PDILT, a Divergent Testis-specific Protein Disulfide Isomerase with a Non-classical SXXC Motif That Engages in Disulfide-dependent Interactions in the Endoplasmic Reticulum

TMX4-driven LINC complex disassembly and asymmetric autophagy of the nuclear envelope upon acute ER stress

The endoplasmic reticulum (ER) is an organelle of nucleated cells that produces proteins, lipids and oligosaccharides. ER volume and activity are increased upon induction of unfolded protein responses (UPR) and are reduced upon activation of ER-phagy programs. A specialized domain of the ER, the nuclear envelope (NE), protects the cell genome with two juxtaposed lipid bilayers, the inner and outer nuclear membranes (INM and ONM) separated by the perinuclear space (PNS). Here we report that expansion of the mammalian ER upon homeostatic perturbations results in TMX4 reductase-driven disassembly of the LINC complexes connecting INM and ONM and in ONM swelling. The physiologic distance between ONM and INM is restored, upon resolution of the ER stress, by asymmetric autophagy of the NE, which involves the LC3 lipidation machinery, the autophagy receptor SEC62 and the direct capture of ONM-derived vesicles by degradative LAMP1/RAB7-positive endolysosomes in a catabolic pathway mechanistically defined as micro-ONM-phagy.

Genetic Determinants of 25-Hydroxyvitamin D Concentrations and Their Relevance to Public Health

Twin studies suggest a considerable genetic contribution to the variability in 25-hydroxyvitamin D (25(OH)D) concentrations, reporting heritability estimates up to 80% in some studies. While genome-wide association studies (GWAS) suggest notably lower rates (13−16%), they have identified many independent variants that associate with serum 25(OH)D concentrations. These discoveries have provided some novel insight into the metabolic pathway, and in this review we outline findings from GWAS studies to date with a particular focus on 35 variants which have provided replicating evidence for an association with 25(OH)D across independent large-scale analyses. Some of the 25(OH)D associating variants are linked directly to the vitamin D metabolic pathway, while others may reflect differences in storage capacity, lipid metabolism, and pathways reflecting skin properties. By constructing a genetic score including these 25(OH)D associated variants we show that genetic differences in 25(OH)D concentrations persist across the seasons, and the odds of having low concentrations (<50 nmol/L) are about halved for individuals in the highest 20% of vitamin D genetic score compared to the lowest quintile, an impact which may have notable influences on retaining adequate levels. We also discuss recent studies on personalized approaches to vitamin D supplementation and show how Mendelian randomization studies can help inform public health strategies to reduce adverse health impacts of vitamin D deficiency.

Protein disulphide isomerase inhibition as a potential cancer therapeutic strategy

The protein disulphide isomerase (PDI) gene family is a large, diverse group of enzymes recognised for their roles in disulphide bond formation within the endoplasmic reticulum (ER). PDI therefore plays an important role in ER proteostasis, however, it also shows involvement in ER stress, a characteristic recognised in multiple disease states, including cancer. While the exact mechanisms by which PDI contributes to tumorigenesis are still not fully understood, PDI exhibits clear involvement in the unfolded protein response (UPR) pathway. The UPR acts to alleviate ER stress through the activation of ER chaperones, such as PDI, which act to refold misfolded proteins, promoting cell survival. PDI also acts as an upstream regulator of the UPR pathway, through redox regulation of UPR stress receptors. This demonstrates the pro‐protective roles of PDI and highlights PDI as a potential therapeutic target for cancer treatment. Recent research has explored the use of PDI inhibitors with PACMA 31 in particular, demonstrating promising anti‐cancer effects in ovarian cancer. This review discusses the properties and functions of PDI family members and focuses on their potential as a therapeutic target for cancer treatment.

Male-Specific Protein Disulphide Isomerase Function is Essential for Plasmodium Transmission and a Vulnerable Target for Intervention

Inhibiting transmission of Plasmodium is an essential strategy in malaria eradication, and the biological process of gamete fusion during fertilization is a proven target for this approach. Lack of knowledge of the mechanisms underlying fertilization have been a hindrance in the development of transmission-blocking interventions. Here we describe a protein disulphide isomerase essential for malarial transmission (PDI-Trans/PBANKA_0820300) to the mosquito. We show that PDI-Trans activity is male-specific, surface-expressed, essential for fertilization/transmission, and exhibits disulphide isomerase activity which is up-regulated post-gamete activation. We demonstrate that PDI-Trans is a viable anti-malarial drug and vaccine target blocking malarial transmission with the use of PDI inhibitor bacitracin (98.21%/92.48% reduction in intensity/prevalence), and anti-PDI-Trans antibodies (66.22%/33.16% reduction in intensity/prevalence). To our knowledge, these results provide the first evidence that PDI function is essential for malarial transmission, and emphasize the potential of anti-PDI agents to act as anti-malarials, facilitating the future development of novel transmission-blocking interventions.

Emerging roles of protein disulfide isomerase in cancer

The protein disulfide isomerase (PDI) family is a group of multifunctional endoplasmic reticulum (ER) enzymes that mediate the formation of disulfide bonds, catalyze the cysteine-based redox reactions and assist the quality control of client proteins. Recent structural and functional studies have demonstrated that PDI members not only play an essential role in the proteostasis in the ER but also exert diverse effects in numerous human disorders including cancer and neurodege-nerative diseases. Increasing evidence suggests that PDI is actively involved in the proliferation, survival, and metastasis of several types of cancer cells. Although the molecular mechanism by which PDI contributes to tumorigenesis and metastasis remains to be understood, PDI is now emerging as a new therapeutic target for cancer treatment. In fact, several attempts have been made to develop PDI inhibitors as anti-cancer drugs. In this review, we discuss the properties and diverse functions of human PDI proteins and focus on recent findings regarding their roles in the state of diseases including cancer and neurodegeneration.

