Overexpression of active Syrian golden hamster prion protein PrPc as a glutathione S-transferase fusion in heterologous systems

Molecular Dynamics Simulations Reveal Novel Interacting Regions of Human Prion Protein to Brucella abortus Hsp60 Protein

The distinctive morphology characteristics of microfold cells (M cells) allow the vaccine antigen not only to interact with immune cells directly, but also to effectively stimulate mucosal immune responses via receptors on its apical surface. Human prion protein, a transmembrane receptor for Brucella abortus Hsp60, is highly expressed on the M cell surface. Nonetheless, this protein tends to express in inclusion body in prokaryotic hosts. In this study, the shorter interacting regions of human prion protein were identified via computational methods such as docking and molecular dynamics simulations to minimize its aggregation tendency. The computational calculations revealed three novel human prion protein-interacting regions, namely PrP125, PrP174, and PrP180. In accordance with in silico prediction, the biologically synthesized peptides fusing with GST tag demonstrated their specific binding to Hsp60 protein via pull-down assay. Hence, this finding laid the groundwork for M-cell targeting candidate validation through these newly identified interacting regions.

A luminescent oxygen channeling biosensor that measures small GTPase activation

We established a homogeneous luminescent oxygen channeling sensor for measuring activation states of small GTPases. The assay quantifies activated GTPases in cell lysates, can be applied to different GTPases, and can be used for multiplex screening. The study will provide guidelines for determining activation states of diverse GTPases in various biological contexts.

Kinetic binding analysis of aptamers targeting HIV-1 proteins by a combination of a microbalance array and mass spectrometry (MAMS)

An enhanced chip-based detection platform was developed by integrating a surface acoustic wave biosensor of the Love-wave type with protein identification by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-ToF MS). The system was applied to characterize the interaction of aptamers with their cognate HIV-1 proteins. The aptamers, which target two proteins of HIV-1, were identified using an automated in vitro selection platform. For aptamers S66A-C6 and S68B-C5, which target the V3 loop of the HIV-1 envelope protein gp120, KD values of 406 and 791 nM, respectively, were measured. Aptamer S69A-C15 was shown to bind HIV-1 reverse transcriptase (HIV-1 RT) with a KD value of 637 nM when immobilized on the biosensor surface. HIV-1 RT was identified with high significance using MALDI-ToF MS even in crude protein mixtures. The V3-loop of gp120 could be directly identified when using chip-bound purified protein samples. From crude protein mixtures, it could be identified indirectly with high significance via its fusion-partner glutathione-S-transferase (GST). Our data show that the combination of the selectivity of aptamers with a sensitive detection by MS enables the reliable and quantitative analysis of kinetic binding events of protein solutions in real time.

Establishment of bovine prion peptide-based monoclonal antibodies for identifying bovine prion

To obtain high titer monoclonal antibodies (McAbs) which can react with mammalian prion protein (PrP), Balb/C mice were immunized with bovine (Bo) PrP peptide (BoPrP 209-228 aa) coupled to keyhole limpet hemocyanin (KLH). The hybridoma cell lines secreting monoclonal antibodies against the peptide were established by cell fusion and cloning. The obtained McAbs were applied to detect recombinant human, bovine and hamster PrP, cellular prion protein (PrP(c)) in normal bovine brain and pathogenic scrapie prion protein (PrP(Sc)) accumulated in the medulla oblongata of bovine spongiform encephalopathy(BSE)specimen with Western blot and immunohistochemical detection, respectively. The current procedure might offer a simple, feasible method to raise high titer antibodies for studying biological features of PrP in mammals, as well as detection of transmissible spongiform encephalopathy (TSE) and diagnosis of BSE, in particular.

Single chain Fv antibodies directed against the 37 kDa/67 kDa laminin receptor as therapeutic tools in prion diseases

Transmissible spongiform encephalopathies are a group of neurological disorders associated with the deposition of PrP(Sc), an abnormal form of the cellular prion protein PrP(c). The 37 kDa/67 kDa laminin receptor (LRP/LR) has been identified as a prion receptor and several lines of evidence strongly suggest that this protein plays a role during prion pathogenesis. Here we report the selection of recombinant single chain antibodies (scFvs) directed against LRP from naïve and synthetic phage scFv libraries for therapeutic application. Western blotting and FACS analysis confirmed a specific LRP/LR recognition pattern of the two selected scFvs S18 and N3. Both scFvs specifically interfered with the PrP/LRP interaction in vitro. High yield production of the scFvs of approx. 1mg/l of culture medium was achieved in E. coli. Passive immunotransfer of the scFv S18 antibody reduced PrP(Sc) levels by approx. 40% in the spleen of scrapie infected C57BL/6 mice 90 days post scFv injection, suggesting that scFv S18 interferes with peripheral PrP(Sc) propagation, without a significant prolongation of incubation and survival times.

