Efficient secretory production of proline/alanine/serine (PAS) biopolymers in Corynebacterium glutamicum yielding a monodisperse biological alternative to polyethylene glycol (PEG).

BACKGROUND: PAS biopolymers are recombinant polypeptides comprising the small uncharged L-amino acids Pro, Ala and/or Ser which resemble the widely used poly-ethylene glycol (PEG) in terms of pronounced hydrophilicity. Likewise, their random chain behaviour in physiological solution results in a strongly expanded hydrodynamic volume. Thus, apart from their use as fusion partner for biopharmaceuticals to achieve prolonged half-life in vivo, PAS biopolymers appear attractive as substitute for PEG-or other poorly degradable chemical polymers-in many areas. As a prerequisite for the wide application of PAS biopolymers at affordable cost, we have established their highly efficient biotechnological production in Corynebacterium glutamicum serving as a well characterized bacterial host organism. RESULTS: Using the CspA signal sequence, we have secreted two representative PAS biopolymers as polypeptides with ~ 600 and ~ 1200 amino acid residues, respectively. Both PAS biopolymers were purified from the culture supernatant by means of a simple downstream process in a truly monodisperse state as evidenced by ESI-MS. Yields after purification were up to ≥ 4 g per liter culture, with potential for further increase by strain optimization as well as fermentation and bioprocess development. Beyond direct application as hydrocolloids or to exploit their rheological properties, such PAS biopolymers are suitable for site-specific chemical conjugation with pharmacologically active molecules via their unique terminal amino or carboxyl groups. To enable the specific activation of the carboxylate, without interference by the free amino group, we generated a blocked N-terminus for the PAS(1200) polypeptide simply by introducing an N-terminal Gln residue which, after processing of the signal peptide, was cyclised to a chemically inert pyroglutamyl group upon acid treatment. The fact that PAS biopolymers are genetically encoded offers further conjugation strategies via incorporation of amino acids with reactive side chains (e.g., Cys, Lys, Glu/Asp) at defined positions. CONCLUSIONS: Our new PAS expression platform using Corynex® technology opens the way to applications of PASylation® technology in multiple areas such as the pharmaceutical industry, cosmetics and food technology.

Structure-guided engineering of Anticalins with improved binding behavior and biochemical characteristics for application in radio-immuno imaging and/or therapy.

Modern strategies in radio-immuno therapy and in vivo imaging require robust, small, and specific ligand-binding proteins. In this context we have previously developed artificial lipocalins, so-called Anticalins, with high binding activity toward rare-earth metal-chelate complexes using combinatorial protein design. Here we describe further improvement of the Anticalin C26 via in vitro affinity maturation to yield CL31, which has a fourfold slower dissociation half-life above 2h. Also, we present the crystallographic analyses of both the initial and the improved Anticalin, providing insight into the molecular mechanism of chelated metal binding and the role of amino acid substitutions during the step-wise affinity maturation. Notably, one of the four structurally variable loops that form the ligand pocket in the lipocalin scaffold undergoes a significant conformational change from C26 to CL31, acting as a lid that closes over the accommodated metal-chelate ligand. A systematic mutational study indicated that further improvement of ligand affinity is difficult to achieve while providing clues on the contribution of relevant side chains in the engineered binding pocket. Unexpectedly, some of the amino acid replacements led to strong increases - more then 10-fold - in the yield of soluble protein from periplasmic secretion in Escherichia coli.

An engineered lipocalin specific for CTLA-4 reveals a combining site with structural and conformational features similar to antibodies.

Biomolecular reagents that enable the specific molecular recognition of proteins play a crucial role in basic research as well as medicine. Up to now, antibodies (immunoglobulins) have been widely used for this purpose. Their predominant feature is the vast repertoire of antigen-binding sites that arise from a set of 6 hypervariable loops. However, antibodies suffer from practical disadvantages because of their complicated architecture, large size, and multiple functions. The lipocalins, on the other hand, have evolved as a protein family that primarily serves for the binding of small molecules. Here, we show that an engineered lipocalin, derived from human Lcn2, can specifically bind the T cell coreceptor CTLA-4 as a prescribed protein target with subnanomolar affinity. Crystallographic analysis reveals that its reshaped cup-like binding site, which is formed by 4 variable loops, provides perfect structural complementarity with this "antigen." Furthermore, comparison with the crystal structure of the uncomplexed engineered lipocalin indicates a pronounced induced-fit mechanism, a phenomenon so far considered typical for antibodies. By recognizing the same epitope on CTLA-4 that interacts with the counterreceptors B7.1/B7.2 on antigen-presenting cells the engineered Lcn2 exhibits strong, cross-species antagonistic activity, as evidenced by biological effects comparable with a CTLA-4-specific antibody. With its proven stimulatory activity on T cells in vivo, the CTLA-4 blocking lipocalin offers potential for immunotherapy of cancer and infectious disease. Beyond that, lipocalins with engineered antigen-binding sites, so-called Anticalins, provide a class of small ( approximately 180 residues), structurally simple, and robust binding proteins with applications in the life sciences in general.