Crystal and solution structures of human protein-disulfide isomerase-like protein of the testis (PDILT) provide insight into its chaperone activity

Protein-disulfide isomerase-like protein of the testis (PDILT), a member of the protein-disulfide isomerase family, is a chaperone essential for the folding of spermatogenesis-specific proteins in male postmeiotic germ cells. However, the structural mechanisms that regulate the chaperone function of PDILTs are unknown. Here, we report the structures of human PDILT (hPDILT) determined by X-ray crystallography to 2.4 Å resolution and small-angle X-ray scattering (SAXS). Distinct from previously reported U-like structures of related PDI family proteins, our structures revealed that hPDILT folds into a compact L-like structure in crystals and into an extended chain-like structure in solution. The hydrophobic regions and the hydrophobic pockets in hPDILT, which are important for substrate recognition, were clearly delineated in the crystal structure. Moreover, our results of the SAXS analysis and of structure-based substitutions and truncations indicated that the C-terminal tail in hPDILT is required for suppression of aggregation of denatured proteins, suggesting that the tail is crucial for the chaperone activity of PDILT. Taken together, our findings have identified the critical regions and conformational changes of PDILT that enable and control its activity. These results advance our understanding of the structural mechanisms involved in the chaperone activity of PDILT.

Factors controlling sperm migration through the oviduct revealed by gene-modified mouse models

Mammalian fertilization is comprised of many steps including sperm survival in the uterus, sperm migration in the female reproductive tract, physiological and morphological changes to the spermatozoa, and sperm-egg interaction in the oviduct. In vitro studies have revealed essential factors for these fertilization steps for over half a century. However, the molecular mechanism of fertilization has recently been revised by the emergence of genetically modified animals. Here, we focus on essential factors for sperm fertilizing ability and describe recent advances in our knowledge of the mechanisms of mammalian fertilization, especially of sperm migration from the uterus into the oviduct.

The control of male fertility by spermatid-specific factors: searching for contraceptive targets from spermatozoon's head to tail

Male infertility due to abnormal spermatozoa has been reported in both animals and humans, but its pathogenic causes, including genetic abnormalities, remain largely unknown. On the other hand, contraceptive options for men are limited, and a specific, reversible and safe method of male contraception has been a long-standing quest in medicine. Some progress has recently been made in exploring the effects of spermatid-specifical genetic factors in controlling male fertility. A comprehensive search of PubMed for articles and reviews published in English before July 2016 was carried out using the search terms 'spermiogenesis failure', 'globozoospermia', 'spermatid-specific', 'acrosome', 'infertile', 'manchette', 'sperm connecting piece', 'sperm annulus', 'sperm ADAMs', 'flagellar abnormalities', 'sperm motility loss', 'sperm ion exchanger' and 'contraceptive targets'. Importantly, we have opted to focus on articles regarding spermatid-specific factors. Genetic studies to define the structure and physiology of sperm have shown that spermatozoa appear to be one of the most promising contraceptive targets. Here we summarize how these spermatid-specific factors regulate spermiogenesis and categorize them according to their localization and function from spermatid head to tail (e.g., acrosome, manchette, head-tail conjunction, annulus, principal piece of tail). In addition, we emphatically introduce small-molecule contraceptives, such as BRDT and PPP3CC/PPP3R2, which are currently being developed to target spermatogenic-specific proteins. We suggest that blocking the differentiation of haploid germ cells, which rarely affects early spermatogenic cell types and the testicular microenvironment, is a better choice than spermatogenic-specific proteins. The studies described here provide valuable information regarding the genetic and molecular defects causing male mouse infertility to improve our understanding of the importance of spermatid-specific factors in controlling fertility. Although a male contraceptive 'pill' is still many years away, research into the production of new small-molecule contraceptives targeting spermatid-specific proteins is the right avenue.

Proteome-wide survey of the autoimmune target repertoire in autoimmune polyendocrine syndrome type 1

Autoimmune polyendocrine syndrome type 1 (APS1) is a monogenic disorder that features multiple autoimmune disease manifestations. It is caused by mutations in the Autoimmune regulator (AIRE) gene, which promote thymic display of thousands of peripheral tissue antigens in a process critical for establishing central immune tolerance. We here used proteome arrays to perform a comprehensive study of autoimmune targets in APS1. Interrogation of established autoantigens revealed highly reliable detection of autoantibodies, and by exploring the full panel of more than 9000 proteins we further identified MAGEB2 and PDILT as novel major autoantigens in APS1. Our proteome-wide assessment revealed a marked enrichment for tissue-specific immune targets, mirroring AIRE’s selectiveness for this category of genes. Our findings also suggest that only a very limited portion of the proteome becomes targeted by the immune system in APS1, which contrasts the broad defect of thymic presentation associated with AIRE-deficiency and raises novel questions what other factors are needed for break of tolerance.

Conserved C272/278 in b domain regulate the function of PDI-P5 to make lysozymes trypsin-resistant forms via significant intermolecular disulfide cross-linking

Protein disulfide isomerase-P5 (P5) is thought to have important functions as an oxidoreductase, however, molecular functions of P5 have not been fully elucidated. We have reported that P5 has insulin reductase activity and inhibits lysozyme refolding by formation of lysozyme multimers with hypermolecular mass inactivated by intermolecular disulfides (hyLYS) in oxidative refolding of reduced denatured lysozyme. To explore the role of each domain of P5, we investigated the effects of domain deletion and Cys-Ala mutants of P5 on insulin reduction and the oxidative refolding of the lysozyme. The mutants of catalytic cysteines, C36/39A, C171/174A, and C36/39/171/174A inhibited the lysozyme refolding almost similarly to P5, and even b domain without catalytic cysteines showed moderate inhibitory effect, suggesting that the b domain played a key role in the inhibition. Western blotting analysis of the refolding products indicated that the catalytic cysteines in both the a and a' domains cross-linked lysozyme comparably to form hyLYS resistant to trypsin, in which b domain was suggested to capture lysozyme for the significant sulfhydryl oxidation. The mutant of the conserved cysteines in b domain, C272/278A, did not form hyLYS, however, showed predominant reductase activity, implying that P5 functioned as a potent sulfhydryl oxidase and a predominant reductase depending on the circumstance around C272/278. These results provide new insight into the molecular basis of P5 function.