Anti-LRP/LR antibody W3 hampers peripheral PrPSc propagation in scrapie infected mice

We identified the 37kDa/67kDa laminin receptor (LRP/LR) as a cell surface receptor for the cellular prion protein (PrP(c)) and the infectious prion protein (PrP(Sc)). Recently, we showed that anti-LRP/LR antibody W3 cured scrapie infected N2a cells. Here, we demonstrate that W3 delivered by passive immunotransfer into C57BL/6 mice reduced the PrP(Sc) content in the spleen significantly by 66%, demonstrating an impairment of the peripheral PrP(Sc) propagation. In addition, we observed a 1.8-fold increase in survival of anti-LRP/LR antibody W3 treated mice (mean survival of 31 days) compared to preimmune serum treated control animals (mean survival of 17 days). We conclude that the significant effect of anti-LRP/LR antibody W3 on the reduction of peripheral PrP(Sc) propagation might be due to the blockage of the prion receptor LRP/LR which is required, as previously shown in vitro, for PrP(Sc) propagation in vivo.

A trans-dominant negative 37kDa/67kDa laminin receptor mutant impairs PrP(Sc) propagation in scrapie-infected neuronal cells

The 37kDa/67kDa laminin receptor (LRP/LR) has been identified as a cell surface receptor for cellular and infectious prion proteins. Here, we show that an N-terminally truncated LRP mutant encompassing the extracellular domain of the LRP/LR (LRP102-295::FLAG) reduces the binding of recombinant cellular huPrP to mouse neuroblastoma cells, and infectious moPrP27-30 to BHK cells, and interferes with the PrP(Sc) propagation in scrapie-infected neuroblastoma cells (N2aSc(+)). A cell-free binding assay demonstrated the direct binding of the LRP102-295::FLAG mutant to both PrP(c) and PrP(Sc). These results, together with the finding that endogenous LRP levels remain unaffected by the expression of the mutant, indicate that the secreted LRP102-295::FLAG mutant may act in a trans-dominant negative manner as a decoy by trapping PrP molecules. The LRP mutant might represent a potential therapeutic tool for the treatment of TSEs.

Seeded conversion of recombinant prion protein to a disulfide-bonded oligomer by a reduction-oxidation process

The infectious form of prion protein, PrP(Sc), self-propagates by its conversion of the normal, cellular prion protein molecule PrP(C) to another PrP(Sc) molecule. It has not yet been demonstrated that recombinant prion protein can convert prion protein molecules from PrP(C) to PrP(Sc). Here we show that recombinant hamster prion protein is converted to a second form, PrP(RDX), by a redox process in vitro and that this PrP(RDX) form seeds the conversion of other PrP(C) molecules to the PrP(RDX) form. The converted form shows properties of oligomerization and seeded conversion that are characteristic of PrP(Sc). We also find that the oligomerization can be reversed in vitro. X-ray fiber diffraction suggests an amyloid-like structure for the oligomerized prion protein. A domain-swapping model involving intermolecular disulfide bonds can account for the stability and coexistence of two molecular forms of prion protein and the capacity of the second form for self-propagation.

Absence of superoxide dismutase activity in a soluble cellular isoform of prion protein produced by baculovirus expression system

A method for expression and purification of a soluble form of histidine (HIS)-tagged murine prion protein (bacMuPrP), which lacks the entire C-terminal cleavage and glycosyl phosphatidyl inositol (GPI) addition site, has been developed using a recombinant baculovirus expression system and purification with Ni-NTA agarose affinity chromatography. In mammalian sources, PrP(C) is attached to the cell membrane by a GPI anchor. However, in our system, bacMuPrP was secreted into the media, enabling its easy purification in abundance. Indirect immunofluorescence studies and immunoblot analysis localized not in cell membrane but in the perinuclear endoplasmic reticulum region in cells and is secreted into the media. Tunicamycin treatment revealed non-glycosylated proteins were secreted into the media, suggesting that glycosylation is not necessary for bacMuPrP secretion. Density-gradient sedimentation analysis demonstrated a sedimentation coefficient of secretory bacMuPrP as 2.3 S, indicating a monomeric form. Although affinity-purified PrP from mouse brain or recombinant prion protein (PrP) produced by Escherichia coli and refolded in the presence of copper has been reported to display superoxide dismutase (SOD) activity, bacMuPrP did not show SOD activity. These results suggest that bacMuPrP has a different biochemical and biophysical characterization from mammalian and bacterial-derived PrP. Furthermore, this simple expression system may provide an adequate source for structural, functional, and biochemical analyses of PrP.