Structural plasticity in the loop region of engineered lipocalins with novel ligand specificities, so-called Anticalins.

Anticalins are generated via combinatorial protein design on the basis of the lipocalin protein scaffold and constitute a novel class of small and robust engineered binding proteins that offer prospects for applications in medical therapy as well as in vivo diagnostics as an alternative to antibodies. The lipocalins are natural binding proteins with diverse ligand specificities which share a simple architecture with a central eight-stranded antiparallel ß-barrel and an α-helix attached to its side. At the open end of the ß-barrel, four structurally variable loops connect the ß-strands in a pair-wise manner and, together, shape the ligand pocket. Using targeted random mutagenesis in combination with molecular selection techniques, this loop region can be reshaped to generate pockets for the tight binding of various ligands ranging from small molecules over peptides to proteins. While such Anticalin proteins can be derived from different natural lipocalins, the human lipocalin 2 (Lcn2) scaffold proved particularly successful for the design of binding proteins with novel specificities and, over the years, more than 20 crystal structures of Lcn2-based Anticalins have been elucidated. In this graphical structural biology review we illustrate the conformational variability that emerged in the loop region of these functionally diverse artificial binding proteins in comparison with the natural scaffold. Our present analysis provides picturesque evidence of the high structural plasticity around the binding site of the lipocalins which explains the proven tolerance toward excessive mutagenesis, thus demonstrating remarkable resemblance to the complementarity-determining region of antibodies (immunoglobulins).

Molecular recognition of structurally disordered Pro/Ala-rich sequences (PAS) by antibodies involves an Ala residue at the hot spot of the epitope.

Pro/Ala-rich sequences (PAS) are polypeptides that were developed as a biological alternative to poly-ethylene glycol (PEG) to generate biopharmaceuticals with extended plasma half-life. Like PEG, PAS polypeptides are conformationally disordered and show high solubility in water. Devoid of any charged or prominent hydrophobic side chains, these biosynthetic polymers represent an extreme case of intrinsically disordered proteins. Despite lack of immunogenicity of PAS tags in numerous animal studies we now succeeded in generating monoclonal antibodies (MAbs) against three different PAS versions. To this end, mice were immunized with a PAS#1, P/A#1 or APSA 40mer peptide conjugated to keyhole limpet hemocyanin as highly immunogenic carrier protein. In each case, one MAb with high binding activity and specificity towards a particular PAS motif was obtained. The apparent affinity was strongly dependent on the avidity effect and most pronounced for the bivalent MAb when interacting with a long PAS repeat. X-ray structural analysis of four representative anti-PAS Fab fragments in complex with their cognate PAS epitope peptides revealed interactions dominated by hydrogen bond networks involving the peptide backbone as well as multiple Van der Waals contacts arising from intimate shape complementarity. Surprisingly, Ala, the L-amino acid with the smallest side chain, emerged as a crucial feature for epitope recognition, contributing specific contacts at the center of the paratope in several anti-PAS complexes. Apart from these insights into how antibodies can recognize feature-less peptides without secondary structure, the MAbs characterized in this study offer valuable reagents for the preclinical and clinical development of PASylated biologics.

Assembly of a functional immunoglobulin Fv fragment in Escherichia coli.

