A general bacterial expression system for functional analysis of cDNA-encoded proteins

Protein fusion tags for efficient expression and purification of recombinant proteins in the periplasmic space of E. coli

Disulfide bonds occurred in majority of secreted protein. Formation of correct disulfide bonds are must for achieving native conformation, solubility and activity. Production of recombinant proteins containing disulfide bond for therapeutic, diagnostic and various other purposes is a challenging task of research. Production of such proteins in the reducing cytosolic compartment of E. coli usually ends up in inclusion bodies formation. Refolding of inclusion bodies can be difficult, time and labor consuming and uneconomical. Translocation of these proteins into the oxidative periplasmic compartment provides correct environment to undergo proper disulfide bonds formation and thus achieving native conformation. However, not all proteins can be efficiently translocated to the periplasm with the help of bacterial signal peptides. Therefore, fusion to a small well-folded and stable periplasmic protein is more promising for periplasmic production of disulfide bonded proteins. In the past decades, several full-length proteins or domains were used for enhancing translocation and solubility. Here, protein fusion tags that significantly increase the yields of target proteins in the periplasmic space are reviewed.

Achieving directed immunostimulating complexes incorporation

In recent years, several studies have been reported with the common aim of generating general expression systems for straightforward production and subsequent coupling of expressed antigens to an adjuvant system. Here, we describe a series of such efforts with a common theme of using gene fusion technology for association of recombinant antigens to immunostimulating complexes (iscoms). In the early stages of vaccine development, uniform antigen preparations are crucial to allow the comparison of immune responses to different antigens, or even subdomains thereof, and we believe that the described systems constitute an important development in this context.

Generation and Characterization of Monoclonal Antibodies Specific for Human FCRLA

FCRLA is a recently identified intracellular protein structurally related to the classic Fc receptors and expressed primarily in the germinal centers of B cells. We generated six monoclonal antibodies (MAbs) specific to the human protein. The MAbs recognize three different epitopes, which were shown to be localized on the D3 domain of the FCRLA molecule. The clones M101 and M616 were demonstrated to be applicable in various immunochemical analyses, such as immunoblotting, immunohistochemistry, and immunoprecipitation. In addition, this pair of antibodies was used for development of a sandwich version of ELISA to quantitatively detect FCRLA in cell lysates. Using these MAbs, we studied FCRLA expression in a panel of human B cell lines, such as Raji, Daudi, Bjab, BL-2, RPMI 1788, RPMI 8226, IM-9, and SKW6.4. It was found that all these lines, except RPMI 8226, produce FCRLA but may vary in the proportion of FCRLA-positive cells. The MAbs we established can be a useful tool to investigate the functional role of FCRLA and its applicability as a B cell development and malignant transformation marker.

Structure-based Design of a Second-generation Lyme Disease Vaccine Based on a C-terminal Fragment of Borrelia burgdorferi OspA

Here, we describe a structure-based approach to reduce the size of an antigen protein for a subunit vaccine. Our method consists of (i) determining the three-dimensional structure of an antigen, (ii) identifying protective epitopes, (iii) generation of an antigen fragment that contains the protective epitope, and (iv) rational design to compensate for destabilization caused by truncation. Using this approach we have successfully developed a second-generation Lyme disease vaccine. Outer surface protein A (OspA) from the Lyme disease spirochete Borrelia burgdorferi elicits protective immunity that blocks transmission of Borrelia from the tick vector to the vaccinated animal, and thus has been a focus of vaccine development. OspA has two globular domains that are connected via a unique single-layer beta-sheet. All anti-OspA monoclonal antibodies that block Borrelia transmission bind to conformational epitopes in the C-terminal domain of OspA, suggesting the possibility of using the C-terminal domain alone as a recombinant protein-based vaccine. The removal of ineffective parts from the OspA antigen may reduce side effects and lead to a safer vaccine. We prepared a C-terminal fragment of OspA by removing approximately 45% of residues from the N terminus. Although the fragment retained the native conformation and affinity to a protective antibody, its vaccine efficacy and conformational stability were significantly reduced with respect to full-length OspA. We successfully stabilized the fragment by replacing amino acid residues involved in buried salt-bridges with residues promoting hydrophobic interactions. The mutations promoted the vaccine efficacy of the redesigned fragment to a level comparable to that of the full-length protein, demonstrating the importance of the antigen stability for OspA's vaccine efficacy. Our strategy should be useful for further refining OspA-based vaccines and developing recombinant vaccines for other diseases.

