Glycosylation site binding protein, a component of oligosaccharyl transferase, is highly similar to three other 57 kd luminal proteins of the ER

Rab GTPase mediated procollagen trafficking in ascorbic acid stimulated osteoblasts

Despite advances in investigating functional aspects of osteoblast (OB) differentiation, especially studies on how bone proteins are deposited and mineralized, there has been little research on the intracellular trafficking of bone proteins during OB differentiation. Collagen synthesis and secretion is the major function of OBs and is markedly up-regulated upon ascorbic acid (AA) stimulation, significantly more so than in fibroblast cells. Understanding the mechanism by which collagen is mobilized in specialized OB cells is important for both basic cell biology and diseases involving defects in bone protein secretion and deposition. Protein trafficking along the exocytic and endocytic pathways is aided by many molecules, with Rab GTPases being master regulators of vesicle targeting. In this study, we used microarray analysis to identify the Rab GTPases that are up-regulated during a 5-day AA differentiation of OBs, namely Rab1, Rab3d, and Rab27b. Further, we investigated the role of identified Rabs in regulating the trafficking of collagen from the site of synthesis in the ER to the Golgi and ultimately to the plasma membrane utilizing Rab dominant negative (DN) expression. We also observed that experimental halting of biosynthetic trafficking by these mutant Rabs initiated proteasome-mediated degradation of procollagen and ceased global protein translation. Acute expression of Rab1 and Rab3d DN constructs partially alleviated this negative feedback mechanism and resulted in impaired ER to Golgi trafficking of procollagen. Similar expression of Rab27b DN constructs resulted in dispersed collagen vesicles which may represent failed secretory vesicles sequestered in the cytosol. A significant and strong reduction in extracellular collagen levels was also observed implicating the functional importance of Rab1, Rab3d and Rab27b in these major collagen-producing cells.

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.

Developmental changes in the expression of glycogenes and the content of N-glycans in the mouse cerebral cortex

Biosynthesis of N-glycans varies significantly among tissues and is strictly regulated spatially and temporally within the tissue. The strict molecular mechanisms that are responsible for control of N-glycan synthesis remain largely unknown. We developed complementary deoxyribonucleic acid (cDNA) macroarray system and analyzed gene expression levels of more than 140 glycosyltransferases and glycosidases in the cerebral cortex from developing and adult mice. We also analyzed the relative amounts of major N-glycans present in the cerebral cortex and examined how the synthesis of N-glycans might be regulated through the expression of these genes. We demonstrated that the content of N-linked oligosaccharides dramatically changed during the course of brain development. Some of these changes could not be explained by alterations in the expression of the corresponding genes. For example, the amount of core fucosylated sugar chains in the early embryonic brain and the expression level of fucosyltransferase VIII, the only gene known to be responsible for core fucosylation, did not change proportionately. This result suggests that post-transcriptional regulation of this gene plays an important role in regulating its enzymatic activity. On the other hand, the amount of beta1,3-galactose residue-containing sugar chains increased postnatally following an increase in the level of beta1,3-galactosyltransferase messenger ribonucleic acid (mRNA). Furthermore, the amount of sugar chains with an outer fucose residue, containing LewisX-BA-2, correlated well with the expression of fusocyltransferase IX mRNA. These findings add to our understanding of the molecular mechanisms responsible for the regulation of N-glycan biosynthesis in the cerebral cortex.

Regulation of ganglioside biosynthesis in the nervous system

Ganglioside biosynthesis is strictly regulated by the activities of glycosyltransferases and is necessarily controlled at the levels of gene transcription and posttranslational modification. Cells can switch between expressing simple and complex gangliosides or between different series within these two groups during brain development. The sequential biosynthesis of gangliosides in parallel enzymatic pathways, however, requires fine-tuned subcellular sequestration and orchestration of glycosyltransferases. A popular model predicts that this regulation is achieved by the vectorial organization of ganglioside biosynthesis: sequential biosynthetic steps occur with the traffic of ganglioside intermediates through subsequent subcellular compartments. Here, we review current models for the subcellular distribution of glycosyltransferases and discuss results that suggest a critical role of N-glycosylation for the processing, transport, and complex formation of these enzymes. In this context, we attempt to illustrate the regulation of ganglioside biosynthesis as well as the biological significance of N-glycosylation as a posttranslational regulatory mechanism. We also review the results of analyses of the 5' regulatory sequences of several glycosyltransferases in ganglioside biosynthesis and provide insights into how their synthesis can be regulated at the level of transcription.

Re-evaluating the role of heat-shock protein-peptide interactions in tumour immunity

Early investigations into the immune surveillance of chemically-induced sarcomas led to two important concepts in tumour immunobiology: one, tumour rejection can be elicited by immune recognition of tumour antigens; and two, tumours express unique sets of antigens, which are known as tumour-specific antigens. The pioneering studies of Srivastava and colleagues led to the proposal that heat-shock proteins (HSPs) function as ubiquitous tumour-specific antigens, with the specificity residing in a population of bound peptides that identify the tissue of origin of the HSP. However, recent findings, including new data on the cell biology of peptide generation and trafficking, have called into question the specificity of tumour rejection that is induced by HSPs.

