ERp72, an abundant luminal endoplasmic reticulum protein, contains three copies of the active site sequences of protein disulfide isomerase

Engineering the assembly pathway of the baculovirus-insect cell expression system

The synthesis of complex biological structures such as antibodies using recombinant DNA technology is a major biotechnological advance. Active murine antibody (IgG) oligomers, composed of two heavy (H) and two light (L) polypeptide chains, have been expressed and secreted by the baculovirus-insect cell expression system. Unfortunately, expression of the functional antibodies is accompanied by the formation of abnormal protein complexes and aggregates in which the polypeptide chains are bound together into incorrect associations. The formation of these abnormal complexes or protein aggregates in insect cells may be caused by insufficient intracellular levels of two catalytic proteins, immunoglobulin binding protein (BiP or GRP78), and protein disulfide isomerase (PDI). Consequently, we obtained the genes coding for murine BiP and PDI and cloned the genes into the baculovirus vector (Autographa californica nuclear polyhedrosis virus) to obtain AcBB-BiP and AcBB-PDI. Infection of Spodoptera frugiperda (Sf-9) insect cells with these two baculoviruses yielded recombinant proteins of the correct size that were recognized by antibodies to these proteins. Cloning these genes into the baculovirus vector is one approach to engineering the assembly pathway in order to lower aggregation and increase production of functionally active proteins and oligomers.

Molecular cloning of Brugia malayi antigens for diagnosis of lymphatic filariasis

Immunological crossreactivity among nematodes has hampered development of specific serodiagnostic assays for lymphatic filariasis. In the present study, we report the molecular cloning and characterization of two filaria-specific recombinant clones (BmM5 and BmM14) with immunodiagnostic potential. BmM5 is a 505-bp cDNA which codes for a protein of 130 residues that ends with an endoplasmic reticulum targeting sequence. BmM14 is closely related to a recently reported clone (SXP-1), and it has 62% homology (deduced amino acid sequence) with a previously described Onchocerca volvulus clone, lambda RAL-2. Glutathione S-transferase fusion proteins of BmM5 and BmM14 were tested in various ELISA formats. The best results were obtained by measuring IgG4 antibodies to the fusion proteins. ELISA studies showed that approximately 90% of 111 sera from Indian and Egyptian patients with brugian and bancroftian filariasis were reactive with both antigens. Nonendemic sera as well as sera from patients with schistosomiasis or intestinal helminths were uniformly nonreactive. Assays based on BmM5 and BmM14 may be useful for large scale screening as an alternative to microfilaria or filarial antigen detection as a means of obtaining a rough index of filariasis endemicity in previously unstudied areas.

Systemic lupus erythematosus is associated with increased auto-antibody titers against calreticulin and grp94, but calreticulin is not the Ro/SS-A antigen

Auto-antibodies against purified human calreticulin were determined by an ELISA in sera from patients with systemic lupus erythematosus (SLE) and from healthy persons or patients without an autoimmune disease. More than 80% of patients with SLE had titers exceeding the highest value obtained in the group without SLE. Almost 30% of the patients had also elevated auto-antibody titers against purified rat grp94, another resident ER-protein of the KDEL-protein family, but not against rat ERp72 (CaBP2), an ER-resident protein of the proteindisulfide isomerase family. It could, however, be excluded that calreticulin is the Ro/SS-A antigen on the basis of the following observations: 1) Calreticulin purified from rat, bovine or human liver contained far less than 1 mol of phosphate per mol of calreticulin, showed an E280/E260-absorption ratio of about 2.0, and did not contain extractable RNA; 2) Sera from patients with SLE did not react with or precipitate endogenous calreticulin from Hep G2 cells; they did, however, precipitate hY-RNA from these cells; 3) Sera from SLE-patients, but not anti-calreticulin antisera precipitated [32P]-hY-RNA from [32P]-labelled Hep G2 cells.