Intra- and interspecies interactions between prion proteins and effects of mutations and polymorphisms

Recently, crystallization of the prion protein in a dimeric form was reported. Here we show that native soluble homogeneous FLAG-tagged prion proteins from hamster, man and cattle expressed in the baculovirus system are predominantly dimeric. The PrP/PrP interaction was confirmed in Semliki Forest virus-RNA transfected BHK cells co-expressing FLAG- and oligohistidine-tagged human PrP. The yeast two-hybrid system identified the octarepeat region and the C-terminal structured domain (aa90-aa230) of PrP as PrP/PrP interaction domains. Additional octarepeats identified in patients suffering from fCJD reduced (wtPrP versus PrP + 9OR) and completely abolished (PrP + 9OR versus PrP + 9OR) the PrP/PrP interaction in the yeast two-hybrid system. In contrast, the Met/Val polymorphism (aa129), the GSS mutation Pro102Leu and the FFI mutation Asp178Asn did not affect PrP/PrP interactions. Proof of interactions between human or sheep and bovine PrP, and sheep and human PrP, as well as lack of interactions between human or bovine PrP and hamster PrP suggest that interspecies PrP interaction studies in the yeast two-hybrid system may serve as a rapid pre-assay to investigate species barriers in prion diseases.

Different isoforms of the non-integrin laminin receptor are present in mouse brain and bind PrP

The prion protein (PrP) plays a central role in prion diseases, and identifying its cellular receptor appears to be of crucial interest. We previously showed in the yeast two-hybrid system that PrP interacts with the 37 kDa precursor (LRP) of the high affinity 67 kDa laminin receptor (LR), which acts as the cellular receptor of PrP in cellular models. However, among the various isoforms of the receptor that have been identified so far, those which are present in the central nervous system and which bind PrP are still unknown. In this study, we have purified mouse brain fractions enriched in the laminin receptor and have performed overlay assays in order to identify those isoforms that interact with the prion protein. We demonstrate (i) the presence, in mouse brain, of several isoforms of the LRP/LR corresponding to different maturation states of the receptor (44, 60, 67 and 220 kDa) and (ii) the binding of all of these isoforms to PrP. Our data strongly support a physiological role of the laminin receptor/PrP interaction in the brain and highlight its relevance for transmissible spongiform encephalopathies.

Recombinant human prion protein mutants huPrP D178N/M129 (FFI) and huPrP+9OR (fCJD) reveal proteinase K resistance

The Semliki-Forest virus (SFV) system was used to overexpress human wild-type and mutant prion proteins as well as FLAG-tagged human and bovine PrP in mammalian cells. The application of recombinant SFV vectors allowed a high-level production of highly glycosylated prion proteins with a molecular weight ranging from 25 to 30 kDa for recombinant wild-type human PrP and from 26 to 32 kDa for wild-type bovine PrP. Further, we report here the generation of recombinant mutant prion proteins that are associated with inherited human prion diseases such as fatal familial insomnia (FFI) and Creutzfeldt-Jakob disease (CJD). Both mutated variants, the FFI-associated PrP carrying a mutation at amino acid position 178 and the CJD-linked form containing an insertion of nine additional octarepeats reveal proteinase K resistance, one of the typical biochemical properties of the infectious scrapie isoform of the prion protein. By contrast, recombinant wild-type PrP was completely proteinase K sensitive when expressed in SFV-transfected BHK cells. The subcellular location of both PrP mutants at the cell surface and in intracellular compartments of transfected BHK cells was similar to that of wild-type PrP. In order to purify recombinant human and bovine PrP from cell lysates, a FLAG-tag was introduced either at the N-terminus behind the signal peptide or at the C-terminus close to the adhesion site of the GPI anchor. N-terminal insertion did not extensively influence the trafficking of the FLAG-tagged protein to the cell surface, whereas insertion close to the GPI attachment site clearly affected the transport of the majority of PrP to the cell membrane, probably resulting in their retention within the secretory pathway. All FLAG-tagged prion proteins were expressed efficiently in BHK cells and showed a typical glycosylation pattern, allowing their rapid and simple purification via anti-FLAG antibody chromatography.

Expression of PrP(C) as HIS-fusion form in a baculovirus system and conversion of expressed PrP-sen to PrP-res in a cell-free system

Conversion of the PrP cellular form (PrP(C)) to the pathogenic form (PrP(Sc)) is the key step in the pathogenesis of transmissible spongiform encephalophathies. Although the mechanism of conformational conversion of PrP proteins remains uncertain, the cell-free conversion reaction and other in vitro PrP amplification tests allow it to be studied under the much quicker and simpler conditions than those of transmission bioassay in vivo. Using baculovirus expression system, wild-type hamster (HaPrP) and human PrP (HuPrP), as well as D178N mutated human PrP (HuPrPm178) were expressed in HIS-fusion form. After 35S-methionine labeling and purification with Ni-NTA agarose affinity chromatography, individual expressed PrP proteins were mixed with PrP(Sc) isolated from hamster brain tissue infected with scrapie 263K. Protease-resistant isoform was detected in the homologous HaPrP reaction, but not in the two heterologous HuPrP preparations, implying a species-specific molecular recognition between PrP(C) and PrP(Sc). HIS-tag in HIS-HaPrP seems to have little effect on the formation of protease-resistant protein in this preparation. This system proposes a simple and protein productive-enriched way for cell-free conversion of prion proteins, as the replacement of native or genetic engineering expressed sole PrP(C) from mammalian or non-mammalian sources.

