the Human Endoplasmic Reticulum: An Update

In mammalian cells, the rough endoplasmic reticulum (ER) plays central roles in the biogenesis of extracellular plus organellar proteins and in various signal transduction pathways. For these reasons, the ER comprises molecular chaperones, which are involved in import, folding, assembly, export, plus degradation of polypeptides, and signal transduction components, such as calcium channels, calcium pumps, and UPR transducers plus adenine nucleotide carriers/exchangers in the ER membrane. The calcium- and ATP-dependent ER lumenal Hsp70, termed immunoglobulin heavy-chain-binding protein or BiP, is the central player in all these activities and involves up to nine different Hsp40-type co-chaperones, i.e., ER membrane integrated as well as ER lumenal J-domain proteins, termed ERj or ERdj proteins, two nucleotide exchange factors or NEFs (Grp170 and Sil1), and NEF-antagonists, such as MANF. Here we summarize the current knowledge on the ER-resident BiP/ERj chaperone network and focus on the interaction of BiP with the polypeptide-conducting and calcium-permeable Sec61 channel of the ER membrane as an example for BiP action and how its functional cycle is linked to ER protein import and various calcium-dependent signal transduction pathways.

Heat-Shock Proteins

Heat-shock proteins (HSPs), or stress proteins, are abundant and highly conserved, present in all organisms and in all cells. Selected HSPs, also known as chaperones, play crucial roles in folding and unfolding of proteins, assembly of multiprotein complexes, transport and sorting of proteins into correct subcellular compartments, cell-cycle control and signaling, and protection of cells against stress and apoptosis. More recently, HSPs have been shown to be key players in immune responses: during antigen presentation as well as cross-priming, they chaperone and transfer antigenic peptides to class I and class II molecules of the major histocompatibility complexes. In addition, extracellular HSPs can stimulate and cause maturation of professional antigen-presenting cells of the immune system, such as macrophages and dendritic cells. They also chaperone several toll-like receptors, which play a central role in innate immune responses. HSPs constitute a large family of proteins that are often classified based on their molecular weight as Hsp10, Hsp40, Hsp60, Hsp70, Hsp90, etc. This unit contains a table that lists common HSPs and summarizes their characteristics including (a) name, (b) subcellular localization, (c) known function, (d) chromosome assignment, (e) brief comments, and (f) references. © 2022 Wiley Periodicals LLC.

A new calnexin modulates antibacterial immune response in obscure puffer Takifugu obscurus

Calnexin (Cnx) is a membrane-bound lectin chaperone of the endoplasmic reticulum. In this study, a novel Cnx homologue from the obscure puffer Takifugu obscurus was characterized, tentatively named ToCnx. The cDNA of ToCnx was 1803 bp, and it contained an open reading frame encoding a polypeptide of 600 amino acid residues with a calculated molecular weight of 67.5 kDa. Multiple alignment of the deduced amino acid sequences of ToCnx and other related fish Cnxs revealed that ToCnx had typical characteristics of fish Cnxs. Sequence comparison and phylogenetic tree analysis showed that ToCnx had the closest relationship with Cnxs from Takifugu flavidus and Takifugu rubripes. ToCnx transcripts were detected in all the tissues examined, and they were mainly expressed in the liver, kidney, and intestine. Upon Vibrio harveyi, Edwardsiella tarda, and Aeromonas hydrophila infection, ToCnx transcripts were all significantly upregulated in the kidneys. The recombinant calreticulin domain of ToCnx (rToCnx) was prepared by prokaryotic expression. In the absence of calcium, rToCnx was able to bind three Gram-negative bacteria (V. harveyi, E. tarda, and A. hydrophila) and two bacterial saccharides, such as lipopolysaccharide and peptidoglycan. In the presence of calcium, rToCnx could agglutinate all the detected microorganisms. In addition, rToCnx possessed the effect of inhibiting the growth of three microbe strains. These observations suggested that ToCnx is an important participant in host immune defense against bacteria.

The New Kid on the Block: HLA-C, a Key Regulator of Natural Killer Cells in Viral Immunity

The human leukocyte antigen system (HLA) is a cluster of highly polymorphic genes essential for the proper function of the immune system, and it has been associated with a wide range of diseases. HLA class I molecules present intracellular host- and pathogen-derived peptides to effector cells of the immune system, inducing immune tolerance in healthy conditions or triggering effective immune responses in pathological situations. HLA-C is the most recently evolved HLA class I molecule, only present in humans and great apes. Differentiating from its older siblings, HLA-A and HLA-B, HLA-C exhibits distinctive features in its expression and interaction partners. HLA-C serves as a natural ligand for multiple members of the killer-cell immunoglobulin-like receptor (KIR) family, which are predominately expressed by natural killer (NK) cells. NK cells are crucial for the early control of viral infections and accumulating evidence indicates that interactions between HLA-C and its respective KIR receptors determine the outcome and progression of viral infections. In this review, we focus on the unique role of HLA-C in regulating NK cell functions and its consequences in the setting of viral infections.