Analysis of the structure and synthesis of GRP94, an abundant stress protein of the endoplasmic reticulum

The synthesis and cotranslational modification of GRP94, an abundant, resident protein of the endoplasmic reticulum belonging to the hsp90 family of stress proteins, has been studied in transient expression studies in COS cells. A fraction of the expressed murine GRP94 was more highly glycosylated than normal, having a greater number of endoglycosidase H (Endo H)-sensitive oligosaccharide moieties than authentic GRP94. To understand better the basis for the appearance of the hyperglycosylated form and determine the acceptor sites that were used for this extra glycosylation, we have used in vitro mutagenesis techniques to construct a set of point mutants and deletion mutants of GRP94. Analysis of the expression of wild-type GRP94 and the mutant proteins has revealed that Asn-196 is the acceptor site used in normal glycosylation of GRP94 and that hyperglycosylation is dependent upon the level of expression of the GRP94 and is occurring at acceptor sites in the carboxy-terminal region of the protein. We have shown, in addition, that Cys-117 is involved in the formation of a disulfide-bonded homodimer of GRP94. Finally, analysis of deletions made from the amino terminus of the mature protein has demonstrated that these alterations change the pattern of usage of the remaining N-glycosylation sites in the mutants.

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.

CaBP2 is a rat homolog of ERp72 with proteindisulfide isomerase activity

Ca-binding protein 2 (CaBP2) has been described previously as an intracisternal calcium-binding microsomal glycoprotein. We report now the primary sequence of this protein as deduced from the corresponding cDNA. The protein possesses a C-terminal -KEEL retention sequence and three repeats of the thioredoxin-like motive -EFYAPNCGHCK-, and represents the rat homolog of ERp72. In contrast to earlier reports on ERp72, CaBP2 possesses significant proteindisulfide isomerase activity. Furthermore, in contrast to ERp72, CaBP2 is a glycoporotein containing O-linked glycans. The amount of CaBP2 in H-35 Reuber hepatoma cells increases in parallel with that of immunoglobin heavy-chain-binding protein under conditions which lead to impaired glycosylation, while the amount of calreticulin, another KDEL-containing glycoprotein, remains almost unchanged.

A thioredoxin gene fusion expression system that circumvents inclusion body formation in the E. coli cytoplasm

We have developed a versatile Escherichia coli expression system based on the use of E. coli thioredoxin (trxA) as a gene fusion partner. The broad utility of the system is illustrated by the production of a variety of mammalian cytokines and growth factors as thioredoxin fusion proteins. Although many of these cytokines previously have been produced in E. coli as insoluble aggregates or "inclusion bodies", we show here that as thioredoxin fusions they can be made in soluble forms that are biologically active. In general we find that linkage to thioredoxin dramatically increases the solubility of heterologous proteins synthesized in the E. coli cytoplasm, and that thioredoxin fusion proteins usually accumulate to high levels. Two additional properties of E. coli thioredoxin, its ability to be specifically released from the E. coli cytoplasm by osmotic shock or freeze/thaw treatments and its intrinsic thermal stability, are retained by some fusions and provide convenient purification steps. We also find that the active-site loop of E. coli thioredoxin can be used as a general site for small peptide insertions, allowing for the high level production of soluble peptides in the E. coli cytoplasm.

Determination of the sequence of an expressible cDNA clone encoding ERp60/calregulin by the use of a novel nested set method

An analysis of the N-terminal sequence of the luminal endoplasmic reticulum protein, ERp60, showed that it was identical to the well-characterized Ca2+-binding protein, calregulin. A full-length, expressible cDNA clone encoding this protein was isolated from a mouse fibroblast cDNA library. A novel nested set strategy for the production of overlapping fragments for DNA sequencing was used to determine the complete nucleotide (nt) sequence of both strands of the ERp60 clone. This method utilizes a series of nonspecific deletion primers in conjunction with a specific site primer to generate the nested set fragments. This procedure possesses several advantages over other nested set techniques, since it does not require (i) the re-cloning of the DNA insert into other vectors, (ii) any prior knowledge of the restriction sites of the nt sequence, or (iii) the transformation and analysis of bacterial subclones. ERp60 has a 17-amino acid (aa) signal sequence and the mature protein contains 399 aa with a calculated M(r) of 46,347.

