Substrate binding induces depolymerization of the C-terminal peptide binding domain of murine GRP78/BiP

Effect of temperature and nucleotide on the binding of BiP chaperone to a protein substrate

BiP (immunoglobulin heavy-chain binding protein) is a Hsp70 monomeric ATPase motor that plays broad and crucial roles in maintaining proteostasis inside the cell. Structurally, BiP is formed by two domains, a nucleotide-binding domain (NBD) with ATPase activity connected by a flexible hydrophobic linker to the substrate-binding domain. While the ATPase and substrate binding activities of BiP are allosterically coupled, the latter is also dependent on nucleotide binding. Recent structural studies have provided new insights into BiP's allostery; however, the influence of temperature on the coupling between substrate and nucleotide binding to BiP remains unexplored. Here, we study BiP's binding to its substrate at the single molecule level using thermo-regulated optical tweezers which allows us to mechanically unfold the client protein and explore the effect of temperature and different nucleotides on BiP binding. Our results confirm that the affinity of BiP for its protein substrate relies on nucleotide binding, by mainly regulating the binding kinetics between BiP and its substrate. Interestingly, our findings also showed that the apparent affinity of BiP for its protein substrate in the presence of nucleotides remains invariable over a wide range of temperatures, suggesting that BiP may interact with its client proteins with similar affinities even when the temperature is not optimal. Thus, BiP could play a role as a "thermal buffer" in proteostasis.

Fic and non-Fic AMPylases: protein AMPylation in metazoans

Protein AMPylation refers to the covalent attachment of an AMP moiety to the amino acid side chains of target proteins using ATP as nucleotide donor. This process is catalysed by dedicated AMP transferases, called AMPylases. Since this initial discovery, several research groups have identified AMPylation as a critical post-translational modification relevant to normal and pathological cell signalling in both bacteria and metazoans. Bacterial AMPylases are abundant enzymes that either regulate the function of endogenous bacterial proteins or are translocated into host cells to hijack host cell signalling processes. By contrast, only two classes of metazoan AMPylases have been identified so far: enzymes containing a conserved filamentation induced by cAMP (Fic) domain (Fic AMPylases), which primarily modify the ER-resident chaperone BiP, and SelO, a mitochondrial AMPylase involved in redox signalling. In this review, we compare and contrast bacterial and metazoan Fic and non-Fic AMPylases, and summarize recent technological and conceptual developments in the emerging field of AMPylation.

Glucose-regulated protein 78 binds to and regulates the melanocortin-4 receptor

The melanocortin-4 receptor (MC4R) belongs to the G protein-coupled receptor (GPCR) family and plays an essential role in the control of energy homeostasis. Here, we identified a novel MC4R-interacting protein, glucose-regulated protein 78 (GRP78), from a pulldown assay using hypothalamic protein extracts and the third intracellular loop of MC4R. We found that MC4R interacted with GRP78 in both the cytosol and at the cell surface and that this interaction increased when MC4R was internalized in the presence of the agonist melanotan-II (MTII). Downregulation of GRP78 using a short interfering RNA approach attenuated MTII-mediated receptor internalization. Reduction in GRP78 expression during tunicamycin-induced endoplasmic reticulum stress also suppressed MTII-mediated internalization of MC4R and cAMP-mediated transcriptional activity. Furthermore, lentiviral-mediated short hairpin RNA knockdown of endogenous GRP78 in the paraventricular nucleus (PVN) of the hypothalamus resulted in an increase in body weight in mice fed a high-fat diet. These results suggest that GRP78 in the PVN binds to MC4R and may have a chaperone-like role in the regulation of MC4R trafficking and signaling.

Changes in quaternary structure cause a kinetic asymmetry of glutamate racemase-catalyzed homocysteic acid racemization

Glutamate racemases (GR) catalyze the racemization of d- and l-glutamate and are targets for the development of antibiotics. We demonstrate that GR from the periodontal pathogen Fusobacterium nucleatum (FnGR) catalyzes the racemization of d-homocysteic acid (d-HCA), while l-HCA is a poor substrate. This enantioselectivity arises because l-HCA perturbs FnGR's monomer-dimer equilibrium toward inactive monomer. The inhibitory effect of l-HCA may be overcome by increasing the total FnGR concentration or by adding glutamate, but not by blocking access to the active site through site-directed mutagenesis, suggesting that l-HCA binds at an allosteric site. This phenomenon is also exhibited by GR from Bacillus subtilis, suggesting that enantiospecific, "substrate"-induced dissociation of oligomers to form inactive monomers may furnish a new inhibition strategy.