Know thy immune self and non-self: Proteomics informs on the expanse of self and non-self, and how and where they arise

T cells play an important role in the adaptive immune response to a variety of infections and cancers. Initiation of a T cell mediated immune response requires antigen recognition in a process termed MHC (major histocompatibility complex) restri ction. A T cell antigen is a composite structure made up of a peptide fragment bound within the antigen‐binding groove of an MHC‐encoded class I or class II molecule. Insight into the precise composition and biology of self and non‐self immunopeptidomes is essential to harness T cell mediated immunity to prevent, treat, or cure infectious diseases and cancers. T cell antigen discovery is an arduous task! The pioneering work in the early 1990s has made large‐scale T cell antigen discovery possible. Thus, advancements in mass spectrometry coupled with proteomics and genomics technologies make possible T cell antigen discovery with ease, accuracy, and sensitivity. Yet we have only begun to understand the breadth and the depth of self and non‐self immunopeptidomes because the molecular biology of the cell continues to surprise us with new secrets directly related to the source, and the processing and presentation of MHC ligands. Focused on MHC class I molecules, this review, therefore, provides a brief historic account of T cell antigen discovery and, against a backdrop of key advances in molecular cell biologic processes, elaborates on how proteogenomics approaches have revolutionised the field.

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.

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.

Cellular Mechanisms Controlling Surfacing of AICL Glycoproteins, Cognate Ligands of the Activating NK Receptor NKp80

AICL glycoproteins are cognate activation-induced ligands of the C-type lectin-like receptor NKp80, which is expressed on virtually all mature human NK cells, and NKp80-AICL interaction stimulates NK cell effector functions such as cytotoxicity and cytokine secretion. Notably, AICL and NKp80 are encoded by adjacent genes in the NK gene complex and are coexpressed by human NK cells. Whereas AICL is intracellularly retained in resting NK cells, exposure of NK cells to proinflammatory cytokines results in AICL surfacing and susceptibility to NKp80-mediated NK fratricide. In this study, we characterize molecular determinants of AICL glycoproteins that cause intracellular retention, thereby controlling AICL surface expression. Cys⁸⁷ residing within the C-type lectin-like domain not only ensures stable homodimerization of AICL glycoproteins by disulfide bonding, but Cys⁸⁷ is also required for efficient cell surface expression of AICL homodimers and essential for AICL-NKp80 interaction. In contrast, cytoplasmic lysines act as negative regulators targeting AICL for proteasomal degradation. One atypical and three conventional N-linked glycosylation sites in the AICL C-type lectin-like domain critically impact maturation and surfacing of AICL, which is strictly dependent on glycosylation of at least one conventional glycosylation site. However, although the extent of conventional N-linked glycosylation positively correlates with AICL surface expression, the atypical glycosylation site impairs AICL surfacing. Stringent control of AICL surface expression by glycosylation is reflected by the pronounced interaction of AICL with calnexin and the impaired AICL expression in calnexin-deficient cells. Collectively, our data demonstrate that AICL expression and surfacing are tightly controlled by several independent cellular posttranslational mechanisms.

Heat Shock Proteins (HSP): Future Trends in Cancer Immunotherapy

Heat Shock Proteins (HSPs) are a large family of highly conserved proteins involved in assisting protein folding and unfolding in the cells. HSPs are expressed constitutively as well as inducibly and, interacting with antigen presenting cells, induce the expression of various cytokines and chemokines as well as the maturation and migration of dendritic cells, thus acting themselves as cytokines. HSP-chaperoned antigenic peptides are also generated within the tumor cells. Such chaperoned peptides are released in the extra cellular medium with an association of HSPs by cell stress, death or tumor cell lyses. HSP-peptide complexes from extra cellular medium are taken up by antigen presenting cells through CD91 receptor and are represented or cross-presented by their MHC class I molecules for specific anti-tumor immune response. In addition, HSPs expressed on the cell surface of tumor cells stimulate αβ T-cells and γδ T-cells as well as natural killer (NK) cells that are first-line defense mechanisms. In this manner, HSPs have the ability to stimulate both arms of the effecter mechanism of the immune system. These unique immunological attributes of HSPs are presently becoming the basis for tumor immunotherapy. Tumor-derived HSP-peptide complexes have been demonstrated to serve as anti-tumor vaccines. To date various approaches of vaccination using HSPs have been developed and tested clinically. These HSP-based vaccine approaches can be combined with hyperthermia and CTLA-4 blockade to enhance their anti-tumor potentiality.

Two endoplasmic reticulum proteins (calnexin and calreticulin) are involved in innate immunity in Chinese mitten crab (Eriocheir sinensis)

Calnexin (Cnx) and calreticulin (Crt), which are important chaperones in the endoplasmic reticulum (ER), participate in the folding and quality control of client proteins. Cnx and Crt identified from Chinese mitten crab (Eriocheir sinensis) are designated as EsCnx and EsCrt, respectively. EsCnx and EsCrt are expressed in the hemocyte, hepatopancrea, gill, and intestine at the mRNA and protein level. Immunofluorescence analysis indicated that EsCnx and EsCRT are located in the ER. Moreover, the mRNA and protein expression levels of EsCnx and EsCrt were altered by challenge with lipopolysaccharides (LPS), peptidoglycans (PGN), Staphyloccocus aureus, and Vibrio parahaemolyticus. Recombinant EsCnx and EsCrt (rEsCnx and rEsCrt, respectively) proteins can bind to various Gram-positive and Gram-negative bacteria, as well as to different polysaccharides (LPS and PGN). rEsCnx and rEsCrt assisted in the clearance of V. parahaemolyticus in vivo, and the clearance efficiency was impaired after silencing of EsCnx and EsCrt. Our results suggest that the two ER proteins are involved in anti-bacterial immunity in E. sinensis.