Glycopeptide export from the endoplasmic reticulum into cytosol is mediated by a mechanism distinct from that for export of misfolded glycoprotein

When glycoproteins formed in the endoplasmic reticulum (ER) are misfolded, they are generally translocated into the cytosol for ubiquitination and are subsequently degraded by the proteasome. This system, the so-called ER-associated glycoprotein degradation, is important for eukaryotes to maintain the quality of glycoproteins generated in the ER. It has been established in yeast that several distinct proteins are involved in this translocation and degradation processes. Small glycopeptides formed in the ER are exported to the cytosol in a similar manner. This glycopeptide export system is conserved from yeast to mammalian cells, suggesting its basic biological significance for eukaryotic cells. These two export systems (for misfolded glycoproteins and glycopeptides) share some properties, such as a requirement for ATP and involvement of Sec61p, a central membrane protein presumably forming a dislocon channel for export of proteins. However, the machinery of glycopeptide export is poorly understood. In this study, various mutants known to have an effect on export/degradation of misfolded glycoproteins were examined for glycopeptide export activity with a newly established assay method. Surprisingly, most of the mutants were found not to exhibit a defect in glycopeptide export. The only gene that was found to be required on efficient export of both types of substrates was PMR1, the gene encoding the medial-Golgi Ca(2+)/Mn(2+)-ion pump. These results provide evidence that although the systems involved in export of misfolded glycoproteins and glycopeptides share some properties, they have exhibited distinct differences.

Complex regulation of thyroid hormone action: multiple opportunities for pharmacological intervention

The thyroid hormone (TH; 3,3',5,5'-tetra-iodothyronine and 3,3',5'-triiodothyronine) regulates growth, development, and critical metabolic functions. Thyroid diseases are among the most prevalent group of metabolic disorders in the Western world. TH exerts effects through complex biological pathways, which offer a wealth of opportunities to pharmacologically intervene in TH signalling at numerous steps. These include biosynthesis, cell-specific uptake or export (involving L-type amino acid transporter, organic anion transporter, organic cation transporter, or multidrug resistance transporter), as well as nuclear targeting and actions (the latter including TH receptor binding and histone acetylation/deacetylation). Such processes represent potentially important pharmacological targets for the design of novel or improved therapies for TH disorders, obesity, and cardiovascular diseases.

Hormone binding by protein disulfide isomerase, a high capacity hormone reservoir of the endoplasmic reticulum

Protein disulfide isomerase (PDI) is a folding assistant of the eukaryotic endoplasmic reticulum, but it also binds the hormones, estradiol, and 3,3',5-triiodo-l-thyronine (T(3)). Hormone binding could be at discrete hormone binding sites, or it could be a nonphysiological consequence of binding site(s) that are involved in the interaction PDI with its peptide and protein substrates. Equilibrium dialysis, fluorescent hydrophobic probe binding (4,4'-dianilino-1,1'-binaphthyl-5,5'-disulfonic acid (bis-ANS)), competition binding, and enzyme activity assays reveal that the hormone binding sites are distinct from the peptide/protein binding sites. PDI has one estradiol binding site with modest affinity (2.1 +/- 0.5 microm). There are two binding sites with comparable affinity for T(3) (4.3 +/- 1.4 microm). One of these overlaps the estradiol site, whereas the other binds the hydrophobic probe, bis-ANS. Neither estradiol nor T(3) inhibit the catalytic or chaperone activity of PDI. Although the affinity of PDI for the hormones estradiol and T(3) is modest, the high local concentration of PDI in the endoplasmic reticulum (>200 microm) would drive hormone binding and result in the association of a substantial fraction (>90%) of the hormones in the cell with PDI. High capacity, low affinity hormone sites may function to buffer hormone concentration in the cell and allow tight, specific binding to the true receptor while preserving a reasonable number of hormone molecules in the very small volume of the cellular environment.

Temporal association of the N- and O-linked glycosylation events and their implication in the polarized sorting of intestinal brush border sucrase-isomaltase, aminopeptidase N, and dipeptidyl peptidase IV

The temporal association between O-glycosylation and processing of N-linked glycans in the Golgi apparatus as well as the implication of these events in the polarized sorting of three brush border proteins has been the subject of the current investigation. O-Glycosylation of pro-sucrase-isomaltase (pro-SI), aminopeptidase N (ApN), and dipeptidyl peptidase IV (DPPIV) is drastically reduced when processing of the mannose-rich N-linked glycans is blocked by deoxymannojirimycin, an inhibitor of the Golgi-located mannosidase I. By contrast, O-glycosylation is not affected in the presence of swainsonine, an inhibitor of Golgi mannosidase II. The results indicate that removal of the outermost mannose residues by mannosidase I from the mannose-rich N-linked glycans is required before O-glycosylation can ensue. On the other hand, subsequent mannose residues in the core chain impose no sterical constraints on the progression of O-glycosylation. Reduction or modification of N- and O-glycosylation do not affect the transport of pro-SI, ApN, or DPPIV to the cell surface per se. However, the polarized sorting of two of these proteins, pro-SI and DPPIV, to the apical membrane is substantially altered when O-glycans are not completely processed, while the sorting of ApN is not affected. The processing of N-linked glycans, on the other hand, has no influence on sorting of all three proteins. The results indicate that O-linked carbohydrates are at least a part of the sorting mechanism of pro-SI and DPPIV. The sorting of ApN implicates neither O-linked nor N-linked glycans and is driven most likely by carbohydrate-independent mechanisms.