FICD acts bifunctionally to AMPylate and de-AMPylate the endoplasmic reticulum chaperone BiP

Protein folding homeostasis in the endoplasmic reticulum (ER) is defended by an unfolded protein response (UPR) that matches ER chaperone capacity to the burden of unfolded proteins. As levels of unfolded proteins decline, a metazoan-specific FIC-domain containing ER-localized enzyme, FICD (HYPE), rapidly inactivates the major ER chaperone BiP by AMPylating T518. Here we show that the single catalytic domain of FICD can also release the attached AMP, restoring functionality to BiP. Consistent with a role for endogenous FICD in de-AMPylating BiP, FICD^-/- hamster cells are hypersensitive to introduction of a constitutively AMPylating, de-AMPylation defective mutant FICD. These opposing activities hinge on a regulatory residue, E234, whose default state renders FICD a constitutive de-AMPylase in vitro. The location of E234 on a conserved regulatory helix and the mutually antagonistic activities of FICD in vivo, suggest a mechanism whereby fluctuating unfolded protein load actively switches FICD from a de-AMPylase to an AMPylase.

Microcystin-LR induced developmental toxicity and apoptosis in zebrafish (Danio rerio) larvae by activation of ER stress response

Recent studies have demonstrated that cyanobacteria-derived Microcystin-LR (MC-LR) can cause developmental toxicity and trigger apoptosis in zebrafish (Danio rerio) larvae, but the underlying mechanisms remain largely unknown. In this study, we tested the hypothesis that the mechanism by which MC-LR induces developmental toxicity is through activation of endoplasmic reticulum (ER) stress. MC-LR (4.0 μM) exposure through submersion caused serious developmental toxicity, such as malformation, growth delay and decreased heart rates in zebrafish larvae, which could be inhibited by ER stress blocker, tauroursodeoxycholic acid (TUDCA, 20 μM). Meanwhile, acridine orange (AO) staining showed TUDCA could rescue cell apoptosis in heart area in zebrafish larvae resulted by MC-LR exposure. Real-time polymerase chain reaction (real-time PCR) analysis demonstrated that MC-LR induced activation of ER stress which consequently triggered apoptosis in zebrafish larvae. Protein expression examined by western blot indicated that MC-LR could activate MAPK8/Bcl-2/Bax pathway and caspase-dependent apoptotic pathway in zebrafish larva and the effects were mitigated by inhibition of ER stress. Taken together, the results observed in this study suggested that ER stress plays a critical role in developmental toxicity and apoptosis in zebrafish embryos exposed to MC-LR.

Physiological modulation of BiP activity by trans-protomer engagement of the interdomain linker

DnaK/Hsp70 chaperones form oligomers of poorly understood structure and functional significance. Site-specific proteolysis and crosslinking were used to probe the architecture of oligomers formed by the endoplasmic reticulum (ER) Hsp70, BiP. These were found to consist of adjacent protomers engaging the interdomain linker of one molecule in the substrate binding site of another, attenuating the chaperone function of oligomeric BiP. Native gel electrophoresis revealed a rapidly-modulated reciprocal relationship between the burden of unfolded proteins and BiP oligomers and slower equilibration between oligomers and inactive, covalently-modified BiP. Lumenal ER calcium depletion caused rapid oligomerization of mammalian BiP and a coincidental diminution in substrate binding, pointing to the relative inertness of the oligomers. Thus, equilibration between inactive oligomers and active monomeric BiP is poised to buffer fluctuations in ER unfolded protein load on a rapid timescale attainable neither by inter-conversion of active and covalently-modified BiP nor by the conventional unfolded protein response.