The Implication and Significance of Beta 2 Microglobulin: A Conservative Multifunctional Regulator

OBJECTIVE

This review focuses on the current knowledge on the implication and significance of beta 2 microglobulin (β2M), a conservative immune molecule in vertebrate.

DATA SOURCES

The data used in this review were obtained from PubMed up to October 2015. Terms of β2M, immune response, and infection were used in the search.

STUDY SELECTIONS

Articles related to β2M were retrieved and reviewed. Articles focusing on the characteristic and function of β2M were selected. The exclusion criteria of articles were that the studies on β2M-related molecules.

RESULTS

β2M is critical for the immune surveillance and modulation in vertebrate animals. The dysregulation of β2M is associated with multiple diseases, including endogenous and infectious diseases. β2M could directly participate in the development of cancer cells, and the level of β2M is deemed as a prognostic marker for several malignancies. It also involves in forming major histocompatibility complex (MHC class I or MHC I) or like heterodimers, covering from antigen presentation to immune homeostasis.

CONCLUSIONS

Based on the characteristic of β2M, it or its signaling pathway has been targeted as biomedical or therapeutic tools. Moreover, β2M is highly conserved among different species, and overall structures are virtually identical, implying the versatility of β2M on applications.

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.

Release from endoplasmic reticulum matrix proteins controls cell surface transport of MHC class I molecules

The anterograde transport of secretory proteins from the endoplasmic reticulum (ER) to the plasma membrane is a multi-step process. Secretory proteins differ greatly in their transport rates to the cell surface, but the contribution of each individual step to this difference is poorly understood. Transport rates may be determined by protein folding, chaperone association in the ER, access to ER exit sites (ERES) and retrieval from the ER-Golgi intermediate compartment or the cis-Golgi to the ER. We have used a combination of folding and trafficking assays to identify the differential step in the cell surface transport of two natural allotypes of the murine major histocompatibility complex (MHC) class I peptide receptor, H-2D(b) and H-2K(b) . We find that a novel pre-ER exit process that acts on the folded lumenal part of MHC class I molecules and that drastically limits their access to ERES accounts for the transport difference of the two allotypes. Our observations support a model in which the cell surface transport of MHC class I molecules and other type I transmembrane proteins is governed by the affinity of all their folding and maturation states to the proteins of the ER matrix.

Transport and quality control of MHC class I molecules in the early secretory pathway

Folding and peptide binding of major histocompatibility complex (MHC) class I molecules have been thoroughly researched, but the mechanistic connection between these biochemical events and the progress of class I through the early secretory pathway is much less well understood. This review focuses on the question how the partially assembled forms of class I (which lack high-affinity peptide and/or the light chain beta-2 microglobulin) are retained inside the cell. Such investigations offer researchers exciting chances to understand the connections between class I structure, conformational dynamics, peptide binding kinetics and thermodynamics, intracellular transport, and antigen presentation.

Co-chaperones of the mammalian endoplasmic reticulum

In mammalian cells, the rough endoplasmic reticulum or ER plays a central role in the biogenesis of most extracellular plus many organellar proteins and in cellular calcium homeostasis. Therefore, this organelle comprises molecular chaperones that are involved in import, folding/assembly, export, and degradation of polypeptides in millimolar concentrations. In addition, there are calcium channels/pumps and signal transduction components present in the ER membrane that affect and are affected by these processes. The ER lumenal Hsp70, termed immunoglobulin-heavy chain binding protein or BiP, is the central player in all these activities and involves up to seven different co-chaperones, i.e. ER-membrane integrated as well as ER-lumenal Hsp40s, which are termed ERj or ERdj, and two nucleotide exchange factors.

Calreticulin: roles in cell-surface protein expression

In order to perform their designated functions, proteins require precise subcellular localizations. For cell-surface proteins, such as receptors and channels, they are able to transduce signals only when properly targeted to the cell membrane. Calreticulin is a multi-functional chaperone protein involved in protein folding, maturation, and trafficking. However, evidence has been accumulating that calreticulin can also negatively regulate the surface expression of certain receptors and channels. In these instances, depletion of calreticulin enhances cell-surface expression and function. In this review, we discuss the role of calreticulin with a focus on its negative effects on the expression of cell-surface proteins.