The design, synthesis, and initial evaluation of benzophenone-containing peptides as potential photoaffinity labels of oligosaccharyltransferase

The benzophenone photophore was incorporated into protected tripeptides and tetrapeptides as photoactivatable probes to study the multimeric enzyme oligosaccharyltransferase (OST). These peptides contain the -Asn-X-Thr- sequon which is required for OST-catalyzed N-glycosylation. Two tripeptides, Bz-Asn-Bpa-Thr-NH2 (3b) and Bz-Asn-Lys[N epsilon-(4-Bz)Bz]-Thr-NH2 (4b), were found to be good OST substrates. They were competitive inhibitors versus standard peptide substrate [14C]Bz-Asn-Leu-Thr-NH2 and their Ki values were determined to be 41 +/- 6 microM and 21 +/- 6 microM, respectively, using synthetic (GlcNAc)2-PP-dolichol.

Divergent and common groups of proteins in glands of venomous snakes

Protein contents of venom-producing glands from the sea-snake Laticauda colubrina (LC) and terrestrial Vipera Russelli (VR) were studied using high-resolution two-dimensional gels: isoelectric focusing followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (IEF/SDS-PAGE) and nonequilibrium pH gradient electrophoresis (NEPHGE) followed by SDS-PAGE. Tentative identities of numerous proteins were established using their amino acid compositions and in certain cases the identities were verified by microsequencing of their N-terminals and internal fragments. As expected, we found several proteins known to be present in the venom of the respective snakes. These include numerous isoforms of phospholipase A2 (PLA2) in both snake glands, various neurotoxins in LC glands and factor IX/factor X-binding protein, hemorrhagic factor and coagulation factor X activating enzyme in Russell's viper glands (VR). Not unexpectedly, we also found a number of cell housekeeping proteins, cytoskeletal proteins, proteins that are necessary for folding, such as heat-shock proteins, protein disulfide-isomerase and peptidyl-prolyl cis/trans isomerases. Unexpectedly, however, the glands of Laticauda colubrina and Russell's viper include a large quantity of antihemorrhagic factor and inhibitor of PLA2, respectively, that have been previously described in snake plasma. The possible reason associated with the presence of these components in venom glands is discussed.

Genome organization of human 48-kDa oligosaccharyltransferase (DDOST)

The enzyme oligosaccharyltransferase (dolichyl-diphosphooligosaccharide-protein glycosyltransferase; EC 2. 4.1.119) (DDOST) catalyzes the transfer of a high-mannose oligosaccharide (GlcNac2Man9Glc3) from a dolichol-linked oligosaccharide donor (dolichol-P-GlcNac2Man9Glc3) onto the asparagine acceptor site within an Asn-X-Ser/Thr consensus motif in nascent polypeptide chains across the membrane of the endoplasmic reticulum. We isolated mouse and human DDOST cDNAs from retinoic acid-treated mouse P19 EC cells and human NT-2 cells, respectively. DDOST mRNA is expressed intensely in heart and pancreas, but at lower levels in brain. Here we show that the human DDOST 48-kDa subunit gene (HGMW-approved symbol DDOST) is organized into 11 exons expanding about 9 kb. This DDOST subunit gene is localized on chromosome 1p36.1 by fluorescence in situ hybridization analysis.

The unusual amino acid triplet Asn-Ile-Cys is a glycosylation consensus site in human alpha-lactalbumin

Human alpha-lactalbumin has not been described as a glycoprotein, despite the fact that several alpha-lactalbumins of both ruminant and nonruminant species are known to be glycosylated. In all these species the glycosylation site is the 45Asn in the usual triplet 45Asn-Gly/Gln-47Ser. We have found that human alpha-lactalbumin is glycosylated and the glycosylation site has been determined by protein sequencing and mass spectrometry. We report an unusual glycosylation site at 71Asn in the triplet 71Asn-Ile-73Cys, which is conserved in all known alpha-lactalbumins except red-necked wallaby. That a relatively small proportion of the protein is glycosylated (about 1%) may reflect the importance of this region of the protein sequence to the molten globule state of alpha-lactalbumin.

Invertebrate phosphatidylinositol-specific phospholipases C and their role in cell signaling

Phosphatidylinositol-specific phospholipase C (PLC) is a family of enzymes that occupy a pivotal role in one of the largest classes of cellular signaling pathways known. Mammalian PLC enzymes have been divided into four major classes and a variety of subclasses based on their structural characteristics and immunological differences. There have been five invertebrate PLC-encoding genes cloned thus far and these fall within three of the four major classes used in categorizing mammalian PLC. Four of these invertebrate genes have been cloned from Drosophila melanogaster and one is from Artemia, a brine shrimp. Structural characteristics of the invertebrate enzymes include the presence of highly conserved Box X and Box Y domains found in major types of mammalian PLC as well as novel features. Two of the invertebrate PLC genes encode multiple splice-variant subtypes which is a newly emerging level of diversity observed in mammalian enzymes. Studies of the invertebrate PLCs have contributed to the identification of the physiological functions of individual isozymes. These identified roles include cellular processes such as phototransduction, olfaction, cell growth and differentiation.