Expression in E. coli and purification of recombinant fragments of wild type and mutant human prion protein

Prion diseases are fatal neurodegenerative disorders of the CNS of men and animals, characterized by spongiform degeneration of the CNS, astrogliosis and deposition of amyloid into the brain. The conversion of a cellular glycoprotein (the prion protein, PrP(C)) into an altered isoform (the prion scrapie, PrP(Sc)), which accumulates within the brain tissue by virtue of its resistance to the intracellular catabolism, is currently believed to represent the etiologic agent responsible for these diseases. Synthetic or recombinant polypeptides are commonly used to elucidate the mechanism of proteins involved in neurodegenerative diseases. Here we describe a procedure, which allows the synthesis and purification in its native folding, of the human prion protein fragment 90-231, corresponding to the protease resistant core of PrP(Sc). We synthesized the polypeptides 90-231 of both the wild type and the E200K mutant isoforms of PrP. Using a gluthatione S-transferase (GST) fusion protein approach, milligram amounts of polypeptides were obtained after expression in E. coli. The recovery of the purified fusion protein was monitored following the evaluation of the GST activity. The PrP fragment was released from the fusion protein immobilized on a glutathione-coupled agarose resin by direct cleavage with thrombin. The recombinant protein was identified by comassie stained acrylamide gel and by immunoblotting employing a monoclonal anti-PrP antibody. The peptide purified by gel filtration chromatography showed mainly an alpha-helix structure, as analysed by circular dichroism (CD) and an intact disulfide bridge. The same procedure was also successfully employed to synthesize and purify the E200K mutant PrP fragment.

Identification of the heparan sulfate binding sites in the cellular prion protein

Data from cell culture and animal models of prion disease support the separate involvement of both heparan sulfate proteoglycans and copper (II) ions in prion (PrP) metabolism. Though direct interactions between prion protein and heparin have been recorded, little is known of the structural features implicit in this interaction or of the involvement of copper (II) ions. Using biosensor and enzyme-linked immunosorbent assay methodology we report direct heparin and heparan sulfate-binding activity in recombinant cellular prion protein (PrP(c)). We also demonstrate that the interaction of recombinant PrP(c) with heparin is weakened in the presence of Cu(II) ions and is particularly sensitive to competition with dextran sulfate. Competitive inhibition experiments with chemically modified heparins also indicate that 2-O-sulfate groups (but not 6-O-sulfate groups) are essential for heparin recognition. We have also identified three regions of the prion protein capable of independent binding to heparin and heparan sulfate: residues 23-52, 53-93, and 110-128. Interestingly, the interaction of an octapeptide-spanning peptide motif amino acids 53-93 with heparin is enhanced by Cu(II) ions. Significantly, a peptide of this sequence is able to inhibit the binding of full-length prion molecule to heparin, suggesting a direct role in heparin recognition within the intact protein. The collective data suggest a complex interaction between prion protein and heparin/heparan sulfate and has implications for the cellular and pathological functions of prion proteins.

Carbonyl formation on a copper-bound prion protein fragment, PrP23-98, associated with its dopamine oxidase activity

The amino-terminal part of prion protein (PrP), containing a series of octapeptide repeats with the consensus sequence PHGGGWGQ, has been implicated in the binding of copper ion. This region possesses amino acid residues susceptible to oxidation, such as histidine, lysine, arginine and proline. In this study, we have investigated copper-catalyzed oxidation of an N-terminal part of human PrP, PrP23-98, that was prepared by the recombinant DNA technique. Carbonyl formations on copper-bound PrP23-98 induced by dopamine and L-ascorbate were analyzed kinetically, and it was found that the redox cycling of PrP23-98-bound copper, especially induced by dopamine, was coupled to the formation of carbonyls on the protein.

The 37-kDa/67-kDa laminin receptor acts as the cell-surface receptor for the cellular prion protein

Recently, we identified the 37-kDa laminin receptor precursor (LRP) as an interactor for the prion protein (PrP). Here, we show the presence of the 37-kDa LRP and its mature 67-kDa form termed high-affinity laminin receptor (LR) in plasma membrane fractions of N2a cells, whereas only the 37-kDa LRP was detected in baby hamster kidney (BHK) cells. PrP co-localizes with LRP/LR on the surface of N2a cells and Semliki Forest virus (SFV) RNA transfected BHK cells. Cell-binding assays reveal the LRP/LR-dependent binding of cellular PrP by neuronal and non-neuronal cells. Hyperexpression of LRP on the surface of BHK cells results in the binding of exogenous PrP. Cell binding is similar in PrP(+/+) and PrP(0/0) primary neurons, demonstrating that PrP does not act as a co-receptor of LRP/LR. LRP/LR-dependent internalization of PrP is blocked at 4 degrees C. Secretion of an LRP mutant lacking the transmembrane domain (aa 86-101) from BHK cells abolishes PrP binding and internalization. Our results show that LRP/LR acts as the receptor for cellular PrP on the surface of mammalian cells.