Single-vector three-frame expression systems for affinity-tagged proteins

An effort is presented to create expression vectors which would allow expression of an inserted gene fragment in three reading frames in a single vector from a single promoter but with three separate ribosome binding sites (RBS). Each expression frame would generate an in-frame fusion with an affinity tag to allow efficient recovery of the produced fusion proteins. In the first generation vector, three identical polyhistidyl tags (His(6)) were used as affinity tags for the three expression frames. In the second generation vector, three different tags, an albumin binding domain derived from streptococcal protein G, an IgG binding Staphylococcus aureus protein A-derived domain (Z) and a His(6) tag, were employed to allow frame-specific affinity recovery. To evaluate the systems, model genes have been inserted in three different frames in both vectors. The first vector was demonstrated to produce fusion proteins in all three frames, whereas for the second, with a much wider spacing between the RBSs and affinity tags, expression could only be demonstrated from the first two translational start sites. For both systems, the first translation start was found to be significantly favored over the others. Nevertheless, we believe that the presented results represent the first successful attempt to create single-vector three-frame expression systems, a concept that could become valuable in future combined cloning-expression vectors.

General expression vectors for Staphylococcus carnosus enabled efficient production of the outer membrane protein A of Klebsiella pneumoniae

General expression vectors, designed for intracellular expression or secretion of recombinant proteins in the non-pathogenic Staphylococcus carnosus, were constructed. Both vector systems encode two different affinity tags, an upstream albumin binding protein and a downstream hexahistidyl peptide, and are furnished with cleavage sites for two site-specific proteases for optional affinity tag removal. To evaluate the novel vectors, the gene encoding the outer membrane protein A (OmpA) of Klebsiella pneumoniae was introduced into the vectors. Efficient production was demonstrated in both systems, although, as expected for OmpA fusions, somewhat better intracellularly, and the fusion proteins could be recovered as full-length products by affinity chromatography.

Rapid screening for improved solubility of small human proteins produced as fusion proteins in Escherichia coli

A prerequisite for structural genomics and related projects is to standardize the process of gene overexpression and protein solubility screening to enable automation for higher throughput. We have tested a methodology to rapidly subclone a large number of human genes and screen these for expression and protein solubility in Escherichia coli. The methodology, which can be partly automated, was used to compare the effect of six different N-terminal fusion proteins and an N-terminal 6*His tag. As a realistic test set we selected 32 potentially interesting human proteins with unknown structures and sizes suitable for NMR studies. The genes were transferred from cDNA to expression vectors using subcloning by recombination. The subcloning yield was 100% for 27 (of 32) genes for which a PCR fragment of correct size could be obtained. Of these, 26 genes (96%) could be overexpressed at detectable levels and 23 (85%) are detected in the soluble fraction with at least one fusion tag. We find large differences in the effects of fusion protein or tag on expression and solubility. In short, four of seven fusions perform very well, and much better than the 6*His tag, but individual differences motivate the inclusion of several fusions in expression and solubility screening. We also conclude that our methodology and expression vectors can be used for screening of genes for structural studies, and that it should be possible to obtain a large fraction of all NMR-sized and nonmembrane human proteins as soluble fusion proteins in E. coli.