An expression system was developed that allows the production of a completely functional antigen-binding fragment of an antibody in Escherichia coli. The variable domains of the phosphorylcholine-binding antibody McPC603 were secreted together into the periplasmic space, where protein folding as well as heterodimer association occurred correctly. Thus, the assembly pathway for the Fv fragment in E. coli is similar to that of a whole antibody in the eukaryotic cell. The Fv fragment of McPC603 was purified to homogeneity with an antigen-affinity column in a single step. The correct processing of both signal sequences was confirmed by amino-terminal protein sequencing. The functionality of the recombinant Fv fragment was demonstrated by equilibrium dialysis. These experiments showed that the affinity constant of the Fv fragment is identical to that of the native antibody McPC603, that there is one binding site for phosphorylcholine in the Fv fragment, and that there is no inactive protein in the preparation. This expression system should facilitate future protein engineering experiments on antibodies.

Anti-fibrotic Effects of CXCR4-Targeting i-body AD-114 in Preclinical Models of Pulmonary Fibrosis.

Idiopathic pulmonary fibrosis (IPF) is a chronic fibrotic lung disease that is prevalent in individuals >50 years of age, with a median survival of 3-5 years and limited therapeutic options. The disease is characterized by collagen deposition and remodeling of the lung parenchyma in a process that is thought to be driven by collagen-expressing immune and structural cells. The G-protein coupled C-X-C chemokine receptor 4, CXCR4, is a candidate therapeutic target for IPF owing to its role in the recruitment of CXCR4+ fibrocytes from the bone marrow to fibrotic lung tissue and its increased expression levels by structural cells in fibrotic lung tissue. We have engineered a novel fully human single domain antibody "i-body" called AD-114 that binds with high affinity to human CXCR4. We demonstrate here that AD-114 inhibits invasive wound healing and collagen 1 secretion by human IPF fibroblasts but not non-diseased control lung fibroblasts. Furthermore, in a murine bleomycin model of pulmonary fibrosis, AD-114 reduced the accumulation of fibrocytes (CXCR4+/Col1+/CD45+) in fibrotic murine lungs and ameliorated the degree of lung injury. Collectively, these studies demonstrate that AD-114 holds promise as a new biological therapeutic for the treatment of IPF.

Bacterial expression of immunoglobulin fragments.

The expression of Ig fragments in Escherichia coli permits rapid access to engineered molecules with antigen-binding properties. While the expression in a functional state by secretion to the periplasm is the standard method for the production of Fv and Fab fragments, single chain Fv fragments are mainly produced by refolding from insoluble aggregates. Although all of these Ig fragments serve as valuable aids in the study of antigen binding, their different biochemical properties must be considered when using them as research tools or for medical applications. In addition to these simple univalent antibody fragments, the bacterial expression of bivalent and bispecific versions and of hybrid proteins with novel effector functions is gaining increasing importance.

Regeneration of lesioned corticospinal tract fibers in the adult rat induced by a recombinant, humanized IN-1 antibody fragment.

Axons in the CNS of higher vertebrates generally fail to regenerate after injury. This lack of regeneration is crucially influenced by neurite growth inhibitory protein constituents of CNS myelin. We have shown previously that a monoclonal antibody (mAb IN-1) capable of binding and neutralizing Nogo-A, a myelin-associated inhibitor of neurite growth, can induce long-distance axonal regeneration and increased structural plasticity with improved functional recovery in rat models of CNS injury. In this paper we demonstrate that a partially humanized, recombinant Fab fragment (rIN-1 Fab) derived from the original mAb IN-1, was able to promote long-distance regeneration of injured axons in the spinal cord of adult rats. When infused into a spinal cord injury site, regrowth of corticospinal fibers in 11 of 18 animals was observed after a survival time of 2 weeks. Regenerating fibers grew for >9 mm beyond the lesion site and arborized profusely in the distal cord. Regenerated fibers formed terminal arbors with varicosities in the spinal cord gray matter, strongly resembling synaptic points of contact to neurons in the spinal cord distal to the lesion. In animals that had received a bovine serum albumin solution or a recombinant IN-1 fragment that had been mutated in the antigen binding site (mutIN-1 Fab), no significant growth beyond normal lesion-induced sprouting was observed. Neutralization of endogenous nerve growth inhibitors represents a novel use of recombinant antibody technology with potential therapeutic applications after traumatic CNS lesions.

Application of neutralizing antibodies against NI-35/250 myelin-associated neurite growth inhibitory proteins to the adult rat cerebellum induces sprouting of uninjured purkinje cell axons.