Different Roles of GRP78 on Cell Proliferation and Apoptosis in Cartilage Development

Eukaryotic cells possess several mechanisms to adapt to endoplasmic reticulum (ER) stress and thereby survive. ER stress activates a set of signaling pathways collectively termed as the unfolded protein response (UPR). We previously reported that Bone morphogenetic protein 2 (BMP2) mediates mild ER stress and activates UPR signal molecules in chondrogenesis. The mammalian UPR protects the cell against the stress of misfolded proteins in the endoplasmic reticulum. Failure to adapt to ER stress causes the UPR to trigger apoptosis. Glucose regulated protein 78 (GRP78), as an important molecular chaperone in UPR signaling pathways, is responsible for binding to misfolded or unfolded protein during ER stress. However the influence on GRP78 in BMP2-induced chondrocyte differentiation has not yet been elucidated and the molecular mechanism underlyng these processes remain unexplored. Herein we demonstrate that overexpression of GRP78 enhanced cell proliferation in chondrocyte development with G1 phase advance, S phase increasing and G2-M phase transition. Furthermore, overexpression of GRP78 inhibited ER stress-mediated apoptosis and then reduced apoptosis in chondrogenesis induced by BMP2, as assayed by cleaved caspase3, caspase12, C/EBP homologous protein (CHOP/DDIT3/GADD153), p-JNK (phosphorylated c-Jun N-terminal kinase) expression during the course of chondrocyte differentiation by Western blot. In addition, flow cytometry (FCM) assay, terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end-labeling (TUNEL) assay and immune-histochemistry analysis also proved this result in vitro and in vivo. It was demonstrated that GRP78 knockdown via siRNA activated the ER stress-specific caspase cascade in developing chondrocyte tissue. Collectively, these findings reveal a novel critical role of GRP78 in regulating ER stress-mediated apoptosis in cartilage development and the molecular mechanisms involved.

Hsp70 oligomerization is mediated by an interaction between the interdomain linker and the substrate-binding domain

Oligomerization in the heat shock protein (Hsp) 70 family has been extensively documented both in vitro and in vivo, although the mechanism, the identity of the specific protein regions involved and the physiological relevance of this process are still unclear. We have studied the oligomeric properties of a series of human Hsp70 variants by means of nanoelectrospray ionization mass spectrometry, optical spectroscopy and quantitative size exclusion chromatography. Our results show that Hsp70 oligomerization takes place through a specific interaction between the interdomain linker of one molecule and the substrate-binding domain of a different molecule, generating dimers and higher-order oligomers. We have found that substrate binding shifts the oligomerization equilibrium towards the accumulation of functional monomeric protein, probably by sequestering the helical lid sub-domain needed to stabilize the chaperone: substrate complex. Taken together, these findings suggest a possible role of chaperone oligomerization as a mechanism for regulating the availability of the active monomeric form of the chaperone and for the control of substrate binding and release.

Tumor-produced secreted form of binding of immunoglobulin protein elicits antigen-specific tumor immunity

Binding of immunoglobulin protein (BiP) is a major molecular chaperone localized in endoplasmic reticulum (ER). It has been demonstrated to interact with nascent Ig. However, contrary to other ER-resident heat shock proteins such as gp96, calreticulin, and ORP150, it is not clear whether tumor-derived BiP plays a role in inducing antitumor immunity. In this study, we show that the tumor-derived secreted form of BiP is capable of inducing antitumor CD8(+) T cell responses. We constructed an ER-retention signal KDEL-deleted mutant of BiP cDNA and transfected it to tumor cells, which resulted in continuous secretion of tumor-derived BiP into the extracellular milieu. We show that this secreted BiP is taken up by bone marrow-derived dendritic cells, and thereafter BiP-associated Ag peptide is cross-presented in association with MHC class I molecules, resulting in elicitation of an Ag-specific CD8(+) T cell response and antitumor effect. This strategy to boost antitumor immune responses shows that a tumor could be its own cellular vaccine via gene modification of the secretion of the tumor Ag-BiP complex.