Identification of interaction domains of the prion protein with its 37-kDa/67-kDa laminin receptor

Cell-binding and internalization studies on neuronal and non-neuronal cells have demonstrated that the 37-kDa/67-kDa laminin receptor (LRP/LR) acts as the receptor for the cellular prion protein (PrP). Here we identify direct and heparan sulfate proteoglycan (HSPG)-dependent interaction sites mediating the binding of the cellular PrP to its receptor, which we demonstrated in vitro on recombinant proteins. Mapping analyses in the yeast two-hybrid system and cell-binding assays identified PrPLRPbd1 [amino acids (aa) 144-179] as a direct and PrPLRPbd2 (aa 53-93) as an indirect HSPG-dependent laminin receptor precursor (LRP)-binding site on PrP. The yeast two-hybrid system localized the direct PrP-binding domain on LRP between aa 161 and 179. Expression of an LRP mutant lacking the direct PrP-binding domain in wild-type and mutant HSPG-deficient Chinese hamster ovary cells by the Semliki Forest virus system demonstrates a second HSPG-dependent PrP-binding site on LRP. Considering the absence of LRP homodimerization and the direct and indirect LRP-PrP interaction sites, we propose a comprehensive model for the LRP-PrP-HSPG complex.

Strain-specific propagation of PrP(Sc) properties into baculovirus-expressed hamster PrP(C)

The conversion of the cellular isoform of the prion protein (PrP(C)) to the abnormal disease-associated isoform (PrP(Sc)) has been simulated in cell-free conversion reactions in which PrP(Sc)-enriched preparations induce the conformational transition of PrP(C) into protease-resistant PrP (PrP-res). We explored the utility of recombinant hamster (Ha)PrP(C) purified from baculovirus-infected insect cells (bacHaPrP(C)) as a replacement for mammalian-derived HaPrP(C) in the conversion reactions. Protease-resistant recombinant HaPrP was generated after incubation of (35)S-bacHaPrP(C) with PrP(Sc)-enriched preparations. Moreover strain-specific PrP-res was also reproduced using insect-cell derived HaPrP(C) and PrP(Sc) from two different strains of hamster-adapted transmissible mink encephalopathy, designated hyper (HY) and drowsy (DY). Two strain-mediated properties were tested: (i) molecular mass of the protease-digested products and (ii) relative resistance to proteinase K (PK) digestion. Similar to in vivo generation of PrP(HY) and PrP(DY), the converted products selectively reproduced both characteristics, with the DY conversion product being smaller in size and less resistant to PK digestion than the HY product. These data demonstrate that non-mammalian sources of recombinant HaPrP can be converted into PK-resistant form and that strain-mediated properties can be transmitted into the newly formed PrP-res.

Recombinant protein expression in Pichia pastoris

The methylotrophic yeast Pichia pastoris is now one of the standard tools used in molecular biology for the generation of recombinant protein. P. pastoris has demonstrated its most powerful success as a large-scale (fermentation) recombinant protein production tool. What began more than 20 years ago as a program to convert abundant methanol to a protein source for animal feed has been developed into what is today two important biological tools: a model eukaryote used in cell biology research and a recombinant protein production system. To date well over 200 heterologous proteins have been expressed in P. pastoris. Significant advances in the development of new strains and vectors, improved techniques, and the commercial availability of these tools coupled with a better understanding of the biology of Pichia species have led to this microbe's value and power in commercial and research labs alike.

Heterologous protein expression in the methylotrophic yeast Pichia pastoris

During the past 15 years, the methylotrophic yeast Pichia pastoris has developed into a highly successful system for the production of a variety of heterologous proteins. The increasing popularity of this particular expression system can be attributed to several factors, most importantly: (1) the simplicity of techniques needed for the molecular genetic manipulation of P. pastoris and their similarity to those of Saccharomyces cerevisiae, one of the most well-characterized experimental systems in modern biology; (2) the ability of P. pastoris to produce foreign proteins at high levels, either intracellularly or extracellularly; (3) the capability of performing many eukaryotic post-translational modifications, such as glycosylation, disulfide bond formation and proteolytic processing; and (4) the availability of the expression system as a commercially available kit. In this paper, we review the P. pastoris expression system: how it was developed, how it works, and what proteins have been produced. We also describe new promoters and auxotrophic marker/host strain combinations which extend the usefulness of the system.