Rainbow trout (Oncorhynchus mykiss) contain two calnexin genes which encode distinct proteins

Calnexin (IP90/P88) is an integral membrane protein of the endoplasmic reticulum that binds newly synthesized N-linked glycoproteins during their folding in the ER including MHC class I molecule. This manuscript reports the identification of two unique cDNA clones of calnexin in rainbow trout. Both encode putative mature proteins of 579 and 592 aa respectively in addition to a 24 aa signal peptide. Sequence analysis revealed that only one of the two cDNA clones encodes a putative ER retention signal, K/QEDDL, followed by a serine phosphorylation site conserved with mammalian homologs. Amino acid sequence alignment illustrated conservation of the calnexin luminal domain, which consists of a globular and a P domain, in both copies. Southern blotting revealed that there are at least two copies of the calnexin gene in the trout genome and northern blotting showed a wide tissue distribution of an estimated 3 kbp calnexin transcript with an additional minor transcript of 2.3 kbp expressed only in head kidney, spleen PBLs and strongly in RTS11. Importantly, the smaller transcript was predominantly upregulated in RTS11 after a 24h treatment with the calcium ionophore A23187. In western blots, calnexin was detected primarily as a 120 kDa protein and upon A23187 treatment; a 100 kDa band was most prominently expressed. These results suggest that in salmonids there are two differentiated versions of the calnexin gene which encode proteins that may have diverged to perform unique biological functions.

Identification of an alternate splice form of tapasin in human melanoma

Assembly of major histocompatibility complex (MHC) class I molecules with peptide in the endoplasmic reticulum requires the assistance of tapasin. Alternative splicing, which is known to regulate many genes, has been reported for tapasin only in the context of mutations. Here, we report on an alternate splice form of tapasin (tpsnΔEx3) derived from a human melanoma cell line that does not appear to be caused by mutations. Excision of exon 3 results in deletion of amino acids 70 to 156 within the beta barrel region, but the membrane proximal Ig domain, the transmembrane domain, and cytoplasmic tail of tapasin are intact. Introduction of tpsnΔEx3 into a tapasin-deficient cell line does not restore MHC class I expression at the cell surface. Similar to a previously described tapasin mutant (tpsnΔN50), tpsnΔEx3 interacts with TAP. Therefore, we used these altered forms of tapasin to test the importance of MHC class I interaction with TAP. In the presence of wild-type tapasin, transfection of tpsnΔN50, but not tpsnΔEx3, reduced MHC class I expression at the cell surface likely due its ability to compete MHC class I molecules from TAP. Together these findings suggest that tumor cells may contain alternate splice forms of tapasin which may regulate MHC class I antigen presentation.

Suppression of MHC class I surface expression by calreticulin's P-domain in a calreticulin deficient cell line

Calreticulin (CRT) is an important chaperone protein, comprising an N-domain, P-domain and C-domain. It is involved in the folding and assembly of multi-component protein complexes in the endoplasmic reticulum, and plays a critical role in MHC class I antigen processing and presentation. To dissect the functional role and molecular basis of individual domains of the protein, we have utilized individual domains to rescue impaired protein assembly in a CRT deficient cell line. Unexpectedly, both P-domain fragment and NP domain of CRT not only failed to rescue defective cell surface expression of MHC class I molecules but further inhibited their appearance on the surface of cells. Formation of the TAP-associated peptide-loading complex and trafficking of the few detectable MHC class I molecules were not significantly impaired. Instead, this further suppression of MHC class I molecules on the cell surface appears due to the complex missing antigenic peptides, the third member of fully assembled MHC class I molecules. Therefore the P-domain of calreticulin appears to play a significant role in antigen presentation by MHC class I molecules.

Lentiviral calnexin-modified dendritic cells promote expansion of high-avidity effector T cells with central memory phenotype

Dendritic cells (DCs) are key immune mediators for the education and activation of effector cytotoxic T lymphocytes (CTLs). Ex vivo manipulation of DCs is an attractive strategy in immunotherapy. The chaperone proteins are known to hold the keys to proper protein folding and antigen processing. However, little is known of the role of molecular chaperones in DC and T-cell functions. We report that DCs expressing supraphysiological levels of calnexin, a chaperone protein, via lentiviral gene transfer stimulated the expansion of high-avidity CTLs with increased central memory phenotype. Microarray RNA profiling and analyses of protein expression with flow cytometry and multiplex enzyme-linked immunosorbent assay indicated that calnexin had a global effect on DCs with up-regulation of immune modulatory signals including costimulatory molecules, cytokines, chemokines and adhesion molecules. Compared with unmodified DCs, calnexin-DCs were capable of activating T cells to exhibit increased functional avidity associated with up-regulation of CCR7 and costimulatory tumour necrosis factor receptor superfamily molecules. These findings demonstrate a prominent role of calnexin in optimizing DC immunity with potential for improving immunotherapy.

A novel calcium oxalate crystal growth inhibitory protein from the seeds of Dolichos biflorus (L.)