Molecular characterization and analysis of a novel protein disulfide isomerase-like protein of Eimeria tenella

Protein disulfide isomerase (PDI) and PDI-like proteins are members of the thioredoxin superfamily. They contain thioredoxin-like domains and catalyze the physiological oxidation, reduction and isomerization of protein disulfide bonds, which are involved in cell function and development in prokaryotes and eukaryotes. In this study, EtPDIL, a novel PDI-like gene of Eimeria tenella, was cloned using rapid amplification of cDNA ends (RACE) according to the expressed sequence tag (EST). The EtPDIL cDNA contained 1129 nucleotides encoding 216 amino acids. The deduced EtPDIL protein belonged to thioredoxin-like superfamily and had a single predicted thioredoxin domain with a non-classical thioredoxin-like motif (SXXC). BLAST analysis showed that the EtPDIL protein was 55–59% identical to PDI-like proteins of other apicomplexan parasites. The transcript and protein levels of EtPDIL at different development stages were investigated by real-time quantitative PCR and western blot. The messenger RNA and protein levels of EtPDIL were higher in sporulated oocysts than in unsporulated oocysts, sporozoites or merozoites. Protein expression was barely detectable in unsporulated oocysts. Western blots showed that rabbit antiserum against recombinant EtPDIL recognized only a native 24 kDa protein from parasites. Immunolocalization with EtPDIL antibody showed that EtPDIL had a disperse distribution in the cytoplasm of whole sporozoites and merozoites. After sporozoites were incubated in complete medium, EtPDIL protein concentrated at the anterior of the sporozoites and appeared on the surface of parasites. Specific staining was more intense and mainly located on the parasite surface after merozoites released from mature schizonts invaded DF-1 cells. After development of parasites in DF-1 cells, staining intensified in trophozoites, immature schizonts and mature schizonts. Antibody inhibition of EtPDIL function reduced the ability of E. tenella to invade DF-1 cells. These results suggested that EtPDIL might be involved in sporulation in external environments and in host cell adhesion, invasion and development of E. tenella.

Structure of the substrate-binding b' domain of the Protein Disulfide Isomerase-Like protein of the Testis

Protein Disulfide Isomerase-Like protein of the Testis (PDILT) is a testis-specific member of the PDI family. PDILT displays similar domain architecture to PDIA1, the founding member of this protein family, but lacks catalytic cysteines needed for oxidoreduction reactions. This suggests special importance of chaperone activity of PDILT, but how it recognizes misfolded protein substrates is unknown. Here, we report the high-resolution crystal structure of the b′ domain of human PDILT. The structure reveals a conserved hydrophobic pocket, which is likely a principal substrate-binding site in PDILT. In the crystal, this pocket is occupied by side chains of tyrosine and tryptophan residues from another PDILT molecule, suggesting a preference for binding exposed aromatic residues in protein substrates. The lack of interaction of the b′ domain with the P-domains of calreticulin-3 and calmegin hints at a novel way of interaction between testis-specific lectin chaperones and PDILT. Further studies of this recently discovered PDI member would help to understand the important role that PDILT plays in the differentiation and maturation of spermatozoids.

Emerging features of ER resident J-proteins in plants

J-proteins are co-chaperone components of the HSP70 system. J-proteins stimulate Hsp70ATPase activity, which is responsible for stabilizing the interaction of Hsp70 with client proteins. J-proteins are localized in various intracellular compartments including the cytoplasm, mitochondria and endoplasmic reticulum (ER). Five types of ER resident J-proteins (ERdjs) have been found in plants (P58, ERdj2, ERdj2A, ERdj3B and ERdj7). Rice OsERdj3A is located in the vacuoleand protein storage vacuoles (PSV, PB-II) under conditions of ER stress. J-proteins that are localized to the vacuole or lysosome are not found in mammals and yeast, suggesting that the presence of OsERdj3A in the vacuole is plant-specific and one of the features unique to plant ERdjs. In this review, we summarize the current state of knowledge andrecent research advancements regarding plant ERdjs, and compare mammalian and yeast ERdjs with plant ERdjs.

Analysis of changes in hepatic gene expression in a murine model of tolerance to acetaminophen hepatotoxicity (autoprotection)

Pretreatment of mice with a low hepatotoxic dose of acetaminophen (APAP) results in resistance to a subsequent, higher dose of APAP. This mouse model, termed APAP autoprotection was used here to identify differentially expressed genes and cellular pathways that could contribute to this development of resistance to hepatotoxicity. Male C57BL/6J mice were pretreated with APAP (400mg/kg) and then challenged 48h later with 600mg APAP/kg. Livers were obtained 4 or 24h later and total hepatic RNA was isolated and hybridized to Affymetrix Mouse Genome MU430_2 GeneChip. Statistically significant genes were determined and gene expression changes were also interrogated using the Causal Reasoning Engine (CRE). Extensive literature review narrowed our focus to methionine adenosyl transferase-1 alpha (MAT1A), nuclear factor (erythroid-derived 2)-like 2 (Nrf2), flavin-containing monooxygenase 3 (Fmo3) and galectin-3 (Lgals3). Down-regulation of MAT1A could lead to decreases in S-adenosylmethionine (SAMe), which is known to protect against APAP toxicity. Nrf2 activation is expected to play a role in protective adaptation. Up-regulation of Lgals3, one of the genes supporting the Nrf2 hypothesis, can lead to suppression of apoptosis and reduced mitochondrial dysfunction. Fmo3 induction suggests the involvement of an enzyme not known to metabolize APAP in the development of tolerance to APAP toxicity. Subsequent quantitative RT-PCR and immunochemical analysis confirmed the differential expression of some of these genes in the APAP autoprotection model. In conclusion, our genomics strategy identified cellular pathways that might further explain the molecular basis for APAP autoprotection.