The endoplasmic reticulum-Golgi intermediate compartment

The recent identification of an endoplasmic reticulum-Golgi intermediate compartment has added to the complexity of the structural and functional organization of the early secretory pathway. Protein sorting along the endoplasmic reticulum-Golgi pathway depends on different signals and mechanisms, some of which guarantee recycling from various levels of the Golgi apparatus to biosynthetically earlier compartments.

Is thioredoxin the physiological vitamin K epoxide reducing agent?

E. coli thioredoxin plus thioredoxin reductase have previously been shown to replace dithiothreitol as the electron donor for mammalian liver microsomal vitamin K epoxide reduction in vitro. Such activity is dependent on detergent disruption of the microsomal membrane integrity. A previously characterized salicylate-inhibitable pathway for electron transfer from endogenous cytosolic reducing agents to the microsomal epoxide reducing warfarin-inhibitable enzyme is not inhibited by known alternate substrates and inhibitors of the thioredoxin system nor by antibodies against thioredoxin.

The carboxylesterase family exhibits C-terminal sequence diversity reflecting the presence or absence of endoplasmic-reticulum-retention sequences

Resident proteins of the endoplasmic reticulum lumen are continuously retrieved from an early Golgi compartment by a receptor-mediated mechanism. The sorting or retention sequence on the endoplasmic reticulum proteins is located at the C-terminus and was initially shown to be the tetrapeptide KDEL in mammalian cells and HDEL in Saccharomyces cerevisiae. The carboxylesterases are a large family of enzymes primarily localized to the lumen of the endoplasmic reticulum. Retention sequences in these proteins have been difficult to identify due to atypical and heterogeneous C-terminal sequences. Utilizing the polymerase chain reaction with degenerate primers, we have identified and characterized the C-termini of four members of the carboxylesterase family from rat liver. Three of the carboxylesterases sequences contained C-terminal sequences (HVEL, HNEL or HTEL) resembling the yeast sorting signal which were reported to be non-functional in mammalian cells. A fourth carboxylesterase contained a distinct C-terminal sequence, TEHT. A full-length esterase cDNA clone, terminating in the sequence HVEL, was isolated and was used to assess the retention capabilities of the various esterase C-terminal sequences. This esterase was retained in COS-1 cells, but was secreted when its C-terminal tetrapeptide, HVEL, was deleted. Addition of C-terminal sequences containing HNEL and HTEL resulted in efficient retention. However, the C-terminal sequence containing TEHT was not a functional retention signal. Both HDEL, the authentic yeast retention signal, and KDEL were efficient retention sequences for the esterase. These studies show that some members of the rat liver carboxylesterase family contain novel C-terminal retention sequences that resemble the yeast signal. At least one member of the family does not contain a C-terminal retention signal and probably represents a secretory form.

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.

Mammalian stress response: induction of the glucose-regulated protein family

The glucose-regulated protein family consists of a set of stress-inducible proteins localized in the endoplasmic reticulum. Since their discovery in 1977, significant advances in our understanding of their structure, function and regulation have been made. Recent findings concerning the physiological roles played by the glucose-regulated proteins, and their regulations at the transcriptional, post-transcriptional, translational and post-translational levels are summarized.

Ligand-induced redistribution of a human KDEL receptor from the Golgi complex to the endoplasmic reticulum

Resident luminal endoplasmic reticulum (ER) proteins carry a targeting signal (usually KDEL in animal cells) that allows their retrieval from later stages of the secretory pathway. In yeast, the receptor that promotes this selective retrograde transport has been identified as the product of the ERD2 gene. We describe here the properties of a human homolog of this protein (hERD2). Overproduction of hERD2 improves retention of a protein with a weakly recognized variant signal (DDEL). Moreover, overexpression of KDEL or DDEL ligands causes a redistribution of hERD2 from the Golgi apparatus to the ER. Mutation of hERD2 alters the ligand specificity of this effect, implying that it interacts directly with the retained proteins. Ligand control of receptor movement may limit retrograde flow and thus minimize fruitless recycling of secretory proteins.

Protein folding in the cell

In the cell, as in vitro, the final conformation of a protein is determined by its amino-acid sequence. But whereas some isolated proteins can be denatured and refolded in vitro in the absence of other macromolecular cellular components, folding and assembly of polypeptides in vivo involves other proteins, many of which belong to families that have been highly conserved during evolution.