Recurrence and persistent side effects of present day treatment for urolithiasis restrict their use, so an alternate, using phytotherapy is being sought. Dolichos biflorus seeds, which are used as dietary food in India, possess antilithiatic properties. In the present study, a novel dimeric antilithiatic protein (98 kDa) from its seeds was purified based on its ability to inhibit calcium oxalate crystallization in vitro. Amino acid analysis of Dolichos biflorus antilithiatic protein showed abundant acidic amino acids. The mascot search engine presented sequence similarity with a calcium binding protein, calnexin of Pisum sativum from the m/z data obtained by MALDI TOF mass spectrometer. Above results demonstrate the anticalcifying/antilithiatic nature of a novel protein from the seeds of Dolichos biflorus and thus open new vistas for using plant proteins as therapeutic agents to treat urolithiasis.

Molecular mechanisms of MHC class I-antigen processing: redox considerations

Major histocompatibility complex (MHC) class I molecules present antigenic peptides to the cell surface for screening by CD8(+) T cells. A number of ER-resident chaperones assist the assembly of peptides onto MHC class I molecules, a process that can be divided into several steps. Early folding of the MHC class I heavy chain is followed by its association with beta(2)-microglobulin (beta(2)m). The MHC class I heavy chain-beta(2)m heterodimer is incorporated into the peptide-loading complex, leading to peptide loading, release of the peptide-filled MHC class I molecules from the peptide-loading complex, and exit of the complete MHC class I complex from the ER. Because proper antigen presentation is vital for normal immune responses, the assembly of MHC class I molecules requires tight regulation. Emerging evidence indicates that thiol-based redox regulation plays critical roles in MHC class I-restricted antigen processing and presentation, establishing an unexpected link between redox biology and antigen processing. We review the influences of redox regulation on antigen processing and presentation. Because redox signaling pathways are a rich source of validated drug targets, newly discovered redox biology-mediated mechanisms of antigen processing may facilitate the development of more selective and therapeutic drugs or vaccines against immune diseases.

Heat-shock proteins

Heat-shock proteins (HSPs), or stress proteins, are highly conserved and present in all organisms and in all cells of all organisms. Selected HSPs, also known as chaperones, play crucial roles in folding/unfolding of proteins, assembly of multiprotein complexes, transport/sorting of proteins into correct subcellular compartments, cell-cycle control and signaling, and protection of cells against stress/apoptosis. More recently, HSPs have been implicated in antigen presentation with the role of chaperoning and transferring antigenic peptides to the class I and class II molecules of the major histocompatibility complexes. In addition, extracellular HSPs can stimulate professional antigen-presenting cells of the immune system, such as macrophages and dendritic cells. HSPs constitute a large family of proteins that are often classified based on their molecular weight: hsp10, hsp40, hsp60, hsp70, hsp90, etc. This unit contains a table that lists common HSPs and summarizes their characteristics including (a) name, (b) subcellular localization, (c) known function, (d) chromosome assignment, (e) brief comments, and (f) references.

Assembly and transport of class I MHC-peptide complexes

Peptides that are presented to T-cells by class I major histocompatibility complex molecules are derived from cytosolic proteins. They are generated in the cytosol and translocated into the endoplasmic reticulum (ER) by the transporters associated with antigen processing (TAP molecules). Competition experiments suggest that TAP molecules can specifically translocate a wide range of peptides from 8-13 amino acids long; longer peptides are less likely to be transported. A photoactivatable peptide derivative has been used to demonstrate that competition for transport into the ER reflects competition for a specific peptide-binding site on the TAP molecule. Class I molecules bind the translocated peptides in the ER thereby allowing their transport to the cell surface. The assembly of the class I-peptide complex in the ER is tightly regulated. The evidence suggests that class I heavy chains first dimerize with beta 2-microglobulin in a process mediated by the chaperone calnexin. The class I-beta 2-microglobulin dimer then physically associates with TAP molecules and is released for transport when it binds a peptide.

An extra molecule in addition to human tapsin is required for surface expression of β2m linked HLA-B4402 on murine cell

Murine MHC class I can be readily expressed on the surface of human cell lines, but human class I molecules are expressed on mouse cells at a reduced level. Both human beta-2-microglobulin (beta(2)m) and tapasin (Tpn) have been demonstrated to be required for proper human MHC class I surface expression. Here we report that besides beta(2)m and tapasin, an extra unidentified component is also critical for the expression of certain human class I alleles. By covalently linking HLA-B4402 heavy chain to beta(2)m (beta(2)m-B44) a pre-assembled class I molecule has been created, which can be efficiently expressed and travel to the surface in human cells. In spite of being able to express inside cells, the linked beta(2)m-B44 molecule does not express on the surface of a murine fibroblast. Further investigation shows that lack of appearance on the surface is not due to quick degradation of unloaded class I, since provision of HLA-B4402 binding peptide could not rescue impaired surface expression. Co-expression with human tapasin does not rescue the defect excluding tapasin as the critical component for expression and indicating that a novel component of human origin is required for efficient surface expression of beta(2)m-B44 in murine cells. Surprisingly, not only did the beta(2)m-B44 construct fail to express on murine cells but also the surface expression of native murine MHC class I Kb was greatly reduced in transfected cells. It is likely that the expressed linked chain competitively associates with a component of class I processing in murine cells, reducing the exit rate of assembled mouse class I molecules. The results together suggest an unknown mechanism, which leads to the trapping of class I molecules in the ER.