A comparison of brain gene expression levels in domesticated and wild animals

Domestication has led to similar changes in morphology and behavior in several animal species, raising the question whether similarities between different domestication events also exist at the molecular level. We used mRNA sequencing to analyze genome-wide gene expression patterns in brain frontal cortex in three pairs of domesticated and wild species (dogs and wolves, pigs and wild boars, and domesticated and wild rabbits). We compared the expression differences with those between domesticated guinea pigs and a distant wild relative (Cavia aperea) as well as between two lines of rats selected for tameness or aggression towards humans. There were few gene expression differences between domesticated and wild dogs, pigs, and rabbits (30-75 genes (less than 1%) of expressed genes were differentially expressed), while guinea pigs and C. aperea differed more strongly. Almost no overlap was found between the genes with differential expression in the different domestication events. In addition, joint analyses of all domesticated and wild samples provided only suggestive evidence for the existence of a small group of genes that changed their expression in a similar fashion in different domesticated species. The most extreme of these shared expression changes include up-regulation in domesticates of SOX6 and PROM1, two modulators of brain development. There was almost no overlap between gene expression in domesticated animals and the tame and aggressive rats. However, two of the genes with the strongest expression differences between the rats (DLL3 and DHDH) were located in a genomic region associated with tameness and aggression, suggesting a role in influencing tameness. In summary, the majority of brain gene expression changes in domesticated animals are specific to the given domestication event, suggesting that the causative variants of behavioral domestication traits may likewise be different.

The human protein disulfide isomerase gene family

Enzyme-mediated disulfide bond formation is a highly conserved process affecting over one-third of all eukaryotic proteins. The enzymes primarily responsible for facilitating thiol-disulfide exchange are members of an expanding family of proteins known as protein disulfide isomerases (PDIs). These proteins are part of a larger superfamily of proteins known as the thioredoxin protein family (TRX). As members of the PDI family of proteins, all proteins contain a TRX-like structural domain and are predominantly expressed in the endoplasmic reticulum. Subcellular localization and the presence of a TRX domain, however, comprise the short list of distinguishing features required for gene family classification. To date, the PDI gene family contains 21 members, varying in domain composition, molecular weight, tissue expression, and cellular processing. Given their vital role in protein-folding, loss of PDI activity has been associated with the pathogenesis of numerous disease states, most commonly related to the unfolded protein response (UPR). Over the past decade, UPR has become a very attractive therapeutic target for multiple pathologies including Alzheimer disease, Parkinson disease, alcoholic and non-alcoholic liver disease, and type-2 diabetes. Understanding the mechanisms of protein-folding, specifically thiol-disulfide exchange, may lead to development of a novel class of therapeutics that would help alleviate a wide range of diseases by targeting the UPR.

The protein disulfide isomerase family: key players in health and disease

SIGNIFICANCE

Protein disulfide isomerase (PDI) and its homologs have essential roles in the oxidative folding and chaperone-mediated quality control of proteins in the secretory pathway. In this review, the importance of PDI in health and disease will be examined, using examples from the fields of lipid homeostasis, hemostasis, infectious disease, cancer, neurodegeneration, and infertility.

RECENT ADVANCES

Recent structural studies, coupled with cell biological, biochemical, and clinical approaches, have demonstrated that PDI family proteins are involved in a wide range of physiological and disease processes.

CRITICAL ISSUES

Critical issues in the field include understanding how and why a PDI family member is involved in a given disease, and defining the physiological client specificity of the various PDI proteins when they are expressed in different tissues.

FUTURE DIRECTIONS

Future directions are likely to include the development of new and more specific reagents to study and manipulate PDI family function. The development of conditional mouse models in concert with clinical data will help us to understand the in vivo function of the different PDIs at the organism level. Taken together with advances in structural biology and biochemical studies, this should help us to further understand and modify PDIs' functional interactions.

Protein disulfide isomerase homolog PDILT is required for quality control of sperm membrane protein ADAM3 and male fertility [corrected]

A disintegrin and metalloproteinase 3 (ADAM3) is a sperm membrane protein critical for both sperm migration from the uterus into the oviduct and sperm primary binding to the zona pellucida (ZP). Here we show that the testis-specific protein disulfide isomerase homolog (PDILT) cooperates with the testis-specific calreticulin-like chaperone, calsperin (CALR3), in the endoplasmic reticulum and plays an indispensable role in the disulfide-bond formation and folding of ADAM3. Pdilt(-/-) mice were male infertile because ADAM3 could not be folded properly and transported to the sperm surface without the PDILT/CALR3 complex. Peculiarly we find that not only Pdilt(-/-), but also Adam3(-/-), spermatozoa effectively fertilize eggs when the eggs are surrounded in cumulus oophorus. These findings reveal that ADAM3 requires testis-specific private chaperones to be folded properly and that the principle role of ADAM3 is for sperm migration into the oviduct but not for the fertilization event. Moreover, the importance of primary sperm ZP binding, which has been thought to be a critical step in mammalian fertilization, should be reconsidered.

YajL, prokaryotic homolog of parkinsonism-associated protein DJ-1, functions as a covalent chaperone for thiol proteome