Ligation of cancer cell surface GRP78 with antibodies directed against its COOH-terminal domain up-regulates p53 activity and promotes apoptosis

Binding of activated α(2)-macroglobulin to GRP78 on the surface of human prostate cancer cells promotes proliferation by activating signaling cascades. Autoantibodies directed against the activated α(2)-macroglobulin binding site in the NH(2)-terminal domain of GRP78 are receptor agonists, and their presence in the sera of cancer patients is a poor prognostic indicator. We now show that antibodies directed against the GRP78 COOH-terminal domain inhibit [(3)H]thymidine uptake and cellular proliferation while promoting apoptosis as measured by DNA fragmentation, Annexin V assay, and clonogenic assay. These antibodies are receptor antagonists blocking autophosphorylation and activation of GRP78. Using 1-LN and DU145 prostate cancer cell lines and A375 melanoma cells, which express GRP78 on their cell surface, we show that antibodies directed against the COOH-terminal domain of GRP78 up-regulate the tumor suppressor protein p53. By contrast, antibody directed against the NH(2)-terminal domain of GRP78 shows negligible effects on p53 expression. PC-3 prostate cancer cells, which do not express GRP78 on their cell surface, are refractory to the effects of anti-GRP78 antibodies directed against either the COOH- or NH(2)-terminal domains. However, overexpression of GRP78 in PC-3 cells causes translocation of GRP78 to the cell surface and promotes apoptosis when these cells are treated with antibody directed against its COOH-terminal domain. Silencing GRP78 or p53 expression by RNA interference significantly blocked the increase in p53 induced by antibodies. Antibodies directed against the COOH-terminal domain may play a therapeutic role in cancer patients whose tumors trigger the production of autoantibodies directed against the NH(2)-terminal domain of GRP78.

ERdj3, a luminal ER DnaJ homologue, binds directly to unfolded proteins in the mammalian ER: identification of critical residues

ERdj3 was identified as a soluble, lumenal DnaJ family member that binds to unassembled immunoglobulin heavy chains along with the BiP chaperone complex in the endoplasmic reticulum of mammalian cells. Here we demonstrated that ERdj3 binds directly to unfolded substrates. Secondary structure predictions suggested that the substrate binding domain of ERdj3 was likely to closely resemble Ydj1, a yeast cytosolic DnaJ family member, which was previously crystallized with a peptide bound to the C-terminal fragment composed of domains I, II, and III. Mutation of conserved residues in domain I, which formed the peptide binding site in Ydj1, affected ERdj3's substrate binding ability in mammalian cells and in vitro binding studies. Somewhat unexpectedly, we found that domain II, which is highly conserved among ERdj3 homologues, but very different from domain II of Ydj1, was also critical for substrate binding. In addition, we demonstrated that ERdj3 forms multimers in cells and found that the conserved carboxy-terminal residue phenylalanine 326 played a critical role in self-assembly. In vitro binding assays revealed that mutation of this residue to alanine diminished ERdj3's substrate binding ability, arguing that multimerization is important for substrate binding. Together, these studies demonstrate that the Ydj1 structure is conserved in another family member and reveal that among this group of DnaJ proteins domain II, which is not present in the closely related type II family members, also plays an essential role in substrate binding.

In vivo analysis of the lumenal binding protein (BiP) reveals multiple functions of its ATPase domain

The endoplasmic reticulum (ER) chaperone binding protein (BiP) binds exposed hydrophobic regions of misfolded proteins. Cycles of ATP hydrolysis and nucleotide exchange on the ATPase domain were shown to regulate the function of the ligand-binding domain in vitro. Here we show that ATPase mutants of BiP with defective ATP-hydrolysis (T46G) or ATP-binding (G235D) caused permanent association with a model ligand, but also interfered with the production of secretory, but not cytosolic, proteins in vivo. Furthermore, the negative effect of BiP(T46G) on secretory protein synthesis was rescued by increased levels of wild-type BiP, whereas the G235D mutation was dominant. Unexpectedly, expression of a mutant BiP with impaired ligand binding also interfered with secretory protein production. Although mutant BiP lacking its ATPase domain had no detrimental effect on ER function, expression of an isolated ATPase domain interfered with secretory protein synthesis. Interestingly, the inhibitory effect of the isolated ATPase was alleviated by the T46G mutation and aggravated by the G235D mutation. We propose that in addition to its role in ligand release, the ATPase domain can interact with other components of the protein translocation and folding machinery to influence secretory protein synthesis.