FCRL, a novel member of the leukocyte Fc receptor family possesses unique structural features

A novel conserved member of the leukocyte Fc receptor (FcR) family was identified in human and mouse. The presumably secreted protein, designated FCRL (FcR-like) is comprised of four domains. The three N-terminal domains are related to the extracellular region of FcgammaRI, with the second (35-37% residue identity) and the third (46-52%) domains showing highest similarity. The C-terminal domain is a unique sequence enriched with proline residues. In humans, alternative transcripts for six FCRL isoforms were revealed. Spleen and tonsils were found to be the major sources of FCRL mRNA in human tissues. Western blotting of tonsil cell lysate using FCRL-specific antibodies recognized a 44-kDa protein produced as a monomer containing free sulfhydryl groups. The monomer, however, was able to form disulfide-linked homo-oligomer upon oxidation. In COS-7 cells transiently transfected with two human FCRL isoforms, both resided intracellularly. Immunohistochemical staining of tonsil sections demonstrated the FCRL expression in germinal centers, suggesting that the protein may be implicated in germinal center-specific stages of B cell development. The phylogenetic analysis of the FCRL relationships with the leukocyte FcR supports a view that the three-domain structure was primordial in the evolution of the family.

Applications of novel affinity cassette methods: use of peptide fusion handles for the purification of recombinant proteins

In this article, recent progress related to the use of different types of polypeptide fusion handles or 'tags' for the purification of recombinant proteins are critically discussed. In addition, novel aspects of the molecular cassette concept are elaborated, together with areas of potential application of these fundamental principles in molecular recognition. As evident from this review, the use of these concepts provides a powerful strategy for the high throughput isolation and purification of recombinant proteins and their derived domains, generated from functional genomic or zeomic studies, as part of the bioprocess technology leading to their commercial development, and in the study of molecular recognition phenomena per se. In addition, similar concepts can be exploited for high sensitivity analysis and detection, for the characterisation of protein bait/prey interactions at the molecular level, and for the immobilisation and directed orientation of proteins for use as biocatalysts/biosensors.

In vivo and in vitro lipidation of recombinant immunogens for direct iscom incorporation

We have previously reported strategies for Escherichia coli production of recombinant immunogens fused to hydrophobic tags to improve their capacity to be incorporated into an adjuvant formulation (J. Immunol. Methods 222 (1999) 171; 238 (2000) 181). Here, we have explored the possibility to use in vivo or in vitro lipidation of recombinant immunogens as means to achieve iscom incorporation through hydrophobic interaction. For the in vivo lipidation strategy, a general expression vector was constructed encoding a composite tag consisting of a sequence (lpp) of the major lipoprotein of E. coli, fused to a dual affinity fusion tag to allow efficient recovery by affinity chromatography. Upon expression in E. coli, fatty acids would be linked to the produced gene products. To achieve in vitro lipidation, the target immunogen would be expressed in frame with an N-terminal His6-ABP affinity tag, in which the hexahistidyl tag was utilized to obtain lipidation via a Cu2+-chelating lipid. A 238 amino acid segment DeltaSAG1, from the central region of the major surface antigen SAG1 of Toxoplasma gondii, served as model immunogen in this study. The two generated fusion proteins, lpp-His6-ABP-DeltaSAG1 and His6-ABP-DeltaSAG1, both expressed at high levels (approximately 5 and 100 mg/l, respectively), could be recovered to high purity by ABP-mediated affinity chromatography, and were evaluated in iscom-incorporation experiments. The His6-ABP-DeltaSAG1 fusion protein was associated to iscom matrix with pre-incorporated chelating lipid. Both fusion proteins were found in the iscom fractions after analytical ultracentrifugation in a sucrose gradient, indicating successful iscom incorporation/association. Iscom formation was further supported by electron microscopy analysis. In addition, these iscom preparations were demonstrated to induce high-titer antigen-specific antibody responses upon immunization of mice. For this particular target immunogen, DeltaSAG1, the induced antibodies demonstrated poor reactivity to the native antigen, although slightly better for the preparation employing the in vitro lipidation strategy, indicating that DeltaSAG1 was suboptimally folded or presented. Nevertheless, we believe that the presented strategies offer convenient alternative ways to achieve efficient adjuvant incorporation for recombinant immunogens.