Assembly of fibronectin into extracellular matrix

The results summarized above suggest that assembly of fibronectin is a fundamental biological process and that knowledge of the process of assembly may reveal new ways by which cells interact with extracellular molecules. Deposition of a fibronectin matrix seems to be regulated as tightly as synthesis of fibronectin or expression of adhesion receptors for fibronectin and is influenced profoundly by two products of blood coagulation--TGF-beta released from platelets and factor XIII activated by thrombin. Fibronectin assembly may be important in all sorts of physiological and pathophysiological processes. Cell A--for instance, a stromal cell--can influence the behavior of cell B--for instance, a lymphocyte--by assembling fibronectin made by cell C--for instance, a hepatocyte. We hope that the testable models of assembly presented in this paper will lead to new understanding of the process of assembly and suggest new modalities for treatment of diseases that result in fibrosis, damaged tissues, and neoplastic growth.

Prolyl 4-hydroxylase and its role in collagen synthesis

Excessive accumulation of collagen in the extracellular matrix has a crucial role in fibrosis. Thus pharmacological inhibition of collagen deposition is likely to be beneficial for patients suffering from fibrotic disorders such as liver cirrhosis. Prolyl 4-hydroxylase catalyzes the formation of 4-hydroxyproline in collagens and other proteins with collagen-like amino acid sequences by the hydroxylation of proline residues in -X-Pro-Gly- sequences. The reaction products, 4-hydroxyproline residues, serve to stabilize the collagen triple helices under physiological conditions. Conversely, collagen chains that contain no 4-hydroxyproline cannot fold into triple helical molecules that are stable at body temperature. The prolyl 4-hydroxylase reaction therefore seems to be a particularly suitable target for the pharmological regulation of excessive collagen formation. The reaction catalyzed by prolyl 4-hydroxylase requires Fe2+, 2-oxoglutarate, O2 and ascorbate and involves an oxidative decarboxylation of 2-oxoglutarate. The active enzyme is an alpha 2 beta 2 tetramer that consists of two types of inactive monomer and has two catalytic sites. Some parts of the catalytic sites may be built up cooperatively of both the alpha and beta subunits, but the alpha subunit appears to contribute the major part. The beta subunit contains the carboxyl-terminal tetrapeptide sequence -Lys-Asp-Glu-Leu which is essential for the retention of a polypeptide within the lumen of the endoplasmic reticulum. Since the alpha subunit lacks the carboxyl-terminal retention signal, one function of the beta subunit in the prolyl 4-hydroxylase tetramer may be to retain the enzyme within the endoplasmic reticulum.(ABSTRACT TRUNCATED AT 250 WORDS)

The retention signal for soluble proteins of the endoplasmic reticulum

The lumen of the endoplasmic reticulum (ER) contains a number of soluble proteins, many of which help the maturation of newly synthesized secretory proteins. Retention of these resident proteins in the ER is dependent on a carboxy-terminal signal, which in animal cells is usually Lys-Asp-Glu-Leu (KDEL). This signal is thought to be recognized by a membrane-bound receptor that continually retrieves the proteins from a later compartment of the secretory pathway and returns them to the ER.

ERD2, a yeast gene required for the receptor-mediated retrieval of luminal ER proteins from the secretory pathway

Resident proteins of the ER lumen carry a specific tetrapeptide signal (KDEL or HDEL) that prevents their secretion. We have previously described the isolation of yeast mutants that fail to retain such resident proteins within the cell. Here we describe ERD2, a gene required for retention. It encodes a 26 kd integral membrane protein whose abundance determines the efficiency and capacity of the retention system. Reduced expression of ERD2 leads to secretion of proteins bearing the HDEL signal, whereas overexpression of ERD2 improves retention both in wild-type cells and in other mutants. These results are consistent with other evidence that ERD2 encodes the HDEL receptor (see accompanying paper). The gene is also required, perhaps indirectly, for normal protein transport through the Golgi, and hence for growth. We discuss possible roles for ERD2 in the secretory pathway.