YajL is the closest Escherichia coli homolog of the Parkinsonism-associated protein DJ-1, a multifunctional oxidative stress response protein whose biochemical function remains unclear. We recently reported the aggregation of proteins in a yajL mutant in an oxidative stress-dependent manner and that YajL exhibits chaperone activity. Here, we show that YajL displays covalent chaperone and weak protein oxidoreductase activities that are dependent on its exposed cysteine 106. It catalyzes reduced RNase oxidation and scrambled RNase isomerization and insulin reduction and forms mixed disulfides with many cellular proteins upon oxidative stress. The formation of mixed disulfides was detected by immunoblotting bacterial extracts with anti-YajL antibodies under nonreducing conditions. Disulfides were purified from bacterial extracts on a YajL affinity column, separated by nonreducing-reducing SDS-PAGE, and identified by mass spectrometry. Covalent YajL substrates included ribosomal proteins, aminoacyl-tRNA synthetases, chaperones, catalases, peroxidases, and other proteins containing cysteines essential for catalysis or FeS cluster binding, such as glyceraldehyde-3-phosphate dehydrogenase, aldehyde dehydrogenase, aconitase, and FeS cluster-containing subunits of respiratory chains. In addition, we show that DJ-1 also forms mixed disulfides with cytoplasmic proteins upon oxidative stress. These results shed light on the oxidative stress-dependent chaperone function of YajL and identify YajL substrates involved in translation, stress protection, protein solubilization, and metabolism. They reveal a crucial role for cysteine 106 and suggest that DJ-1 also functions as a covalent chaperone. These findings are consistent with several defects observed in yajL or DJ-1 mutants, including translational defects, protein aggregation, oxidative stress sensitivity, and metabolic deficiencies.

Protein folding and modification in the mammalian endoplasmic reticulum

Analysis of the human genome reveals that approximately a third of all open reading frames code for proteins that enter the endoplasmic reticulum (ER), demonstrating the importance of this organelle for global protein maturation. The path taken by a polypeptide through the secretory pathway starts with its translocation across or into the ER membrane. It then must fold and be modified correctly in the ER before being transported via the Golgi apparatus to the cell surface or another destination. Being physically segregated from the cytosol means that the ER lumen has a distinct folding environment. It contains much of the machinery for fulfilling the task of protein production, including complex pathways for folding, assembly, modification, quality control, and recycling. Importantly, the compartmentalization means that several modifications that do not occur in the cytosol, such as glycosylation and extensive disulfide bond formation, can occur to secreted proteins to enhance their stability before their exposure to the extracellular milieu. How these various machineries interact during the normal pathway of folding and protein secretion is the subject of this review.

The tissue-specific Rep8/UBXD6 tethers p97 to the endoplasmic reticulum membrane for degradation of misfolded proteins

The protein known as p97 or VCP in mammals and Cdc48 in yeast is a versatile ATPase complex involved in several biological functions including membrane fusion, protein folding, and activation of membrane-bound transcription factors. In addition, p97 plays a central role in degradation of misfolded secretory proteins via the ER-associated degradation pathway. This functional diversity of p97 depends on its association with various cofactors, and to further our understanding of p97 function it is important that these cofactors are identified and analyzed. Here, we isolate and characterize the human protein named Rep8 or Ubxd6 as a new cofactor of p97. Mouse Rep8 is highly tissue-specific and abundant in gonads. In testes, Rep8 is expressed in post-meiotic round spermatids, whereas in ovaries Rep8 is expressed in granulosa cells. Rep8 associates directly with p97 via its UBX domain. We show that Rep8 is a transmembrane protein that localizes to the ER membrane with its UBX domain facing the cytoplasm. Knock-down of Rep8 expression in human cells leads to a decreased association of p97 with the ER membrane and concomitantly a retarded degradation of misfolded ER-derived proteasome substrates. Thus, Rep8 tethers p97 to the ER membrane for efficient ER-associated degradation.

Chemical stress on protein disulfide isomerases and inhibition of their functions

Protein disulfide isomerase (PDI) is a folding assistant in the endoplasmic reticulum (ER) of eukaryotic cells. PDI has multiple roles, acting as a chaperone, a binding partner of other proteins, and a hormone reservoir as well as a disulfide isomerase in the formation of disulfide bonds. PDI only interacts covalently with the cysteines of its substrates, but also binds a variety of peptides/proteins and small chemical ligands such as thyroid hormone. Oxidative stress and nitrosative stress can cause damage to chaperones, protein misfolding, and neurodegenerative disease, by affecting the functional integrity of PDI. There are 20 putative PDI-family members in the ER of human cells, but their functional differentiation is far from complete. This review discusses recent advances in our understanding of the mammalian PDI family of enzymes and focuses on their functional properties and interaction with substrates and small chemical ligands.

Disulfide bonds in ER protein folding and homeostasis

Proteins that are expressed outside the cell must be synthesized, folded, and assembled in a way that ensures they can function in their designate location. Accordingly, these proteins are primarily synthesized in the endoplasmic reticulum (ER), which has developed a chemical environment more similar to that outside the cell. This organelle is equipped with a variety of molecular chaperones and folding enzymes that both assist the folding process, while at the same time exerting tight quality control measures that are largely absent outside the cell. A major post-translational modification of ER-synthesized proteins is disulfide bridge formation, which is catalyzed by the family of protein disulfide isomerases. As this covalent modification provides unique structural advantages to extracellular proteins, multiple pathways to disulfide bond formation have evolved. However, the advantages that disulfide bonds impart to these proteins come at a high cost to the cell. Very recent reports have shed light on how the cell can deal with or even exploit the side reactions of disulfide bond formation to maintain homeostasis of the ER and its folding machinery.

Calsperin is a testis-specific chaperone required for sperm fertility

Calnexin (CANX) and calreticulin (CALR) are homologous lectin chaperones located in the endoplasmic reticulum and cooperate to mediate nascent glycoprotein folding. In the testis, calmegin (CLGN) and calsperin (CALR3) are expressed as germ cell-specific counterparts of CANX and CALR, respectively. Here, we show that Calr3(-/-) males produced apparently normal sperm but were infertile because of defective sperm migration from the uterus into the oviduct and defective binding to the zona pellucida. Whereas CLGN was required for ADAM1A/ADAM2 dimerization and subsequent maturation of ADAM3, a sperm membrane protein required for fertilization, we show that CALR3 is a lectin-deficient chaperone directly required for ADAM3 maturation. Our results establish the client specificity of CALR3 and demonstrate that the germ cell-specific CALR-like endoplasmic reticulum chaperones have contrasting functions in the development of male fertility. The identification and understanding of the maturation mechanisms of key sperm proteins will pave the way toward novel approaches for both contraception and treatment of unexplained male infertility.