Prions in Saccharomyces and Podospora spp.: protein-based inheritance

Genetic evidence showed two non-Mendelian genetic elements of Saccharomyces cerevisiae, called [URE3] and [PSI], to be prions of Ure2p and Sup35p, respectively. [URE3] makes cells derepressed for nitrogen catabolism, while [PSI] elevates the efficiency of weak suppressor tRNAs. The same approach led to identification of the non-Mendelian element [Het-s] of the filamentous fungus Podospora anserina, as a prion of the het-s protein. The prion form of the het-s protein is required for heterokaryon incompatibility, a normal fungal function, suggesting that other normal cellular functions may be controlled by prions. [URE3] and [PSI] involve a self-propagating aggregation of Ure2p and Sup35p, respectively. In vitro, Ure2p and Sup35p form amyloid, a filamentous protein structure, high in beta-sheet with a characteristic green birefringent staining by the dye Congo Red. Amyloid deposits are a cardinal feature of Alzheimer's disease, non-insulin-dependent diabetes mellitus, the transmissible spongiform encephalopathies, and many other diseases. The prion domain of Ure2p consists of Asn-rich residues 1 to 80, but two nonoverlapping fragments of the molecule can, when overproduced, induce the de nova appearance of [URE3]. The prion domain of Sup35 consists of residues 1 to 114, also rich in Asn and Gln residues. While runs of Asn and Gln are important for [URE3] and [PSI], no such structures are found in PrP or the Het-s protein. Either elevated or depressed levels of the chaperone Hsp104 interfere with propagation of [PSI]. Both [URE3] and [PSI] are cured by growth of cells in millimolar guanidine HCl. [URE3] is also cured by overexpression of fragments of Ure2p or fusion proteins including parts of Ure2p.

Immunological characterization of the sheep prion protein expressed as fusion proteins in Escherichia coli

The prion protein (PrP) from sheep was produced in large quantities of entire protein in Escherichia coli after fusion with a carboxy-terminal hexahistidine sequence. In contrast, amino-terminal fusion with glutathione S-transferase (GST) revealed a high susceptibility toward cleavage of the protein. Both recombinant proteins were recognised, at variable levels, in Western blots using a panel of antibodies against the 40-56, 89-104, 98-113 and 112-115 sequences of the prion protein, similarly to the abnormal prion protein extracted from scrapie-infected sheep. Interestingly, monoclonal antibody 3F4 was found to react with these three proteins in Western blot.

Chicken prion tandem repeats form a stable, protease-resistant domain

Prion-linked diseases, such as mad cow disease, scrapie, and the human genetic disorder Creutzfeldt-Jakob disease, are fatal neurodegenerative diseases correlated with changes in the secondary structure of neural prion protein. We expressed recombinant chicken prion protein in Escherichia coli and purified the protein to homogeneity. Circular dichroism spectra of the 26 kDa recombinant protein closely resemble those of prion protein purified directly from healthy hamster brain. The chicken prion protein exists as a soluble, monodisperse monomer but can be forced to multimerize following lyophilization and resuspension. We analyzed the chicken prion protein domain structure by proteolysis and show that, unlike the mammalian homologues, the chicken prion protein N-terminal tandem amino acid repeats form a stable, protease-resistant domain. This domain probably represents a physiologically functional unit. As tested by both mass spectrometry and circular dichroism, the mature chicken prion protein does not bind copper, unlike synthetic peptides from the chicken prion N-terminus, suggesting that binding copper is not the physiological activity of the chicken prion. However, copper strongly destabilizes the prion protein and depresses the melting temperature by 30 degreesC, presumably by binding to the unfolded form of the prion protein. The chicken prion N-terminus may have evolved to fold without a cofactor, unlike mammalian prion proteins, whose N-termini are disordered without cofactors such as copper present. Chicken prion offers an alternative to intractable mammalian prions for structural studies of the amino-terminal domain.

Secretion and purification of HCV E1 protein forms as glutathione-S-transferase fusion in the baculovirus insect cell system

We have expressed the E1 protein of Hepatitis C Virus (HCV) in a new recombinant form by using a baculovirus transfer vector directing the expression of proteins fused to the carboxy-terminus of glutathione-S-transferase (GST). The E1 domain was expressed varying at its carboxy terminus in order to retain (GST-E1) or delete (GST-E1b) the C-terminal hydrophobic region that may be involved in membrane association. Following infection with the recombinant virus, GST-E1b was efficiently secreted into the culture media and could be purified in a single step with the minimum of denaturation by glutathione affinity chromatography. The purified product was specifically immunoprecipitated by HCV positive human sera suggesting the maintenance of an immuno-relevant tertiary structure despite removal of the hydrophobic anchor. By contrast, cells infected with a recombinant baculovirus expressing GST-E1 gave a fusion protein with an appropriate molecular weight but also a series of polypeptides of lower molecular weight consistent with cleavage at the C-terminus of E1. GST-E1 was not secreted into the medium and was associated predominantly with the membrane fraction following cell disruption; the lower molecular weight forms were soluble and secreted.

Complementary hydropathy identifies a cellular prion protein receptor

Prions, the etiological agents for infectious degenerative encephalopathies, act by entering the cell and inducing conformational changes in PrPC (a normal cell membrane sialoglycoprotein), which result in cell death. A specific cell-surface receptor to mediate PrPC and prion endocytosis has been predicted. Complementary hydropathy let us generate a hypothetical peptide mimicking the receptor binding site. Antibodies raised against this peptide stain the surface of mouse neurons and recognize a 66-kDa membrane protein that binds PrPC both in vitro and in vivo. Furthermore, both the complementary prion peptide and antiserum against it inhibit the toxicity of a prion-derived peptide toward neuronal cells in culture. Such reagents might therefore have therapeutic applications.