Age-related alteration of proline hydroxylase and collagen-binding heat shock protein (HSP47) expression in human fibroblasts

Changes in the expression of alpha and beta subunits of proline 4-hydroxylase (PH alpha and PH beta) and HSP47, implicated as a molecular chaperone specific for procollagen processing, were examined in human embryonal fibroblasts in relation to in vitro aging. For this purpose a model with treatments causing the decreased hydroxylation of proline residues in procollagens was used. In cells at a low population doubling level (PDL) induction of PH alpha, PH beta, and HSP47 by depletion of ascorbate or addition of alpha-alpha' dipyridyl could be clearly demonstrated by immunoprecipitation and Northern blotting. In contrast, the induction of PH alpha and HSP47 expression was markedly attenuated in high PDL cells, indicating an age-related decrease in response to procollagen retention in the ER caused by hypohydroxylation of proline residues of procollagens.

A Drosophila gene that encodes a member of the protein disulfide isomerase/phospholipase C-alpha family

Screening of a Drosophila genomic DNA library at reduced stringency hybridization conditions using a rat PLC alpha cDNA probe yielded a gene which encodes a member of the protein disulfide isomerase/PLC alpha family. The gene has been localized to band 74C on the left arm of the third chromosome and has been designated dpdi. Northern analysis shows that the dpdi gene encodes a transcript that is 2.3 kb in length and is present throughout development as well as in both heads and bodies of adults. The deduced dpdi protein is 496 amino acids in length and contains two domains exhibiting high similarity to thioredoxin, two regions that are similar to the hormone binding domain of human estrogen receptor, and a sequence of four amino acids (KDEL) at the C-terminus which has been described by others as being responsible for retention of proteins in the endoplasmic reticulum. Overall, dpdi contains a higher similarity to rat protein disulfide isomerase (53% identical) than to rat PLC alpha (30% identical). However, it is unclear whether dpdi functions in vivo as a PDI or as a PLC, or both. Drosophila, with its well characterized genetics and the ability to generate mutants in a gene that has been cloned, provides an excellent system in which to resolve this issue.

Dollo's law and the death and resurrection of genes

Dollo's law, the concept that evolution is not substantively reversible, implies that the degradation of genetic information is sufficiently fast that genes or developmental pathways released from selective pressure will rapidly become nonfunctional. Using empirical data to assess the rate of loss of coding information in genes for proteins with varying degrees of tolerance to mutational change, we show that, in fact, there is a significant probability over evolutionary time scales of 0.5-6 million years for successful reactivation of silenced genes or "lost" developmental programs. Conversely, the reactivation of long (> 10 million years)-unexpressed genes and dormant developmental pathways is not possible unless function is maintained by other selective constraints; the classic example of the resurrection of "hen's teeth" is most likely an experimental artifact, and the experimental reactivation of the Archaeopteryx limb developmental program has been shown to be a misinterpretation. For groups undergoing adaptive radiations, lost features may "flicker" on and off, resulting in a distribution of character states that does not reflect the phylogeny of the group.

Association of Hsp47, Grp78, and Grp94 with procollagen supports the successive or coupled action of molecular chaperones

Hsp47, Grp78, and Grp94 have been implicated with procollagen maturation events. In particular, Hsp47 has been shown to nascent procollagen alpha 1(I) chains in the course of synthesis and/or translocation into the endoplasmic reticulum (ER). Although, Hsp47 binding to gelatin and collagen has previously been suggested to be independent of ATP. Grp78 and Grp94 are known to dissociate from its substrates by an ATP-dependent release mechanism. The early association of Hsp47 with procollagen and its relatively late release suggested that other chaperones, Grp78 and Grp94, interact successively or concurrently with Hsp47. Herein, we examined how these events occur in cells metabolically stressed by depletion of ATP. In cells depleted of ATP, the release of Hsp47, Grp78, and Grp94 from maturing procollagen is delayed. Thus, in cells experiencing metabolic stress, newly synthesized procollagen unable to properly fold became stably bound to a complex of molecular chaperones. In that Hsp47, Grp78, and Grp94 could be recovered with nascent procollagen and as oligomers in ATP depleted cells suggests that these chaperones function in a series of coupled or successive reactions.

The Onchocerca volvulus homologue of the multifunctional polypeptide protein disulfide isomerase

Protein disulfide isomerase (PDI) functions to catalyze the formation of correct disulfide bonds in nascent proteins, and also acts as one of the subunits of prolyl-4 hydroxylase, the enzyme responsible for the oxidative maturation of procollagen. Since the cuticle of parasitic nematodes consists primarily of a network of collagen molecules which are connected through intermolecular disulfide bonds, PDI might be expected to be involved in the process of cuticle biosynthesis. The isolation and characterization of a cDNA encoding the PDI homologue of Onchocerca volvulus is described. This cDNA contains a single, long open reading frame that encodes sequence motifs identical to the two known active sites of PDI for isomerase activity. The O. volvulus PDI appears to be encoded by a single copy gene. Both in situ hybridization and immunolocalization data suggest that PDI is both spatially and temporally regulated in O. volvulus. The pattern of spatial and temporal regulation is consistent with the involvement of PDI in the biosynthesis of the parasite cuticle. The parasite protein appears to be an antigen recognized by a minority of individuals exposed to O. volvulus.