Coupling of antibodies via protein Z on modified polyoma virus-like particles

Therapeutic application of virus-based delivery systems often implies a change of the tropism of these vectors. This can be achieved by insertion of polypeptides (e.g., antibody fragments) in viral coat proteins. Such fusion proteins have only been used in viral vectors so far and, as part of a virus, they have not been available for a detailed biophysical characterization. We analyzed a fusion protein called VP1-Z, which is based on the polyoma virus coat protein VP1 and protein Z. Protein Z is an engineered antibody-binding domain derived from protein A from Staphylococcus aureus. The fusion VP1-Z was constructed by insertion of protein Z in the HI-loop of VP1. As wild-type VP1, VP1-Z formed pentameric capsomers and assembled to VLPs in vitro. The stability of these particles was very similar compared to that of VLPs of wild-type VP1. Protein Z was fully structured in the fusion protein and was still capable of binding antibodies on the surface of VLPs of VP1-Z. Using this fusion protein, we could change the tropism of polyoma VLPs toward cells presenting on their surface the antigen of the coupled antibody.

The spatial and temporal expression of Tekt1, a mouse tektin C homologue, during spermatogenesis suggest that it is involved in the development of the sperm tail basal body and axoneme

Tektins comprise a family of filament-forming proteins that are known to be coassembled with tubulins to form ciliary and flagellar microtubules. Recently we described the sequence of the first mammalian tektin protein, Tekt1 (from mouse testis), which is most homologous with sea urchin tektin C. We have now investigated the temporal and spatial expression of Tekt1 during mouse male germ cell development. By in situ hybridization analysis TEKT1 RNA expression is detected in spermatocytes and in round spermatids in the mouse testis. Immunofluorescence microscopy analysis with anti-Tekt1 antibodies showed no distinct labeling of any subcellular structure in spermatocytes, whereas in round spermatids anti-Tekt1 antibodies co-localize with anti-ANA antibodies to the centrosome. At a later stage, elongating spermatids display a larger area of anti-Tektl staining at their caudal ends; as spermiogenesis proceeds, the anti-Tekt1 staining disappears. Together with other evidence, these results provide the first intraspecies evidence that Tekt1 is transiently associated with the centrosome, and indicates that Tekt1 is one of several tektins to participate in the nucleation of the flagellar axoneme of mature spermatozoa, perhaps being required to assemble the basal body.

Improved systems for hydrophobic tagging of recombinant immunogens for efficient iscom incorporation

We have previously reported a strategy for production in Escherichia coli of recombinant immunogens fused to a hydrophobic tag to improve their capacity to associate with an adjuvant formulation [Andersson et al., J. Immunol. Methods 222 (1999) 171]. Here, we describe a further development of the previous strategy and present significant improvements. In the novel system, the target immunogen is produced with an N-terminal affinity tag suitable for affinity purification, and a C-terminal hydrophobic tag, which should enable association through hydrophobic interactions of the immunogen with an adjuvant system, here being immunostimulating complexes (iscoms). Two different hydrophobic tags were evaluated: (i) a tag denoted M, derived from the membrane-spanning region of Staphylococcus aureus protein A (SpA), and (ii) a tag denoted MI consisting of the transmembrane region of hemagglutinin from influenza A virus. Furthermore, two alternative affinity tags were evaluated; the serum albumin-binding protein ABP, derived from streptococcal protein G, and the divalent IgG-binding ZZ-domains derived from SpA. A malaria peptide M5, derived from the central repeat region of the Plasmodium falciparum blood-stage antigen Pf155/RESA, served as model immunogen in this study. Four different fusion proteins, ABP-M5-M, ABP-M5-MI, ZZ-M5-M and ZZ-M5-MI, were thus produced, affinity purified and evaluated in iscom-incorporation experiments. All of the fusion proteins were found in the iscom fractions in analytical ultracentrifugation, indicating iscom incorporation. This was further supported by electron microscopy analysis showing that iscoms were formed. In addition, these iscom preparations were demonstrated to induce M5-specific antibody responses upon immunisation of mice, confirming the successful incorporation into iscoms. The novel system for hydrophobic tagging of immunogens, with optional affinity and hydrophobic tags, gave expression levels that were increased ten to fifty-fold, as compared to the earlier reported system. We believe that the presented strategy would be a convenient way to achieve efficient adjuvant association for recombinant immunogens.