The human 37-kDa laminin receptor precursor interacts with the prion protein in eukaryotic cells

Prions are thought to consist of infectious proteins that cause transmissible spongiform encephalopathies. According to overwhelming evidence, the pathogenic prion protein PrPSc converts its host encoded isoform PrPC into insoluble aggregates of PrPSc, concomitant with pathological modifications (for review, see refs. 1-3). Although the physiological role of PrPC is poorly understood, studies with PrP knockout mice demonstrated that PrPC is required for the development of prion diseases. Using the yeast two-hybrid technology in Saccharomyces cerevisiae, we identified the 37-kDa laminin receptor precursor (LRP) as interacting with the cellular prion protein PrPC. Mapping analysis of the LRP-PrP interaction site in S. cerevisiae revealed that PrP and laminin share the same binding domain (amino acids 161 to 180) on LRP. The LRP-PrP interaction was confirmed in vivo in insect (Sf9) and mammalian cells (COS-7). The LRP level was increased in scrapie-infected murine N2a cells and in brain and spleen of scrapie-infected mice. In contrast, the LRP concentration was not significantly altered in these organs from mice infected with the bovine spongiform encephalopathic agent (BSE), which have a lower PrPSc accumulation. LRP levels, however, were dramatically increased in brain and pancreas, slightly increased in the spleen and not altered in the liver of crapie-infected hamsters. These data show that enhanced LRP concentrations are correlated with PrPSc accumulation in organs from mice and hamsters. The laminin receptor precursor, which is highly conserved among mammals and is located on the cell surface, may act as a receptor or co-receptor for the prion protein on mammalian cells.

The prion model for [URE3] of yeast: spontaneous generation and requirements for propagation

The genetic properties of the non-Mendelian element, [URE3], suggest that it is a prion (infectious protein) form of Ure2p, a mediator of nitrogen regulation in Saccharomyces cerevisiae. Into a ure2Delta strain (necessarily lacking [URE3]), we introduced a plasmid overproducing Ure2p. This induced the frequent "spontaneous generation" of [URE3], with properties identical to the original [URE3]. Altering the translational frame only in the prion-inducing domain of URE2 shows that it is Ure2 protein (and not URE2 RNA) that induces appearance of [URE3]. The proteinase K-resistance of Ure2p is unique to [URE3] strains and is not seen in nitrogen regulation of normal strains. The prion-inducing domain of Ure2p (residues 1-65) can propagate [URE3] in the absence of the C-terminal part of the molecule. In contrast, the C-terminal part of Ure2p cannot be converted to the prion (inactive) form without the prion-inducing domain covalently attached. These experiments support the prion model for [URE3] and extend our understanding of its propagation.

RNA aptamers specifically interact with the prion protein PrP

We have isolated RNA aptamers which are directed against the recombinant Syrian golden hamster prion protein rPrP23-231 (rPrPc) fused to glutathione S-transferase (GST). The aptamers did not recognize the fusion partner GST or the fusion protein GST::rPrP90-231 (rPrP27-30), which lacks 67 amino acids from the PrP N terminus. The aptamer-interacting region of PrPc was mapped to the N-terminal amino acids 23 to 52. Sequence analyses suggest that the RNA aptamers may fold into G-quartet-containing structural elements. Replacement of the G residues in the G quartet scaffold with uridine residues destroyed binding to PrP completely, strongly suggesting that the G quartet motif is essential for PrP recognition. Individual RNA aptamers interact specifically with prion protein in brain homogenates from wild-type mice (C57BL/6), hamsters (Syrian golden), and cattle as shown by supershifts obtained in the presence of anti-PrP antibodies. No interaction was observed with brain homogenates from PrP knockout mice (prn-p(0/0)). Specificity of the aptamer-PrP interaction was further confirmed by binding assays with antisense aptamer RNA or a mutant aptamer in which the guanosine residues in the G tetrad scaffold were replaced by uridine residues. The aptamers did not recognize PrP27-30 in brain homogenates from scrapie-infected mice. RNA aptamers may provide a first milestone in the development of a diagnostic assay for the detection of transmissible spongiform encephalopathies.

The complete mature bovine prion protein highly expressed in Escherichia coli: biochemical and structural studies

According to the 'protein only' hypothesis, modification of the 3-dimensional fold of the constituent cellular protein, PrP(C), into the disease-associated isoform, PrP(Sc), is the cause of neurodegenerative diseases in animals and humans. Here we describe the high-level synthesis in Escherichia coli, and purification in the monomeric form, of a histidine-tagged full-length mature PrP (25-249) of bovine brain, termed His-PrP. Based on biochemical and spectroscopic data, His-PrP displays characteristics expected for the PrP(C) isoform. The reported expression system should allow the production of quantities of bovine PrP(C) sufficient to permit 3-dimensional structure determinations.