Thiol-proteindisulfide-oxidoreductase (proteindisulfide isomerase): a new plasma membrane constituent of mature human B lymphocytes

The thiol-proteindisulfide-oxidoreductase (TPO, EC 1.8.4.2., proteindisulfide isomerase, EC 5.3.4.1.) is known as an cytoplasmatic enzyme, and is thought to be involved in the post-translational folding of disulfide containing proteins. Using monoclonal and polyclonal antibodies the authors were able to prove that this enzyme or an unknown homologous protein is localized also to the plasma membrane of B lymphocytes. In peripheral blood from healthy donors 11% of the mononuclear cells (PBMNC) expressed this surface antigen whereas in PBMNC of patients with B-cell chronic lymphocytic leukaemia 76% of the MNC were positive. This value correlates well with the known B-cell markers CD19 and CD20. However, this antigen is different from all known clustered B-cell markers. Immunoprecipitation analysis of PHA-stimulated PBMNC and of cells from patients suffering from chronic lymphocytic leukaemia revealed a membrane protein with the same molecular weight (61 kDa) as the TPO. These data suggest that this enzyme is present not only in the cytoplasm but also on the surface of B cells and that it is possibly involved in the regulation of the SH-SS status of the cell membrane proteins of B lymphocytes.

Production of human protein disulfide isomerase by Bacillus brevis

Human protein disulfide isomerase (PDI; EC 5.3.4.1) was expressed and secreted into the culture medium using Bacillus brevis as host and pNU200 which codes the promoter and signal sequence of major cell wall protein of B. brevis as vector. The accumulation of recombinant human PDI (rhPDI) reached about 5 mg l-1 in the late exponential phase of the bacterial growth. The purified rhPDI was found to be exactly processed at the carboxyl terminus of the signal sequence. It was as active as natural PDI derived from human placenta as determined by its ability to reactivate scrambled ribonuclease that was a fully oxidized mixture containing randomly formed disulfide bonds. The activity was significantly accelerated in the presence of dithiothreitol or a mixture of reduced and oxidized glutathione. These indicate that the characteristics of rhPDI are similar to those reported for mammalian PDI and that it can be used for refolding inactive proteins having incorrect disulfide bonds.

The same 15 kDa proteolipid subunit is a constituent of two different proteins in Torpedo, the acetylcholine releasing protein mediatophore and the vacuolar H+ ATPase

Using the monoclonal antibody 15K1, we have studied, at the cellular and subcellular levels, the distribution of a 15 kDa proteolipid, identified as the subunit of mediatophore, a presynaptic membrane protein able to release acetylcholine when activated by calcium. Aside from the electric lobe, the antigen distribution in the brain of Torpedo paralleled that of the synaptic vesicle antigen SV2 and did not appear to be related to that of acetylcholine and choline acetyltransferase. The 15 kDa proteolipid antigen was therefore present in all nerve endings and not restricted to cholinergic ones. At the ultrastructural level, on cholinergic nerve endings, the antigen was detected associated to synaptic vesicles and, to a lesser extent, to the presynaptic plasma membrane. Indeed, considering the high sequence homology between the mediatophore subunit (Birman et al., 1990) and the proteolipid subunit of the vacuolar type H+ ATPase, a major enzyme constituent of synaptic vesicles, this distribution was not surprising. To determine whether antibody 15K1 recognizes the vacuolar type H+ ATPase, we chose a non neuronal cell type which possesses a high content of this enzyme, the kidney proton secreting epithelial cells. Indeed, antibody 15K1 intensely labelled the apical plasma membrane of mitochondria rich epithelial cells in kidney tubules. A high density of the antigen was also found associated to intracellular membrane structures such as lysosomal multivesicular bodies, both in kidney epithelial cells and in electromotoneurons. The 15 kDa proteolipid antigen was associated with other vacuolar H+ ATPase subunits in kidney membranes which was not the case in presynaptic plasma membranes. This illustrates that the 15 kDa proteolipid antigen is a constituent of two different protein complexes, which exhibit very different functional properties.

Identification of genomic sequences that mediate the induction of the endoplasmic reticulum stress protein, ERp72, by protein traffic

ERp72, a resident protein of the endoplasmic reticulum (ER) is both a stress protein and a member of the protein disulfide isomerase family of proteins. Analysis of the murine ERp72 promoter region revealed the presence of potential transcriptional control elements characteristic of the promoters of mammalian ER proteins. These include multiple CCAAT elements and Sp1 and AP-2 consensus sequences. Functional analysis of mutations in the ERp72 promoter and 5'-flanking region revealed an 82-bp fragment that is sufficient to mediate the stimulation observed for ERp72 either by stress or by the expression of incompletely assembled immunoglobulin mu heavy chain in the ER. This 82-bp fragment contains two CCAAT elements but little additional homology to protein traffic-responsive sequences of other members of the ER stress family. This suggests that the ERp72 gene contains a novel element that is the target of an intracellular signaling pathway initiated by protein traffic in the ER.

Interaction of transcription factor Sp1 with the promoter of the gene for the multifunctional protein disulphide isomerase polypeptide

Protein disulphide isomerase (PDI) is a unique polypeptide which resides in the lumen of the endoplasmic reticulum and also functions as the beta-subunit of prolyl 4-hydroxylase, as a cellular thyroid hormone-binding protein, as the smaller subunit of the microsomal triacylglycerol transfer protein complex, as a dehydroascorbate reductase and as a protein that binds various peptides in a specific manner. We have recently demonstrated that the promoter of the PDI gene contains six CCAAT boxes and other elements which are needed for efficient transcription. We now demonstrate that purified human recombinant transcription factor Sp1 interacts with two perfect GGGCGG sequences and three other GC-rich elements of the PDI promoter. Sp1 also appears to participate in the regulation of PDI gene expression, since overexpression of Sp1 stimulated PDI promoter activity in HeLa cells and mutations introduced into each of these Sp1-binding sites separately reduced the promoter strength, although even the largest decrease was only about 50%. These results support our view that expression of the gene for this polypeptide with multiple functions is secured by several regulatory elements, some of which are functionally redundant.