Phage display derived human monoclonal antibodies isolated by binding to the surface of live primary breast cancer cells recognize GRP78

Clinical trials using monoclonal antibodies (mAb) against cell-surface markers have yielded encouraging therapeutic results in several cancer types. Generally, however, anticancer antibodies are only efficient against a subpopulation of cancers, and there is a strong need for identification of novel targets and human antibodies against them. We have isolated single-chain human mAbs from a large naïve antibody phage display library by panning on a single-cell suspension of freshly isolated live cancer cells from a human breast cancer specimen, and these antibodies were shown to specifically recognize cancer-associated cell-surface proteins. One of the isolated human antibody fragments, Ab39, recognizes a cell-surface antigen expressed on a subpopulation of cancer cell lines of different origins. Immunohistochemical analysis of a large panel of cancerous and normal tissues showed that Ab39 bound strongly to several cancers, including 45% breast carcinomas, 35% lung cancers, and 86% melanomas, but showed no or weak binding to normal tissues. A yeast two-hybrid screen of a large human testis cDNA library identified the glucose-regulated protein of 78 kDa (GRP78) as the antigen recognized by Ab39. The interaction was confirmed by colocalization studies and antibody competition experiments that also mapped the epitope recognized by Ab39 to the COOH terminus of GRP78. The expression of GRP78 on the surface of cancer cells, but not normal cells, makes it an attractive target for cancer therapies including mAb-based immunotherapy. Our results suggest that the human antibody Ab39 may be a useful starting point for further genetic optimization that could render it a useful diagnostic and therapeutic reagent for a variety of cancers.

Neurospora crassa FKBP22 Is a Novel ER Chaperone and Functionally Cooperates with BiP

FK506 binding proteins (FKBPs) belong to the family of peptidyl prolyl cis-trans isomerases (PPIases) catalyzing the cis/trans isomerisation of Xaa-Pro bonds in oligopeptides and proteins. FKBPs are involved in folding, assembly and trafficking of proteins. However, only limited knowledge is available about the roles of FKBPs in the endoplasmic reticulum (ER) and their interaction with other proteins. Here we show the ER located Neurospora crassa FKBP22 to be a dimeric protein with PPIase and a novel chaperone activity. While the homodimerization of FKBP22 is mediated by its carboxy-terminal domain, the amino-terminal domain is a functional FKBP domain. The chaperone activity is mediated by the FKBP domain but is exhibited only by the full-length protein. We further demonstrate a direct interaction between FKBP22 and BiP, the major Hsp70 chaperone in the ER. The binding to BiP is mediated by the FKBP domain of FKBP22. Interestingly BiP enhances the chaperone activity of FKBP22. Both proteins form a stable complex with an unfolded substrate protein and thereby prevent its aggregation. These results suggest that BiP and FKBP22 form a folding helper complex with a high chaperoning capacity in the ER of Neurospora crassa.

The effect of the hexahistidine-tag in the oligomerization of HSC70 constructs

The hexahistidine is a fusion tag used for the isolation of proteins via an immobilized metal-ion affinity chromatography (IMAC). In the present study, we have purified and analyzed two constructs of the heat shock protein HSC70 in the presence or the absence of the His-tag (C30WT-His(+)/C30WT and C30DeltaL-His(+)/C30DeltaL). The oligomerization properties of the constructs were analyzed by size exclusion chromatography (SEC) and analytical ultracentrifugation (AU). Results from SEC analysis indicated that the His-tag promotes the dimerization of C30DeltaL-His(+) but has no effect on the elution profile of C30WT-His(+), compared to their respective untagged forms C30DeltaL and C30WT. These observations were also confirmed by AU analysis which indicates that C30DeltaL is stabilized in the dimeric form in the presence of the His-tag. These results emphasize the need to remove the His-tag before structural characterization of some recombinant proteins.