Expression and purification of the seven nonstructural proteins of the flavivirus Kunjin in the E. coli and the baculovirus expression systems

All seven nonstructural (ns) proteins of the flavivirus Kunjin (KUN) ranging from NS1 to NS5 were expressed either alone or as fusion proteins with Glutathione-S-transferase (GST). High level expression of recombinant proteins was achieved in Spodoptera frugiperda (Sf9) cells using the baculovirus expression system in contrast to the low level of expression in E. coli. The order of the level of expression of the recombinant fusion proteins per 4 x 10(7) Sf9 cells was: GST-NS5 (yields approximately 4-5 mg) > GST-delta NS3 (approximately 1-2 mg) > GST-4A (approximately 1 mg) > GST-2B (approximately 0.5-1 mg) > GST-2A (approximately 0.5 mg) > GST-4B (approximately 0.1-0.2 mg). NS1 protein was expressed in a native form at the level of approximately 2-4 mg per 4 x 10(7) Sf9 cells. All the GST-fusion proteins were purified by adsorption on Glutathione Sepharose (GS) beads from solubilized lysates of Sf9 cells infected with the recombinant baculoviruses, or of E. coli cultures transformed with the expression plasmid and induced with IPTG. Only delta NS3 protein was recovered intact by removing GST from the fusion protein by digestion with Factor Xa protease. Attempts to cleave off the GST moiety from all the other purified recombinant proteins resulted either in inefficient cleavage or in degradation of the proteins. No GST-NS5 but from 20 to 50% of the purified GST-NS2A, GST-NS2B, GST-delta NS3, GST-NS4A, and GST-NS4B was eluted off the GS beads by adding glutathione. Thus, KUN purified recombinant proteins, either in eluted form or while immobilized on GS beads, could be used to raise monospecific antibodies, to perform functional assays or to participate in protein-protein or RNA-protein binding reactions.

Prion protein PrPc interacts with molecular chaperones of the Hsp60 family

Prions mediate the pathogenesis of certain neurodegenerative diseases, including bovine spongiform encephalopathy in cattle and Creutzfeldt-Jakob disease in humans. The prion particle consists mainly, if not entirely, of PrPSc, a posttranslationally modified isoform of the cellular host-encoded prion protein (PrPc). It has been suggested that additional cellular factors might be involved in the physiological function of PrPc and in the propagation of PrPSc. Here we employ a Saccharomyces cerevisiae two-hybrid screen to search for proteins which interact specifically with the Syrian golden hamster prion protein. Screening of a HeLa cDNA library identified heat shock protein 60 (Hsp60), a cellular chaperone as a major interactor for PrPc. The specificity of the interaction was confirmed in vitro for the recombinant proteins PrPc23-231 and rPrP27-30 fused to glutathione S-transferase with recombinant human Hsp60 as well as the bacterial GroEL. The interaction site for recombinant Hsp60 and GroEL proteins was mapped between amino acids 180 and 210 of the prion protein by screening with a set of recombinant PrPc fragments. The binding of Hsp60 and GroEL occurs within a region which contains parts of the putative alpha-helical domains H3 and H4 of the prion protein.

High-level expression and characterization of a purified 142-residue polypeptide of the prion protein

The major, and possible only, component of the infectious prion is the scrapie prion protein (PrPSc); the protease resistant core of PrPSc is PrP 27-30, a protein of approximately 142 amino acids. PrPSc is derived from the cellular PrP isoform (PrPC) by a post-transliatonal process in which a profound conformational change occurs. Syrian hamster (SHa) PrP genes of varying length ranging from the N- and C- terminally truncated 90-228 up to the full-length mature protein 23-231 were inserted into various secretion and intracellular expression vectors that were transformed into Escherichia coli deficient for proteases. Maximum expression was obtained for a truncated SHaPrP containing residues 90-231, which correspond to the sequence of PrP 27-30; disruption of the bacteria using a microfluidizer produced the highest yields of this protein designated rPrP. After solubilization of rPrP in 8 M GdnHC1, it was purified by size exclusion chromatography and reversed phase chromatography. During purification the recovery was approximately 50%, and from each liter of E. coli culture, approximately 50 mg of purified rPrP was obtained. Expression of the longer species containing the basic N-terminal region was less successful and was not pursued further. The primary structure of rPrP was verified by Edman sequencing and mass spectrometry, and secondary structure determined by circular dichroism and Fourier transform infrared spectroscopy. When rPrP was purified under reducing conditions, it had a high beta-sheet content and relatively low solubility similar to PrPSc, particularly at pH values > 7. Refolding of rPrP by oxidation to form a disulfide bond between the two Cys residues of this polypeptide produced a soluble protein with a high alpha-helical content similar to PrPC. These multiple conformations of rPrP are reminiscent of the structural plurality that characterizes the naturally occurring PrP isoforms. The high levels of purified rPrP which can now be obtained should facilitate determination of the multiple tertiary structures that Prp can adopt.