cDNA for R-cognin: homology with a multifunctional protein

Retina cognin (R-cognin) is a developmentally regulated 50-kDa protein that was isolated from chicken embryo retina cell membranes. It mediates the adhesion and reaggregation in vitro of retina cells from chicken and mouse embryos, but not of cells from other tissues, and may be involved in neuronal differentiation. We report here the cloning of a cDNA for R-cognin. A chicken embryo retina cDNA library was constructed in lambda gt11 vector and was screened with polyclonal R-cognin antiserum, yielding several immunoreactive clones. Antiserum prepared to the R-cognin-beta-galactosidase fusion protein produced by one recombinant lysogen recognized the 50-kDa R-cognin protein derived from retina cell membranes. This antiserum inhibited the reaggregation of dissociated retina cells and immunostained chicken embryo retina tissue in a pattern similar to that obtained with R-cognin antiserum. In vitro translation of RNA from a cDNA subclone yielded a 50-kDa protein that was recognized by R-cognin antiserum on a Western blot. By these criteria we identify the cDNA clone as representative of the gene encoding R-cognin. This cDNA is nearly identical to a major portion of the cDNA for the multifunctional protein that is the beta subunit of prolyl 4-hydroxylase and has both protein disulfide isomerase activity and thyroid hormone-binding activity. These findings demonstrate that R-cognin differs from other cell adhesion molecules and suggest possible mechanisms for its action in cell adhesion and neuronal differentiation.

Members of the ALDH gene family are lens and corneal crystallins

Many of the major lens proteins, known as crystallins, responsible for the structural integrity and functional utility of this visual tissue have been previously shown to be recruited proteins. This phenomena of a protein that is expressed and functions elsewhere acquiring a new function in another tissue has been termed 'gene sharing'. It is now becoming obvious that the cornea of vertebrates has similarly acquired proteins, and that at least one corneal protein, ALDH3 belongs to a gene family that has been previously identified as a lens crystallin. The recognition that both lens and corneal crystallins exist is a novel concept that has implications that involve the process by which multifunctional gene products have evolved. Members of the ALDH gene family function in both the cornea and lens as crystallins and the acquisition of multifunctionality by this gene family is unique. Based on our analysis we have deduced a supragene family relationship between the thiol protein esterases, aldehyde dehydrogenases, and the taxon-specific crystallins. Evolution of a complex organ such as the vertebrate eye is not a sequential and gradual process such as the Darwinian Giraffe's neck, since the eye can provide selective advantage only as a complete organ. Catastrophic theory proposes that the complex vertebrate eye with its lens, and focussing mechanism arose from the primitive eye spot which contained originally only the photoreceptor system by a one step event. In the evolution of the vertebrate eye it is evolutionarily plausible that several pre-existing proteins have been recruited to perform a structural role for this complex organ. It is also incumbent in evolutionary thought that any inherent enzymatic activity associated with this protein would be purely an incidental addition to the organ. However, the fact that most of these have pyridine nucleotide binding capacity, which is presumed important in giving protection from UV exposure, is noteworthy. Finally, to construct the vertebrate eye in one step from the existing visual pigment system such as the eyespot of unicellular organisms the following criteria would apparently be advantageous: (1) high water solubility; (2) transparency; and (3) common genetic regulatory elements (e.g. promoters/enhancers). Although it is an important observation that certain members of the aldehyde dehydrogenase gene family are present as structural proteins in the cornea and lens, it is not surprising that the phenomenon of gene sharing extends to another ocular tissue such as the cornea. In this context, it will be interesting to note if similar multifunctional gene products will be found as frequently in organs other than the eye.

Purification and characterization of rat brain cytosolic 3,5,3'-triiodo-L-thyronine-binding protein. Evidence for binding activity dependent on NADPH, NADP and thioredoxin

A rat brain cytosolic 3,5,3'-triiodo-L-thyronine-(T3)-binding protein (CTBP) was purified using, successively, carboxymethyl-Sephadex, DEAE-Spherodex, T3-Sepharose-4B affinity chromatography and Sephacryl S-200. The molecular mass determined by SDS/PAGE wa 58 kDa. The binding characteristics determined by Scatchard analysis revealed a single class of binding sites with a Ka of 1.56 nM-1 and a maximal binding capacity of 7500 nmol T3/g protein. The relative binding affinities of iodothyronine analogues were D-T3 > L-T3 > L-T4 > 3,3'-5-triiodothyroacetic acid > reverse T3. The optimum pH for binding was 7.5. Purified brain CTBP was reversibly inactivated by charcoal. NADPH, NADP and thioredoxin restored binding activity to a level higher than that of the control; this effect was concentration dependent. Maximal activation was observed at 25 nM NADPH. NADP was effective only in the presence of 1 mM dithiothreitol; maximal activity was obtained at 10 nM NADP. At concentrations higher than 50 nM NADP, the binding gradually decreased. Thioredoxin in the presence of 1 mM dithiothreitol activated CTBP; maximal binding was obtained with 4 microM thioredoxin. In the presence of NADPH, NADP or thioredoxin the maximal binding capacity increased 2-4 times and the Ka was 2.6 nM-1. These results show that the activity of purified cytosolic brain T3-binding protein may be modulated by NADPH, NADP or thioredoxin.