ERp57 interacts with conserved cysteine residues in the MHC class I peptide-binding groove

The oxidoreductase ERp57 is a component of the major histocompatibility complex (MHC) class I peptide-loading complex. ERp57 can interact directly with MHC class I molecules, however, little is known about which of the cysteine residues within the MHC class I molecule are relevant to this interaction. MHC class I molecules possess conserved disulfide bonds between cysteines 101-164, and 203-259 in the peptide-binding and alpha3 domain, respectively. By studying a series of mutants of these conserved residues, we demonstrate that ERp57 predominantly associates with cysteine residues in the peptide-binding domain, thus indicating ERp57 has direct access to the peptide-binding groove of MHC class I molecules during assembly.

Increased proteolysis of diphtheria toxin by human monocytes after heat shock: a subsidiary role for heat-shock protein 70 in antigen processing

The expression of heat-shock proteins (hsp) increases after exposure to various stresses including elevated temperatures, oxidative injury, infection and inflammation. As molecular chaperones, hsp have been shown to participate in antigen processing and presentation, in part through increasing the stability and expression of major histocompatibility complex molecules. Heat shock selectively increases human T-cell responses to processed antigen, but does not affect T-cell proliferation induced by non-processed antigens. Here, we have analysed the mechanisms by which stress such as heat shock, and the ensuing hsp over-expression affect the processing of diphtheria toxin (DT) in peripheral blood monocytes. We found that heat shock increased DT proteolysis in endosomes and lysosomes while the activities of the cathepsins B and D, classically involved in DT proteolysis, were decreased. These effects correlated with the heat-shock-mediated increase in hsp 70 expression observed in endosomes and lysosomes. Actinomycin D or blocking anti-hsp 70 antibodies abolished the heat-shock-mediated increase in DT proteolysis. These data indicate that the increased expression of hsp 70 constitutes a subsidiary mechanism that facilitates antigen proteolysis in stressed cells. Confirming these data, presentation by formaldehyde-fixed cells of DT proteolysates that were obtained with endosomes and lysosomes from heat-shocked peripheral blood monocytes showed higher stimulation of T cells than those generated with endosomes and lysosomes from control peripheral blood monocytes.

α-Tocopherol induces calnexin in renal tubular cells: Another protective mechanism against free radical-induced cellular damage

Pre-administration of alpha-tocopherol is protective against oxidative renal tubular damage and subsequent carcinogenesis by ferric nitrilotriacetate (Fe-NTA) in rats. We searched for mechanisms other than the scavenging effect of alpha-tocopherol with microarray analyses, which implicated calnexin, a chaperone for glycoproteins. Renal mRNA levels of calnexin significantly increased 3h after an injection of Fe-NTA in rats fed a standard diet whereas those fed an alpha-tocopherol-supplemented diet showed an increase prior to injection, but after injection showed a decrease in renal calnexin mRNA levels, with unaltered protein levels. In experiments using LLC-PK1 cells, addition of alpha-tocopherol was protective against oxidative stress by H2O2, concomitant with calnexin induction. Knockdown of calnexin by siRNA significantly reduced this protection. Furthermore, COS-7 cells transfected with the calnexin gene were more resistant to H2O2. Together with the fact that alpha-tocopherol induced N-acetylglucosaminyltransferase 3, our data suggest that alpha-tocopherol modifies glycoprotein metabolism partially by conferring mild ER stress. This adds another molecular mechanism of alpha-tocopherol toward cancer prevention.

Human cytomegalovirus-encoded US2 and US11 target unassembled MHC class I heavy chains for degradation

Surface MHC class I molecules serve important immune functions as ligands for both T and NK cell receptors for the elimination of infected and malignant cells. In order to reach the cell surface, MHC class I molecules have to fold properly and form trimers consisting of a heavy chain (HC), a beta2-microglobulin light chain and an 8-10-mer peptide. A panel of ER chaperones facilitates the folding and assembly process. Incorrectly assembled or folded MHC class I HCs are detected by the ER quality-control system and transported to the cytosol for degradation by proteasomes. In human cytomegalovirus-infected cells, two viral proteins are synthesized, US2 and US11, which target MHC class I HCs for proteasomal degradation. It is unknown at which stage of MHC class I folding and complex formation US2 and US11 come into play. In addition, it is unclear if the disposal takes place via the same pathway through which proteins are removed that fail to pass ER quality control. In this study, we show with a beta2m-deficient cell line that US2 and US11 both target unassembled HCs for degradation. This suggests that US2 and US11 both act at an early stage of MHC class I complex formation. In addition, our data indicate that US11-mediated degradation involves mechanisms that are similar to those normally used to remove terminally misfolded HCs.

Impaired assembly of the major histocompatibility complex class I peptide-loading complex in mice deficient in the oxidoreductase ERp57

The thiol-oxidoreductase ERp57 is an integral component of the peptide-loading complex of the major histocompatibility complex (MHC) class I pathway, but its function is unknown. To investigate its function in antigen presentation, we generated ERp57-deficient mice. Death in utero caused by ubiquitous ERp57 deletion was prevented by specific deletion in the B cell compartment. We demonstrate that ERp57 was central for recruitment of MHC class I molecules into the loading complex. In ERp57-deficient cells, we found short-lived interaction of MHC class I molecules with the loading complex. Thus, in the steady state, very few MHC class I molecules were present in the loading complex. Surface H-2K(b)-peptide expression and stability were reduced, and presentation of a model antigen was decreased. Our results indicate that ERp57 does not influence the redox state of MHC class I molecules but is an essential structural component required for stable assembly of the peptide-loading complex.