Drosophila vitelline membrane assembly: a critical role for an evolutionarily conserved cysteine in the "VM domain" of sV23

The vitelline membrane (VM), the oocyte proximal layer of the Drosophila eggshell, contains four major proteins (VMPs) that possess a highly conserved "VM domain" which includes three precisely spaced, evolutionarily conserved, cysteines (CX⁷CX⁸C). Focusing on sV23, this study showed that the three cysteines are not functionally equivalent. While substitution mutations at the first (C123S) or third (C140S) cysteines were tolerated, females with a substitution at the second position (C131S) were sterile. Fractionation studies showed that sV23 incorporates into a large disulfide linked network well after its secretion ceases, suggesting that post-depositional mechanisms are in place to restrict disulfide bond formation until late oogenesis, when the oocyte no longer experiences large volume increases. Affinity chromatography utilizing histidine tagged sV23 alleles revealed small sV23 disulfide linked complexes during the early stages of eggshell formation that included other VMPs, namely sV17 and Vml. The early presence but late loss of these associations in an sV23 double cysteine mutant suggests that reorganization of disulfide bonds may underlie the regulated growth of disulfide linked networks in the vitelline membrane. Found within the context of a putative thioredoxin active site (CXXS) C131, the critical cysteine in sV23, may play an important enzymatic role in isomerizing intermolecular disulfide bonds during eggshell assembly.

Protein quality control in the ER: the recognition of misfolded proteins

The mechanism, in molecular terms of protein quality control, specifically of how the cell recognizes and discriminates misfolded proteins, remains a challenge. In the secretory pathway the folding status of glycoproteins passing through the endoplasmic reticulum is marked by the composition of the N-glycan. The different glycoforms are recognized by specialized lectins. The folding sensor UGGT acts as an unusual molecular chaperone and covalently modifies the Man9 N-glycan of a misfolded protein by adding a glucose moiety and converts it to Glc1Man9 that rebinds the lectin calnexin. However, further links between the folding status of a glycoprotein and the composition of the N-glycan are unclear. There is little unequivocal evidence for other proteins in the ER recognizing the N-glycan and also acting as molecular chaperones. Nevertheless, based upon a few examples, we suggest that this function is carried out by individual proteins in several different complexes. Thus, calnexin binds the protein disulfide isomerase ERp57, that acts upon Glc1Man9 glycoproteins. In another example the protein disulfide isomerase ERdj5 binds specifically to EDEM (which is probably a mannosidase) and a lectin OS9, and reduces the disulfide bonds of bound glycoproteins destined for ERAD. Thus the glycan recognition is performed by a lectin and the chaperone function performed by a specific partner protein that can recognize misfolded proteins. We predict that this will be a common arrangement of proteins in the ER and that members of protein foldase families such as PDI and PPI will bind specifically to lectins in the ER. Molecular chaperones BiP and GRp94 will assist in the folding of proteins bound in these complexes as well as in the folding of non-glycoproteins.

Protein disulfide isomerase: a critical evaluation of its function in disulfide bond formation

Disulfide bond formation is probably involved in the biogenesis of approximately one third of human proteins. A central player in this essential process is protein disulfide isomerase or PDI. PDI was the first protein-folding catalyst reported. However, despite more than four decades of study, we still do not understand much about its physiological mechanisms of action. This review examines the published literature with a critical eye. This review aims to (a) provide background on the chemistry of disulfide bond formation and rearrangement, including the concept of reduction potential, before examining the structure of PDI; (b) detail the thiol-disulfide exchange reactions that are catalyzed by PDI in vitro, including a critical examination of the assays used to determine them; (c) examine oxidation and reduction of PDI in vivo, including not only the role of ERo1 but also an extensive assessment of the role of glutathione, as well as other systems, such as peroxide, dehydroascorbate, and a discussion of vitamin K-based systems; (d) consider the in vivo reactions of PDI and the determination and implications of the redox state of PDI in vivo; and (e) discuss other human and yeast PDI-family members.

Pancreas-specific protein disulfide isomerase has a cell type-specific expression in various mouse tissues and is absent in human pancreatic adenocarcinoma cells: implications for its functions

Members of the protein disulfide isomerase (PDI) family play a critical role in catalyzing the formation of disulfide bonds in secretory proteins, and most of these enzymes have a wide tissue distribution. However, the pancreas-specific PDI homolog was previously suggested to be exclusively expressed in the pancreas (thus commonly referred to as PDIp). In the present study, we found that PDIp was also highly expressed in several other tissues in mice, including the stomach, cecum, ileum, adrenal glands, epididymis, and prostate. Notably, in the digestive organs, such as the stomach and pancreas, very high levels of PDIp were selectively expressed in the digestive enzyme-secreting cells (e.g., gastric chief cells and pancreatic acinar cells). This observation suggests that PDIp may function as a protein-folding catalyst for secretory digestive enzymes. In ileum, PDIp was exclusively expressed in Paneth cells. In addition, high levels of PDIp expression were also detected in normal human pancreas, but its expression was mostly absent in human pancreatic duct adenocarcinoma and pancreatic cancer cell lines. The absence of PDIp expression in pancreatic adenocarcinoma may serve as an additional biomarker for pancreatic cancer.

Efficient IgM assembly and secretion require the plasma cell induced endoplasmic reticulum protein pERp1

Plasma cells daily secrete their own mass in antibodies, which fold and assemble in the endoplasmic reticulum (ER). To reach these levels, cells require pERp1, a novel lymphocyte-specific small ER-resident protein, which attains expression levels as high as BiP when B cells differentiate into plasma cells. Although pERp1 has no homology with known ER proteins, it does contain a CXXC motif typical for oxidoreductases. In steady state, the CXXC cysteines are locked by two parallel disulfide bonds with a downstream C(X)(6)C motif, and pERp1 displays only modest oxidoreductase activity. pERp1 emerged as a dedicated folding factor for IgM, associating with both heavy and light chains and promoting assembly and secretion of mature IgM.