Genetic immunization for the recovery and purification of recombinant proteins

A system that uses genetic immunization for recombinant protein recovery and purification is described. The genetic sequence encoding a target protein is subcloned into both a eukaryotic and a prokaryotic vector. With the eukaryotic construct, a rabbit is genetically immunized and specific polyclonal antibodies to the encoded protein raised. The prokaryotic construct is used for bacterial transformation and expression of recombinant protein. Recovery and purification of target recombinant protein are obtained by passing the lysate of expressing bacteria through an immunoaffinity column prepared with the polyclonal antibodies raised in the genetically immunized animal. This method allows purification of recombinant protein without fusion tails and can be applied to purify any protein whose encoding genetic sequence is known.

Enhancement of antibody responses by DNA immunization using expression vectors mediating efficient antigen secretion

The immune responses elicited in mice, after intradermal (i.d.) immunization with plasmids encoding secreted or intracellular forms of HIV-1 nef, HIV-1 tat or C. pneumoniae omp2 proteins, respectively, were compared. To mediate secretion of these proteins the genes were fused to a heterologous signal sequence from murine heavy chain IgG. The nef- and omp2-specific antibody responses were dramatically increased when mice were inoculated with the plasmid encoding the secreted form of these proteins. In contrast, HIV-1 tat comprising an internal strong nuclear targeting sequence could not be induced to secretion and subsequently no enhanced antibody response was observed. Slight improvement of the HIV-1 nef antibody response was achieved after co-inoculation with a granulocyte-macrophage colony-stimulating factor (GM-CSF) expression vector. Further, nef-specific T-cell responses were induced after nef DNA injections, and were of Th1-like phenotype regardless of whether the nef protein was secreted or not. The system described in this study, using a plasmid vector with a strong heterologous signal sequence that mediate efficient antigen secretion in vivo, may have wide applicability for the induction of high antibody levels to normally non-secreted antigens.

Characterization of a novel nucleolar protein that transiently associates with the condensed chromosomes in mitotic cells

We report the isolation and characterization of a murine gene encoding a conserved mammalian nucleolar protein. The protein, called Tsg118, has a predicted molecular mass of 59.4 kDa and a high content of basic amino acids. A homologous human gene was localized to chromosome 16p12.3. The Tsg118 protein is predominantly expressed in proliferating somatic cells and in male germ cells. Indirect immunofluorescence microscopy analysis using an affinity-purified anti-Tsg118 serum shows colocalization of Tsg118 and a known nucleolar protein, fibrillarin, to the dense fibrillar component of the nucleolus. The nucleolar localization of the Tsg118 protein appears to be temporally restricted to the interphase stages of the somatic cell cycle and to the meiotic phase of spermatogenesis. We find that the Tsg118 protein localizes to the nucleolus in both proliferating and serum-starved cells. Interestingly, as the nucleolar signal disappears in mitotic cells, the Tsg118 protein instead becomes associated with the surface of the condensed chromosomes.

Solid-phase differential display and bacterial expression systems in selection and functional analysis of cDNAs

Differential gene expression can be expected during activation and differentiation of cells as well as during pathological conditions, such as cancer. A number of strategies have been described to identify and understand isolated differentially expressed genes. The differential display methodology has rapidly become a widely used technique to identify differentially expressed mRNAs. In this chapter we described a variant of the differential display method based on solid-phase technology. The solid-phase procedure offers an attractive alternative to solution-based differential display because minute amounts of sample can be analyzed in considerably less time than previously. The employed solid support, monodisperse super paramagnetic beads, which circumvents precipitation and centrifugations steps, has also allowed for optimization of the critical enzymatic and preparative steps in the differential display methodology. We also described how bacterial expression can be used as a means to elucidate gene function. An efficient dual-expression system was presented, together with a basic concept describing how parallel expression of selected portions of cDNAs can be used for production of cDNA-encoded proteins as parts of affinity-tagged fusion proteins. The fusion proteins are suitable both for the generation of antibodies reactive to the target cDNA-encoded protein and for the subsequent affinity enrichment of such antibodies. Affinity-enriched antibodies have proved to be valuable tools in various assays, including immunoblotting and immunocytochemical staining, and can thus be used to localize the target cDNA-encoded protein to certain cells in a tissue section or even to a specific cell compartment or organelle within a cell. High-resolution localization of a cDNA-encoded protein would provide valuable information toward the understanding of protein function.