Site-directed mutagenesis of human protein disulphide isomerase: effect on the assembly, activity and endoplasmic reticulum retention of human prolyl 4-hydroxylase in Spodoptera frugiperda insect cells

Protein disulphide isomerase (PDI) is a highly unusual multifunctional polypeptide, identical to the beta-subunit of prolyl 4-hydroxylase. It has two -Cys-Gly-His-Cys- sequences which represent two independently acting catalytic sites of PDI activity. We report here on the expression in baculovirus vectors of various mutant PDI/beta-subunits together with a wild-type alpha-subunit of the human prolyl 4-hydroxylase alpha 2 beta 2 tetramer in Spodoptera frugiperda insect cells. When either one or both of the -Cys-Gly-His-Cys- sequences was converted to -Ser-Gly-His-Cys-, a tetramer was formed as with wild-type PDI/beta-subunit. This tetramer was fully active prolyl 4-hydroxylase. The data demonstrate that PDI activity of the PDI/beta-subunit is not required for tetramer assembly or for the prolyl 4-hydroxylase activity of the tetramer, and thus other sequences of the PDI/beta-subunit may be critical for keeping the alpha-subunits in a catalytically active, non-aggregated conformation. Measurements of the PDI activities of tetramers containing the various mutant PDI/beta-subunits demonstrated that the activity of the wild-type tetramer is almost exclusively due to the C-terminal PDI catalytic sites, which explains the finding that the PDI activity of the PDI/beta-subunit present in the tetramer is about half that in the free polypeptide.(ABSTRACT TRUNCATED AT 250 WORDS)

The yeast EUG1 gene encodes an endoplasmic reticulum protein that is functionally related to protein disulfide isomerase

The product of the EUG1 gene of Saccharomyces cerevisiae is a soluble endoplasmic reticulum protein with homology to both the mammalian protein disulfide isomerase (PDI) and the yeast PDI homolog encoded by the essential PDI1 gene. Deletion or overexpression of EUG1 causes no growth defects under a variety of conditions. EUG1 mRNA and protein levels are dramatically increased in response to the accumulation of native or unglycosylated proteins in the endoplasmic reticulum. Overexpression of the EUG1 gene allows yeast cells to grow in the absence of the PDI1 gene product. Depletion of the PDI1 protein in Saccharomyces cerevisiae causes a soluble vacuolar glycoprotein to accumulate in its endoplasmic reticulum form, and this phenotype is only partially relieved by the overexpression of EUG1. Taken together, our results indicate that PDI1 and EUG1 encode functionally related proteins that are likely to be involved in interacting with nascent polypeptides in the yeast endoplasmic reticulum.

The yeast EUG1 gene encodes an endoplasmic reticulum protein that is functionally related to protein disulfide isomerase

The product of the EUG1 gene of Saccharomyces cerevisiae is a soluble endoplasmic reticulum protein with homology to both the mammalian protein disulfide isomerase (PDI) and the yeast PDI homolog encoded by the essential PDI1 gene. Deletion or overexpression of EUG1 causes no growth defects under a variety of conditions. EUG1 mRNA and protein levels are dramatically increased in response to the accumulation of native or unglycosylated proteins in the endoplasmic reticulum. Overexpression of the EUG1 gene allows yeast cells to grow in the absence of the PDI1 gene product. Depletion of the PDI1 protein in Saccharomyces cerevisiae causes a soluble vacuolar glycoprotein to accumulate in its endoplasmic reticulum form, and this phenotype is only partially relieved by the overexpression of EUG1. Taken together, our results indicate that PDI1 and EUG1 encode functionally related proteins that are likely to be involved in interacting with nascent polypeptides in the yeast endoplasmic reticulum.

Tunicamycin, puromycin and brefeldin A influence the subcellular distribution of neuropeptides in hypothalamic magnocellular neurones of rat

Magnocellular neurones in the supraoptic nuclei of normal Long Evans and homozygous Brattleboro rats were examined electron-microscopically after intracisternal injections of tunicamycin, puromycin, or brefeldin A. Moderate (50 micrograms) or high (200 micrograms) doses of tunicamycin caused the formation of electron-dense filamentous accretions in the endoplasmic reticulum (ER) cisterns of vasopressin neurones, but only the high dose of tunicamycin also caused accretions to form in the ER of some oxytocin neurones. Immunogold labelling of ultrathin sections from tunicamycin-treated rats revealed that, in about 5% of vasopressin neurones, the accretions could be immunogold-labelled for vasopressin and its associated neurophysin. However, in the majority of vasopressin neurones, the sections required trypsinisation before immunolabelling of the accretions could be detected. Small accretions in the ER of oxytocin neurones did not label for oxytocin or its neurophysin without prior trypsinisation, whereas larger accretions in other oxytocin cells could be labelled without prior trypsin treatment. Administration of puromycin resulted in the formation of small ER accretions in both vasopressin and oxytocin neurones. These accretions were immunolabelled with antisera, respectively, to vasopressin and oxytocin, but neurophysin-immunoreactivity was in most cases absent and was not revealed by treatment with trypsin, suggesting that neurophysin-immunoreactive epitopes were absent from truncated peptides forming the accretions. Brefeldin A caused dilatation of ER cisterns and disruption of the Golgi apparatus in both oxytocin and vasopressin neurones, but did not cause accretions to form in the ER.