The seven dirty little secrets of major histocompatibility complex class I antigen processing

Every field has its dirty little secrets (DLSs): assumptions based on flimsy evidence, findings that directly contradict prevailing models or so beg comprehension that they cannot even seed reasonable alternative hypotheses. Although our natural tendency is to hug these DLSs, they should be exposed, for it is these gaps in our understanding that point to the path to enlightenment. Here, I discuss some of the DLSs of major histocompatibility complex class I antigen processing.

Caenorhabditis elegans calnexin is N-glycosylated and required for stress response

Calnexin, a type I integral Ca(2+)-binding protein in the endoplasmic reticulum (ER) membrane, has been implicated in various biological functions including chaperone activity, calcium homeostasis, phagocytosis, and ER stress-induced apoptosis. Caenorhabditis elegans CNX-1 is expressed in the H-shaped excretory cell, intestine, dorsal and ventral nerve cord, spermatheca, and head and tail neurons throughout development. A cnx-1 null mutant displays temperature-sensitive developmental and reproductive defects, and retarded growth under stress. Moreover, a double knockout mutant of calnexin and calreticulin exhibits additive severe defects. Interestingly, both cnx-1 transcript and protein levels are elevated under stress conditions suggesting that CNX-1 may be important for stress-induced chaperoning functions in C. elegans. Glycosidase treatment and site-directed mutagenesis confirmed that CeCNX-1 is N-glycosylated at two asparagine residues of Asn(203) and Asn(571). When transgenic animals from cnx-1 mutant were generated, a glycosylation defective construct failed to rescue phenotypes of cnx-1 mutant suggesting that glycosylation is important for calnexin's functions in C. elegans.

β2-Microglobulin required for cell surface expression of blastocyst MHC

Blastocyst MHC is a mouse MHC class Ib gene that is selectively expressed in blastocysts and placenta like human HLA-G, which protect fetal trophoblasts and some tumor cells from NK cell attack, and in TAP-dependent expression on the cell surface. We expressed blastocyst MHC cDNA in beta2-deficient EL-4 S3 or beta2m-transfected EL-4 S3 cells. In parental EL-4 S3 cells, only 47-kDa blastocyst MHC protein was expressed and retained in the cytoplasm. However, additional 51-kDa blastocyst MHC protein was expressed on the surface of beta2m-transfected EL-4 S3 cells. The 51-kDa protein was resistant to Endo-H, whereas the 47-kDa protein was sensitive for Endo-H. The results suggested that beta2m as well as TAP was necessary for the transportation of blastocyst MHC from endoplasmic reticulum to cell surfaces through the Golgi apparatus, similar to other MHC class I molecules.

Different MHC class I heavy chains compete with each other for folding independently of beta 2-microglobulin and peptide

We reported previously that different MHC class I molecules can compete with each other for cell surface expression in F(1) hybrid and MHC class I transgenic mice. In this study, we show that the competition also occurs in transfected cell lines, and investigate the mechanism. Cell surface expression of an endogenous class I molecule in Chinese hamster ovary (CHO) cells was strongly down-regulated when the mouse K(d) class I H chain was introduced by transfection. The competition occurred only after K(d) protein translation, not at the level of RNA, and localization studies of a CHO class I-GFP fusion showed that the presence of K(d) caused retention of the hamster class I molecule in the endoplasmic reticulum. The competition was not for beta(2)-microglobulin, because a single chain version of K(d) that included mouse beta(2)-microglobulin also had a similar effect. The competition was not for association with TAP and loading with peptide, because a mutant form of the K(d) class I H chain, not able to associate with TAP, caused the same down-regulation of hamster class I expression. Moreover, K(d) expression led to a similar level of competition in TAP2-negative CHO cells. Competition for cell surface expression was also found between different mouse class I H chains in transfected mouse cells, and this competition prevented association of the H chain with beta(2)-microglobulin. These unexpected new findings show that different class I H chains compete with each other at an early stage of the intracellular assembly pathway, independently of beta(2)-microglobulin and peptide.

Human cytomegalovirus inhibits tapasin-dependent peptide loading and optimization of the MHC class I peptide cargo for immune evasion

The immune evasion protein US3 of human cytomegalovirus binds to and arrests MHC class I molecules in the endoplasmic reticulum (ER). However, substantial amounts of class I molecules still escape US3-mediated ER retention, suggesting that not all class I alleles are affected equally by US3. Here, we identify tapasin inhibition as the mechanism of MHC retention by US3. US3 directly binds tapasin and inhibits tapasin-dependent peptide loading, thereby preventing the optimization of the peptide repertoire presented by class I molecules. Due to the allelic specificity of tapasin toward class I molecules, US3 affects only class I alleles that are dependent on tapasin for peptide loading and surface expression. Accordingly, tapasin-independent class I alleles selectively escape to the cell surface.