General expression vectors for production of hydrophobically tagged immunogens for direct iscom incorporation

A new general strategy for the production of recombinant protein immunogens has been investigated. The rationale involves the production of a recombinant immunogen as fused to a composite tag comprising one domain suitable for affinity purification and a hydrophobic tag designed for direct incorporation through hydrophobic interaction of the affinity-purified immunogen into an adjuvant system, in this case immunostimulating complexes (iscoms). Three different hydrophobic tags were evaluated: (i) a tag denoted IW containing stretches of hydrophobic isoleucine (I) and tryptophan (W) residues; (ii) a tag denoted MI consisting of the transmembrane region of hemagglutinin from influenza A virus; and (iii) a tag denoted PD designed to be pH-dependent in such a way that an amphiphatic alpha-helix would be formed at low pH. As an affinity tag, an IgG-binding domain Z derived from Staphylococcus aureus protein A (SpA) was used, and a malaria peptide M5, derived from the central repeat region of the Plasmodium falciparum blood-stage antigen Pf155/RESA, served as a model immunogen in this study. Three different fusion proteins, IW-Z-M5, MI-Z-M5 and PD-Z-M5, were produced in Escherichia coli, and after affinity purification these were evaluated in iscom-incorporation experiments. Two of the fusion proteins, IW-Z-M5 and MI-Z-M5 were found in the iscom fraction following preparative ultracentrifugation, indicating iscom incorporation. This was further supported by electron microscopy analysis showing that iscoms were formed. Furthermore, these iscom preparations were demonstrated to induce efficient M5-specific antibody responses upon immunization of mice, confirming successful incorporation into iscoms. The implications of these results for the design and production of subunit vaccines are discussed.

High-level production and isotope labeling of snake neurotoxins, disulfide-rich proteins

The aim of this work was to produce and to label snake neurotoxins, disulfide-rich proteins. A mutant of a snake toxin, erabutoxin a, was used as a model. Its N-terminal part was fused to ZZ, a synthetic IgG-binding domain of protein A (B. Nilsson et al., 1987, Protein Eng. 1, 107-113), thus preventing degradation in the bacterial cytoplasm and providing a simple affinity-purification method on IgG Sepharose. A soluble fusion protein was obtained with a yield of 60 mg/L, corresponding to 20 mg/L toxin. The toxin moiety was folded on the column while the hybrid was still bound. The oxidoreducing conditions for the refolding were optimized and were found to be oxidative but with a need for reducing molecules. The concentration of the hybrid bound to the column could be increased up to 3.3 mg/ml without significantly altering the folding process. CNBr cleavage of the fusion protein followed by a purification step yielded about 2 mg of biologically active toxin mutant per gram of dry cell weight. This procedure was applied to produce 55 mg of a toxin uniformly labeled with 15N.

Multiple affinity domains for the detection, purification and immobilization of recombinant proteins

Affinity systems based on specific molecular recognition are valuable tools for detection, purification and immobilization of recombinant proteins. Here, novel multipartite affinity fusion vectors were assembled and investigated to allow flexible binding and elution conditions. The rationale for the assembly of different combinations of affinity domains was to take advantage of the wide variety of molecular interactions of these domains for purification, solubilization, detection and immobilization. In total, seven different affinity tags representing five different types of tag-ligand interactions were studied: (i) monoclonal antibodies-peptides (T7-tag and FLAG peptide); (ii) streptavidin-peptide (Strep-tag); (iii) hexahistidyl-metal ions (His6-tag; (iv) bacterial receptors-serum proteins (staphyloccal protein A-Fc and streptococcal protein G-serum albumin); (v) streptavidin-biotin (in vivo biotinylated peptide). Selected tags were evaluated for the production and purification of Escherichia coli DNA polymerase I (Klenow fragment). On the basis of the results, a vector (pAff2c) was assembled using a novel combination of affinity domains: (i) an in vivo biotinylated peptide; (ii) a His6 sequence, and (iii) a highly soluble serum albumin binding region. Using these three affinities, a wide variety of conditions can be employed for both the binding and the elution steps.