Retention of Misfolded Mutant Transthyretin by the Chaperone BiP/GRP78 Mitigates Amyloidogenesis

Carriers of the D18G transthyretin (TTR) mutation display an unusual central nervous system (CNS) phenotype with late onset of disease. D18G TTR is monomeric and highly prone to misfold and aggregate even at physiological conditions. Extremely low levels of mutant protein circulate both in human serum and cerebrospinal fluid, indicating impaired secretion of D18G TTR. Recent data show efficient selective ER-associated degradation (ERAD) of D18G TTR. One essential component of the ER-assisted folding machinery is the molecular chaperone BiP. Co-expression of BiP and D18G TTR, or BiP and wild-type (wt) TTR, or mutants A25T TTR and L55P TTR in Escherichia coli showed that only D18G TTR was significantly captured by BiP. Negligible capture of wt TTR and L55P TTR was seen and a sixfold smaller amount of A25T TTR bound to BiP compared to D18G TTR. These data correlate very well with thermodynamic and kinetic stability of the TTR variants, indicating that folding efficiency is inversely correlated to BiP capture. The complexes between BiP and D18G TTR were stable and could be isolated through affinity chromatography. Analytical ultracentrifugation and size-exclusion chromatography revealed that D18G TTR and BiP formed a mixture of 1:1 complexes and large soluble oligomers. The stoichiometry of captured D18G TTR versus BiP increased with increasing size of the oligomers. This indicates that BiP either worked as a molecular shepherd collecting the aggregation-prone mutant into stable oligomers or that BiP could bind to oligomers formed from misfolded mutant protein. Sequence analysis of bound TTR peptides to BiP revealed a bound sequence corresponding to residues 88-103 of TTR, comprising beta-strand F in the folded TTR monomer constituting part of the hydrogen bonding tetramer interface in native TTR. The F-strand has also been suggested as a possible elongation region of amyloid fibrils, implicating how substoichiomeric amounts of BiP could sequester prefibrillar amyloidogenic oligomers through binding to this part of TTR. BiP binding to D18G TTR was abolished by addition of ATP. The released D18G TTR completely misfolded into amyloid aggregates as shown by ThT fluorescence and Congo red binding.

Approaches to the isolation and characterization of molecular chaperones

Molecular chaperones are integral components of the cellular machinery involved in ensuring correct protein folding and the continued maintenance of protein structure. An understanding of these ubiquitous molecules is key to finding cures to protein misfolding diseases such as Alzheimer's and Creutzfeldt-Jacob diseases. In addition, further understanding of chaperones will enhance our comprehension of the way the body copes with the environmental stresses that humans encounter daily. Our laboratory and our collaborators specialize in the production and characterization of chaperones from a wide variety of sources in order to gain a fuller understanding of how chaperones function in the cell. In this review, we primarily use the Hsp70/Hsp40 chaperone pair as an example to discuss recent advances in technology and reductions in cost that lend themselves to chaperone purification from both native and recombinant sources. Common assays to assess purified chaperone activity are also discussed.

Biophysical characterization of ERp29. Evidence for a key structural role of cysteine 125

ERp29 is a major resident of the endoplasmic reticulum (ER) that seemingly plays an important role in most animal cells. Although a protein-folding association is widely supported, ERp29's specific molecular function remains unknown. A chaperone activity was postulated from evidence that ERp29 forms multimers like the classical ER chaperones, but conflicting results have emerged from our recent studies. Here a biophysical approach was used to clarify this issue and also reveal a key structural role for ERp29's characteristic cysteine, Cys-125. Applying hydrodynamic parameters derived from sedimentation and dynamic light-scattering analyses, a model of ERp29's quaternary structure was assembled from existing tertiary substructures. Comparison with Windbeutel, an ERp29-like protein from fruit fly with specialized chaperone activity, revealed similar tri-lobar gross structures but some finer differences consistent with functional divergence. Solubility and hydrophobic probe assays revealed moderate surface hydrophobicity, which was reduced in mutant ERp29 in which serine replaced Cys-125. This mutant was also relatively labile to proteolytic degradation, providing two reasons for the strict conservation of Cys-125. No multimerization was observed with untagged ERp29, which existed as tight homodimers (K(d) < 50 nm), whereas His-tagged ERp29 artifactually formed 670-kDa oligomers. These findings distinguish ERp29 biophysically from its peers in the ER including Windbeutel, endorsing our postulate that ERp29 adds a distinct type of folding activity to the ER machinery. By invoking novel functional associations for Cys-125 and the adjoining linker, new clues about how ERp29 might work have also arisen.