Molecular cloning and characterization of soybean protein disulfide isomerase family proteins with nonclassic active center motifs

Protein disulfide isomerase (PDI) and other PDI family proteins are members of the thioredoxin superfamily and are thought to play important roles in disulfide bond formation and isomerization in the endoplasmic reticulum (ER). The exact functions of PDI family proteins in plants remain unknown. In this study, we cloned two novel PDI family genes from soybean leaf (Glycine max L. Merrill cv. Jack). The cDNAs encode proteins of 520 and 523 amino acids, and have been denoted GmPDIL-3a and GmPDIL-3b, respectively. GmPDIL-3a and GmPDIL-3b are the first plant ER PDI family proteins reported to contain the nonclassic redox center motif CXXS/C, and both proteins are ubiquitously expressed in the plant body. However, recombinant GmPDIL-3a and GmPDIL-3b did not function as oxidoreductases or as molecular chaperones in vitro, although a proportion of each protein formed complexes in both thiol-dependent and thiol-independent ways in the ER. Expression of GmPDIL-3a and GmPDIL-3b in the cotyledon increased during seed maturation when synthesis of storage proteins was initiated. These results suggest that GmPDIL-3a and GmPDIL-3b may play important roles in the maturation of the cotyledon by mechanisms distinct from those of other PDI family proteins.

Solution structure of the bb' domains of human protein disulfide isomerase

Protein disulfide isomerase is the most abundant and best studied of the disulfide isomerases that catalyze disulfide bond formation in the endoplasmic reticulum, yet the specifics of how it binds substrate have been elusive. Protein disulfide isomerase is composed of four thioredoxin-like domains (abb'a'). Cross-linking studies with radiolabeled peptides and unfolded proteins have shown that it binds incompletely folded proteins primarily via its third domain, b'. Here, we determined the solution structure of the second and third domains of human protein disulfide isomerase (b and b', respectively) by triple-resonance NMR spectroscopy and molecular modeling. NMR titrations identified a large hydrophobic surface within the b' domain that binds unfolded ribonuclease A and the peptides mastoparan and somatostatin. Protein disulfide isomerase-catalyzed refolding of reduced ribonuclease A in vitro was inhibited by these peptides at concentrations equal to their affinity to the bb' fragment. Our findings provide a structural basis for previous kinetic and cross-linking studies which have shown that protein disulfide isomerase exhibits a saturable, substrate-binding site.

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.

A disulfide-bond A oxidoreductase-like protein (DsbA-L) regulates adiponectin multimerization

Impairments in adiponectin multimerization lead to defects in adiponectin secretion and function and are associated with diabetes, yet the underlying mechanisms remain largely unknown. We have identified an adiponectin-interacting protein, previously named GST-kappa, by yeast 2-hybrid screening. The adiponectin-interacting protein contains 2 thioredoxin domains and has very little sequence similarity to other GST isoforms. However, this protein shares high sequence and secondary structure homology to bacterial disulfide-bond A oxidoreductase (DsbA) and is thus renamed DsbA-like protein (DsbA-L). DsbA-L is highly expressed in adipose tissue, and its expression level is negatively correlated with obesity in mice and humans. DsbA-L expression in 3T3-L1 adipocytes is stimulated by the insulin sensitizer rosiglitazone and inhibited by the inflammatory cytokine TNFalpha. Overexpression of DsbA-L promoted adiponectin multimerization while suppressing DsbA-L expression by RNAi markedly and selectively reduced adiponectin levels and secretion in 3T3-L1 adipocytes. Our results identify DsbA-L as a key regulator for adiponectin biosynthesis and uncover a potential new target for developing therapeutic drugs for the treatment of insulin resistance and its associated metabolic disorders.

Protein quality control in the early secretory pathway

Eukaryotic cells are able to discriminate between native and non-native polypeptides, selectively transporting the former to their final destinations. Secretory proteins are scrutinized at the endoplasmic reticulum (ER)-Golgi interface. Recent findings reveal novel features of the underlying molecular mechanisms, with several chaperone networks cooperating in assisting the maturation of complex proteins and being selectively induced to match changing synthetic demands. 'Public' and 'private' chaperones, some of which enriched in specializes subregions, operate for most or selected substrates, respectively. Moreover, sequential checkpoints are distributed along the early secretory pathway, allowing efficiency and fidelity in protein secretion.

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.

A developmentally regulated chaperone complex for the endoplasmic reticulum of male haploid germ cells

Glycoprotein folding is mediated by lectin-like chaperones and protein disulfide isomerases (PDIs) in the endoplasmic reticulum. Calnexin and the PDI homologue ERp57 work together to help fold nascent polypeptides with glycans located toward the N-terminus of a protein, whereas PDI and BiP may engage proteins that lack glycans or have sugars toward the C-terminus. In this study, we show that the PDI homologue PDILT is expressed exclusively in postmeiotic male germ cells, in contrast to the ubiquitous expression of many other PDI family members in the testis. PDILT is induced during puberty and represents the first example of a PDI family member under developmental control. We find that PDILT is not active as an oxido-reductase, but interacts with the model peptide Delta-somatostatin and nonnative bovine pancreatic trypsin inhibitor in vitro, indicative of chaperone activity. In vivo, PDILT forms a tissue-specific chaperone complex with the calnexin homologue calmegin. The identification of a redox-inactive chaperone partnership defines a new system of testis-specific protein folding with implications for male fertility.