Characterization of the human prolyl 4-hydroxylase tetramer and its multifunctional protein disulfide-isomerase subunit synthesized in a baculovirus expression system

Prolyl 4-hydroxylase (EC 1.14.11.2), an alpha 2 beta 2 tetramer, catalyzes the posttranslational formation of 4-hydroxyproline in collagens. The enzyme can easily be dissociated into its subunits, but all attempts to associate a tetramer from the dissociated subunits in vitro have been unsuccessful. Molecular cloning of the catalytically important alpha subunit has identified two types of cDNA clone due to mutually exclusive alternative splicing. The beta subunit is a highly unusual multifunctional polypeptide, being identical to the enzyme protein disulfide-isomerase (EC 5.3.4.1). We report here on expression of the alpha and beta subunits of prolyl 4-hydroxylase and a fully active enzyme tetramer in Spodoptera frugiperda insect cells by baculovirus vectors. When the beta subunit was expressed alone, the polypeptide produced was found in a 0.1% Triton X-100 extract of the cell homogenate and was a fully active protein disulfide-isomerase. When either form of the alpha subunit was expressed alone, only traces of the alpha subunit could be extracted from the cell homogenate with 0.1% Triton X-100, and 1% SDS was required to obtain efficient solubilization. These alpha subunits had no prolyl 4-hydroxylase activity. When the cells were coinfected with both alpha- and beta-subunit-producing viruses, an enzyme tetramer was formed, but significant amounts of alpha and beta subunits remained unassociated. The recombinant tetramer was indistinguishable from that isolated from vertebrate tissue in terms of its specific activity and kinetic constants for cosubstrates and the peptide substrate. The two alternatively spliced forms of the alpha subunit gave enzyme tetramers with identical catalytic properties. Baculovirus expression seems to be an excellent system for mass production of the enzyme tetramer and for detailed investigation of the mechanisms involved in the association of the monomers.

The expression of murine protein disulfide isomerase in Escherichia coli

Protein disulfide isomerase (PDI), a luminal enzyme of the endoplasmic reticulum (ER), is thought to be involved in the process that assures that the correct disulfide bonds form as a newly synthesized protein folds into its appropriate three-dimensional structure (Freeman, 1984). In recent years, the ER has been shown to have at least two additional, distinct PDI-related luminal proteins (Bennett et al., 1988; Mazzarella et al., 1990). As a potential first step toward an investigation of the structure and function of PDI and of the PDI-related proteins as well, we have developed a bacterial expression system in Escherichia coli capable of synthesizing significant levels of enzymatically active PDI under the control of the inducible tac promoter. We have observed that the use of this bacterial expression system is complicated by the fact that there is a significant amount of internal initiation of protein synthesis within the PDI coding sequence and the fact that all of the PDI-related expression products are found equally distributed between the cytoplasmic and periplasmic fractions due to a single peptide-independent mechanism. Our studies with this system have demonstrated that at least some truncated PDI molecules containing the carboxy-terminal most active site have significant PDI activity.

Identification of the three-dimensional thioredoxin motif: related structure in the ORF3 protein of the Staphylococcus aureus mer operon

We have developed a computerized search pattern for recognition of the three-dimensional redox site of thioredoxins based on primary and predicted secondary structure. This pattern, developed in the ARIADNE protein expert system, is used to search for thioredoxin-like tertiary structural motif among proteins for which the only structural information is the primary sequence. The pattern was trained on 102 protein sequences (25 functionals and 77 controls); it matches all 25 members of the functional set under cutoff conditions that include only 2 members of the control set, for a sensitivity of 1.0 and a specificity of 0.97. The pattern matches only one of the two thioredoxin-like domains in protein disulfide isomerases (PDIs) and their analogues, suggesting that the C-terminal domain is more structurally similar to thioredoxin than the N-terminal domain. The Escherichia coli DsbA protein, a possible PDI analogue, appears to be more structurally similar to the N-terminal thioredoxin-like domain of PDIs. Thioredoxin-like redox functionality has been proposed for lutropin and follitropin, in part on the basis of their having -Cys-X-Pro-Cys- sequences. None match our pattern; all lack a predicted alpha-helix pattern element immediately after the active site. Hypothetical proteins in the National Biomedical Research Foundation Protein Identification Resource database were searched for matches to the pattern. The most interesting match was a hypothetical protein (161 residues) from the third open reading frame in the Staphylococcus aureus mer operon, which is involved in mercury detoxification. The match to our pattern and the hydrophobicity distribution in aligned elements of secondary structure not in our pattern strongly suggest that it has thioredoxin-like structure.

Oligosaccharyltransferase activity is associated with a protein complex composed of ribophorins I and II and a 48 kd protein

Oligosaccharyltransferase catalyzes the N-linked glycosylation of asparagine residues on nascent polypeptides in the lumen of the rough endoplasmic reticulum (RER). A protein complex composed of 66, 63, and 48 kd subunits copurified with oligosaccharyltransferase from canine pancreas. The 66 and 63 kd subunits were shown by protein immunoblotting to be identical to ribophorin I and II, two previously identified RER glycoproteins that colocalize with membrane-bound ribosomes. The transmembrane segment of ribophorin I was found to be homologous to a recently proposed dolichol recognition consensus sequence. Based on a revision of the consensus sequence, we propose a model for the interaction of dolichol with the glycosyltransferases that catalyze the assembly and transfer of lipid-linked oligosaccharides.