Dependence of endoplasmic reticulum-associated degradation on the peptide binding domain and concentration of BiP

ER-associated degradation (ERAD) removes defective and mis-folded proteins from the eukaryotic secretory pathway, but mutations in the ER lumenal Hsp70, BiP/Kar2p, compromise ERAD efficiency in yeast. Because attenuation of ERAD activates the UPR, we screened for kar2 mutants in which the unfolded protein response (UPR) was induced in order to better define how BiP facilitates ERAD. Among the kar2 mutants isolated we identified the ERAD-specific kar2-1 allele (Brodsky et al. J. Biol. Chem. 274, 3453-3460). The kar2-1 mutation resides in the peptide-binding domain of BiP and decreases BiP's affinity for a peptide substrate. Peptide-stimulated ATPase activity was also reduced, suggesting that the interdomain coupling in Kar2-1p is partially compromised. In contrast, Hsp40 cochaperone-activation of Kar2-1p's ATPase activity was unaffected. Consistent with UPR induction in kar2-1 yeast, an ERAD substrate aggregated in microsomes prepared from this strain but not from wild-type yeast. Overexpression of wild-type BiP increased substrate solubility in microsomes obtained from the mutant, but the ERAD defect was exacerbated, suggesting that simply retaining ERAD substrates in a soluble, retro-translocation-competent conformation is insufficient to support polypeptide transit to the cytoplasm.

Interaction of the chaperone BiP with an antibody domain: implications for the chaperone cycle

BiP is an Hsp70 homologue found in the endoplasmic reticulum of eukaryotic cells. Like other Hsp70 chaperones, BiP interacts with its substrate proteins in an ATP-dependent manner. The functional analysis has so far been performed mainly with short, synthetic peptides. Here, we present an experimental system that allows to study the partial reactions of the BiP chaperone cycle for a natural substrate protein domain in its soluble, stably unfolded conformation. This unfolded antibody domain forms a binary complex with BiP in the absence of ATP. The dissociation of the BiP dimer seems to be the rate-limiting step in this reaction. The BiP-C(H)3 complexes dissociate rapidly in the presence of ATP. The affinity for BiP-binding peptides and the non-native antibody domain was determined to be similar, suggesting that only the peptide binding site is involved in these interactions. Furthermore, these results imply that, also in the context of the antibody domain, an extended peptide sequence is recognized. However, the accessibility of the BiP-binding site in the non-native protein seems to influence the kinetics of complex formation.

Thioredoxin fold as homodimerization module in the putative chaperone ERp29: NMR structures of the domains and experimental model of the 51 kDa dimer

BACKGROUND

ERp29 is a ubiquitously expressed rat endoplasmic reticulum (ER) protein conserved in mammalian species. Fold predictions suggest the presence of a thioredoxin-like domain homologous to the a domain of human protein disulfide isomerase (PDI) and a helical domain similar to the C-terminal domain of P5-like PDIs. As ERp29 lacks the double-cysteine motif essential for PDI redox activity, it is suggested to play a role in protein maturation and/or secretion related to the chaperone function of PDI. ERp29 self-associates into 51 kDa dimers and also higher oligomers.

RESULTS

3D structures of the N- and C-terminal domains determined by NMR spectroscopy confirmed the thioredoxin fold for the N-terminal domain and yielded a novel all-helical fold for the C-terminal domain. Studies of the full-length protein revealed a short, flexible linker between the two domains, homodimerization by the N-terminal domain, and the presence of interaction sites for the formation of higher molecular weight oligomers. A gadolinium-based relaxation agent is shown to present a sensitive tool for the identification of macromolecular interfaces by NMR.

CONCLUSIONS

ERp29 is the first eukaryotic PDI-related protein for which the structures of all domains have been determined. Furthermore, an experimental model of the full-length protein and its association states was established. It is the first example of a protein where the thioredoxin fold was found to act as a specific homodimerization module, without covalent linkages or supporting interactions by further domains. A homodimerization module similar as in ERp29 may also be present in homodimeric human PDI.