Chaperones and folding of MHC class I molecules in the endoplasmic reticulum

In this review we discuss the influence of chaperones on the general phenomena of folding as well as on the specific folding of an individual protein, MHC class I. MHC class I maturation is a highly sophisticated process in which the folding machinery of the endoplasmic reticulum (ER) is heavily involved. Understanding the MHC class I maturation per se is important since peptides loaded onto MHC class I molecules are the base for antigen presentation generating immune responses against virus, intracellular bacteria as well as tumours. This review discusses the early stages of MHC class I maturation regarding BiP and calnexin association, and differences in MHC class I heavy chain (HC) interaction with calnexin and calreticulin are highlighted. Late stage MHC class I maturation with focus on the dedicated chaperone tapasin is also discussed.

The early expression of glycoprotein B from herpes simplex virus can be detected by antigen-specific CD8+ T cells

The immune response to cutaneous herpes simplex virus type 1 (HSV-1) infection begins with remarkable rapidity. Activation of specific cytotoxic T lymphocytes (CTL) begins within hours of infection, even though the response within the draining lymph nodes peaks nearly 5 days later. HSV gene products are classified into three main groups, alpha, beta, and gamma, based on their kinetics and requirements for expression. In C57BL/6 mice, the immunodominant epitope from HSV is derived from glycoprotein B (gB(498-505)). While gB is considered a gamma or "late" gene product, previous reports have indicated that some level of gene expression may occur soon after infection. Using brefeldin A as a specific inhibitor of viral antigen presentation to major histocompatibility complex class I-restricted CTL, we have formally addressed the timing of gB peptide expression in an immunologically relevant manner following infection. Presentation of gB peptide detected by T-cell activation was first observed within 2 h of infection. Comparison with another viral epitope expressed early during infection, HSV-1 ribonucleotide reductase, demonstrated that gB is presented with the same kinetics as this classical early-gene product. Moreover, this rapidity of gB expression was further illustrated via rapid priming of naïve transgenic CD8(+) T cells in vivo after HSV-1 infection of mice. These results establish that gB is expressed rapidly following HSV-1 infection, at levels capable of effectively stimulating CD8(+) T cells.

Virus subversion of the MHC class I peptide-loading complex

Many viral proteins modulate class I expression, yet, in general, their mechanisms of specific class I recognition are poorly understood. The mK3 protein of gamma(2)-Herpesvirus 68 targets the degradation of nascent class I molecules via the ubiquitin/proteasome pathway. Here, we identify cellular components of the MHC class I assembly machinery, TAP and tapasin, that are required for mK3 function. mK3 failed to regulate class I in TAP- or tapasin-deficient cells, and mK3 interacted with TAP/tapasin, even in the absence of class I. Expression of mK3 resulted in the ubiquitination of TAP/tapasin-associated class I, and mutants of class I incapable of TAP/tapasin interaction were unaffected by mK3. Thus, mK3 subverts TAP/tapasin to specifically target class I molecules for destruction.

Sbh1p, a subunit of the Sec61 translocon, interacts with the chaperone calnexin in the yeast Yarrowia lipolytica

The core component of the translocation apparatus, Sec61p or alpha, was previously cloned in Yarrowia lipolytica. Using anti-Sec61p antibodies, we showed that most of the translocation sites are devoted to co-translational translocation in this yeast, which is similar to the situation in mammalian cells but in contrast to the situation in Saccharomyces cerevisiae, where post-translational translocation is predominant. In order to characterize further the minimal translocation apparatus in Y. lipolytica, the beta Sec61 complex subunit, Sbh1p, was cloned by functional complementation of a Deltasbh1, Deltasbh2 S. cerevisiae mutant. The secretion of the reporter protein is not impaired in the Y. lipolytica sbh1 inactivated strain. We screened the Y. lipolytica two-hybrid library to look for partners of this translocon component. The ER-membrane chaperone protein, calnexin, was identified as an interacting protein. By a co-immunoprecipitation approach, we confirmed this association in Yarrowia and then showed that the S. cerevisiae Sbh2p protein was a functional homologue of YlSbh1p. The interaction of Sbh1p with calnexin was shown to occur between the lumenal domain of both proteins. These results suggest that the beta subunit of the Sec61 translocon may relay folding of nascent proteins to their translocation.

Antigen degradation or presentation by MHC class I molecules via classical and non-classical pathways

Major histocompatibility complex (MHC) class I molecules usually present endogenous peptides at the cell surface. This is the result of a cascade of events involving various dedicated proteins like the peptide transporter associated with antigen processing (TAP) and the ER chaperone tapasin. However, alternative ways for class I peptide loading exist which may be highly relevant in a process called cross-priming. Both pathways are described here in detail. One major difference between these pathways is that the proteases involved in the generation of peptides are different. How proteases and peptidases influence peptide generation and degradation will be discussed. These processes determine the amount of peptides available for TAP translocation and class I binding and ultimately the immune response.