The endoplasmic reticulum: folding, calcium homeostasis, signaling, and redox control

The endoplasmic reticulum (ER) plays a major role in regulating synthesis, folding, and orderly transport of proteins. It is also essentially involved in various cellular signaling processes, primarily by its function as a dynamic Ca(2+) store. Compared to the cytosol, oxidizing conditions are found in the ER that allow oxidation of cysteine residues in nascent polypeptide chains to form intramolecular disulfide bonds. However, compounds and enzymes such as PDI that catalyze disulfide bonds become reduced and have to be reoxidized for further catalytic cycles. A number of enzymes, among them products of the ERO1 gene, appear to provide oxidizing equivalents, and oxygen appears to be the final oxidant in aerobic living organisms. Thus, protein oxidation in the ER is connected with generation of reactive oxygen species (ROS). Changes in the redox state and the presence of ROS also affect the Ca(2+) homeostasis by modulating the functionality of ER-based channels and buffering chaperones. In addition, a close relationship exists between oxidative stress and ER stress, which both may activate signaling events leading to a rebalance of folding capacity and folding demand or to cell death. Thus, redox homeostasis appears to be a prerequisite for proper functioning of the ER.

Conservation and diversity of the cellular disulfide bond formation pathways

Two pathways for the formation of biosynthetic protein disulfide bonds have been characterized in the endoplasmic reticulum (ER) of eukaryotes. In the major pathway, the membrane-associated flavoprotein Ero1 generates disulfide bonds for transfer to protein disulfide isomerase (PDI), which is responsible for directly introducing disulfide bonds into secretory proteins. In a minor fungal-specific protein oxidation pathway, the membrane-associated flavoprotein Erv2 can catalyze disulfide bond formation via the transfer of oxidizing equivalents to PDI. Genomic sequencing has revealed an abundance of enzymes sharing homology with Ero1, Erv2, or PDI. Herein the authors discuss the functional, mechanistic, and potential structural similarities between these homologs and the core enzymes of the characterized ER oxidation pathways. In addition they speculate about the possible differences between these enzymes that may explain why the cell contains multiple proteins dedicated to a single process. Finally, the eukaryotic ER protein oxidation and reduction pathways are compared to the corresponding prokaryotic periplasmic pathways, to highlight the functional, mechanistic, and structural similarities that exist between the pathways in these two kingdoms despite very low primary sequence homology between the protein and small molecule components.

An introduction to thiol redox proteins in the endoplasmic reticulum and a review of current electrochemical methods of detection of thiols

This aim of this paper is to expound the complexity of thiol redox systems in the endoplasmic reticulum of eukaryotic cells to the electroanalytical community. A summary of the state of the art in electrochemical methods for detection of thiols gives an insight into the challenges that need to be addressed to bridge the disparity between current analytical techniques and applications in a 'real' biological scenario.

Versatility of the endoplasmic reticulum protein folding factory

The endoplasmic reticulum (ER) is dedicated to import, folding and assembly of all proteins that travel along or reside in the secretory pathway of eukaryotic cells. Folding in the ER is special. For instance, newly synthesized proteins are N-glycosylated and by default form disulfide bonds in the ER, but not elsewhere in the cell. In this review, we discuss which features distinguish the ER as an efficient folding factory, how the ER monitors its output and how it disposes of folding failures.

Tissue-specific expression and dimerization of the endoplasmic reticulum oxidoreductase Ero1beta

Endoplasmic reticulum oxidoreductases (Eros) are essential for the formation of disulfide bonds. Understanding disulfide bond catalysis in mammals is important because of the involvement of protein misfolding in conditions such as diabetes, arthritis, cancer, and aging. Mammals express two related Ero proteins, Ero1alpha and Ero1beta. Ero1beta is incompletely characterized but is of physiological interest because it is induced by the unfolded protein response. Here, we show that Ero1beta can form homodimers and mixed heterodimers with Ero1alpha, in addition to Ero-PDI dimers. Ero-Ero dimers require the Ero active site, occur in vivo, and can be modeled onto the Ero1p crystal structure. Our data indicate that the Ero1beta protein is constitutively strongly expressed in the stomach and the pancreas, but in a cell-specific fashion. In the stomach, selective expression of Ero1beta occurs in the enzyme-producing chief cells. In pancreatic islets, Ero1beta expression is high, but is inversely correlated with PDI and PDIp levels, demonstrating that cell-specific differences exist in the regulation of oxidative protein folding in vivo.

The human protein disulphide isomerase family: substrate interactions and functional properties

The process of disulphide bond formation in the endoplasmic reticulum of eukaryotic cells was one of the first mechanisms of catalysed protein folding to be discovered. Protein disulphide isomerase (PDI) is now known to catalyse all of the reactions that are involved in native disulphide bond formation, but despite more than 40 years of study, its mechanism of action is still not fully understood. This review discusses recent advances in our understanding of the human PDI family of enzymes and focuses on their functional properties, substrate interactions and some recently identified family members.

Identification and Characterization of a Novel Thioredoxin-related Transmembrane Protein of the Endoplasmic Reticulum

The endoplasmic reticulum (ER) contains a number of thiol-disulfide oxidoreductases of the protein-disulfide isomerase (PDI) family that catalyze the formation of disulfide bonds in newly synthesized proteins. Here we describe the identification and characterization of a novel member of the human PDI family, TMX3 (thioredoxin-related transmembrane protein 3). The TMX3 gene encodes a protein of 454 amino acid residues that contains a predicted N-terminal signal sequence, a single domain with sequence similarity to thioredoxin and a CGHC active site sequence, a potential transmembrane domain, and a C-terminal KKKD tetrapeptide sequence that matches the classical KKXX-type consensus sequence for ER retrieval of type I transmembrane proteins. Endogenous TMX3 contains endoglycosidase H-sensitive glycans, localizes to the ER by immunofluorescence microscopy, and is present in the membrane fraction after alkaline extraction of the ER luminal content. The TMX3 transcript is found in a variety of tissues and is not up-regulated by the unfolded protein response. Circular dichroism spectroscopy of the recombinantly expressed luminal domain of TMX3 showed features typical of a properly folded protein of the alpha/beta type. The redox potential of recombinant luminal TMX3 was determined to -0.157 V, similar to the values previously found for PDI and ERp57. Interestingly, TMX3 showed oxidase activity, and in human tissue-culture cells the protein was found partially in the oxidized form, potentially suggesting a function of the protein as a dithiol oxidase.