Isolation, expression, and characterization of fully functional nontoxic BiP/GRP78 mutants

Mammalian BiP/GRP78 and Escherichia coli DnaK belong to the highly conserved hsp70 family and function as molecular chaperones in the endoplasmic reticulum or the cytosol, respectively. Induction of murine BiP/GRP78 expression in E. coli leads to growth arrest and cell death, independent of the bacterial strain and vector used. Analysis of various BiP constructs and mutants shows that the dominant-lethal phenotype is induced specifically by the expression of the 13.7-kDa C-terminal domain and abolished by a single substitution in that region. Deletion of that region also results in nontoxic gene products that can be overexpressed and purified to homogeneity. The nontoxic mutants are highly expressed in E. coli, representing up to 20% of the soluble fraction. They are catalytically active, depolymerize upon binding ATP or synthetic peptide, and interact with the J-domain of the DnaJ-like accessory protein, MTJ1, with near wild-type affinity. Our data indicate that the cytotoxic effect encountered during overexpression of recombinant proteins can be caused by a single domain and can be alleviated by a specific mutation or deletion in that region without altering the catalytic properties of the enzyme.

Interaction of murine BiP/GRP78 with the DnaJ homologue MTJ1

The activity of Hsp70 proteins is regulated by accessory proteins, among which the most studied are the members of the DnaJ-like protein family. BiP/GRP78 chaperones the translocation and maturation of secreted and membrane proteins in the endoplasmic reticulum. No DnaJ-like partner has been described so far to regulate the function of mammalian BiP/GRP78. We show here that murine BiP/GRP78 interacts with the lumenal J domain of the murine transmembrane protein MTJ1 (J-MTJ1). J-MTJ1 stimulates the ATPase activity of BiP/GRP78 at stoichiometric concentrations. The C-terminal tail of BiP/GRP78 is not required for the interaction with J-MTJ1, leaving the function of this portion of the molecule still unclear. Physical interactions between J-MTJ1 and BiP/GRP78 are stable and can be abolished by a single histidine --> glutamine substitution in the highly conserved HPD motif shared by all DnaJ-like proteins. The J-MTJ1 fragment, but not the mutant J-MTJ1:H89Q fragment, stimulates the ATPase activity of Escherichia coli DnaK, although at a higher concentration than its genuine partner DnaJ. Full-length DnaJ does not stimulate BiP over the range of concentrations investigated. These results indicate that the J domain of MTJ1 is sufficient for its interaction with BiP/GRP78 and cannot be substituted by E. coli DnaJ.

Role and regulation of the ER chaperone BiP

BiP, an HSP70 molecular chaperone located in the lumen of the endoplasmic reticulum (ER), binds newly-synthesized proteins as they are translocated into the ER and maintains them in a state competent for subsequent folding and oligomerization. BiP is also an essential component of the translocation machinery, as well as playing a role in retrograde transport across the ER membrane of aberrant proteins destined for degradation by the proteasome. BiP is an abundant protein under all growth conditions, but its synthesis is markedly induced under conditions that lead to the accumulation of unfolded polypeptides in the ER. This attribute provides a marker for disease states that result from misfolding of secretory and transmembrane proteins.

Specificity of peptide-induced depolymerization of the recombinant carboxy-terminal fragment of BiP/GRP78

In the present study, we have used a non-denaturing gel electrophoresis assay to characterize the specificity of the peptide-induced depolymerization process of the isolated recombinant C-terminal domain (C30) of the molecular chaperone BiP, in the presence of specific synthetic peptides and with the neuropeptide Substance P. In the absence of peptidic ligand, C30 self-associates readily into multiple oligomeric species. Upon peptide addition, C30 oligomers convert into dimers, then into monomers. Our data indicate that the algorithm we previously developed to predict putative BiP binding sites in any protein sequence is also a good indicator as to whether a peptide can efficiently induce depolymerization of the C-terminal peptide binding domain and stimulate the ATPase activity of the full-length protein.