The myelin-associated proteins NI-35/250 exert a powerful inhibition on axon regeneration, but their function exerted on intact neurons is still unclear. In the adult CNS these proteins are thought to regulate axon growth processes to confine plasticity within restricted regions and to prevent the formation of aberrant connections. We have recently shown that application of neutralizing IN-1 antibody Fab fragment against NI-35/250 proteins to the adult cerebellum induces the expression of injury/growth-associated markers in intact Purkinje cells. Here, we asked whether these cellular modifications are accompanied by growth phenomena of Purkinje neurites. A single intraparenchymal application of IN-1 Fab fragment to the adult cerebellum induces a profuse sprouting of Purkinje axons along their intracortical course. The newly formed processes spread to cover most of the granular layer depth. A significant axon outgrowth is evident 2 d after injection; it tends to increase at 5 and 7 d, but it is almost completely reversed after 1 month. No axonal modifications occur in control Fab-treated cerebella. The IN-1 Fab fragment-induced cellular changes and axon remodeling are essentially reproduced by applying affinity-purified antibody 472 raised against a peptide sequence of the recombinant protein NI-220, thus confirming the specificity of the applied treatments on these myelin-associated molecules. Functional neutralization of NI-35/250 proteins induces outgrowth from uninjured Purkinje neurites in the adult cerebellum. Together with previous observations, this suggests that these molecules regulate axonal plasticity to maintain the proper targeting of terminal arbors within specific gray matter regions.

Lipocalins as a scaffold.

The concept of scaffolds that can be equipped with artificial biochemically active sites has gained recent interest in the field of protein design. Members of the lipocalin protein family represent promising model systems in this respect. Especially prototypic lipocalins, such as the retinol-binding protein or the bilin-binding protein (BBP), exhibit a structurally simple one-domain fold with a conformationally well conserved beta-barrel as their central motif. This type of supersecondary structure is made of a cylindrically closed beta-sheet of eight antiparallel strands. At the open end of the barrel the beta-strands are connected by four loops in a pairwise manner so that a pocket for the ligand is formed. In a rational protein design study a metal-binding site was functionally grafted on the solvent-exposed surface of the beta-barrel, whereby the rigid backbone conformation permitted the spatially defined arrangement of three His side chains. In a combinatorial protein design approach, the natural ligand pocket of a lipocalin was reshaped. In this manner variants of the BBP were engineered which exhibit high affinity and remarkable specificity for haptens like fluorescein and digoxigenin. The so-called 'anticalins', i.e. artificial lipocalins recognizing prescribed ligands, could provide an interesting alternative to recombinant antibody fragments. Consequently, the use of lipocalins as a scaffold opens new applications for members of this functionally diverse protein family in biotechnology and medicine.

The rational construction of an antibody against cystatin: lessons from the crystal structure of an artificial Fab fragment.

In a protein design study the artificial antibody M41 was modelled with its binding site complementary to the protease inhibitor cystatin, which was chosen as a structurally well-characterized "antigen". The modelling of M41 took advantage of the crystal structure of the anti-lysozyme antibody HyHEL-10 as a structural template. Its combining site was reshaped by replacing 19 amino acid side-chains in the hypervariable loops. In addition, ten amino acid residues were substituted in the framework regions. The crystal structure of the corresponding antibody model M41, which was produced as an F(ab) fragment in Escherichia coli, was determined at a resolution of 1.95 A. The crystals exhibited symmetry of the space group P2(1)2(1)2(1) (a = 96.5 A; b = 103.5 A; c = 113.6 A) with two F(ab) fragments in the asymmetric unit, which were independently refined (final R-factor 21.7%). The resulting coordinates were used for a detailed comparison with the modelled protein structure. It was found that the mutual arrangement of the six complementarity-determining regions as well as most of their backbone conformation had been correctly predicted. One major difference that was detected for the conformation of a five residue insertion in complementarity-determining region L1 could be explained by an erroneously defined segment in the structure of the antibody 4-4-20, which had been used as a template for this loop. In the light of more recent crystallographic data it appears that this segment adopts a new canonical structure. Apart from this region, most of the side-chains in the antigen-binding site had been properly placed in the M41 model. There was however one important exception concerning Trp H98, whose side-chain conformation had been kept as it appeared in HyHEL-10. The differing orientation of this residue in the model compared with the crystal structure of the artificial F(ab) fragment M41 explains why an antigen affinity could not be detected so far. The detailed analysis of this and other, more subtle deviations suggests how to make this F(ab) fragment function by introducing a few additional amino acid changes into M41.

Functional expression of the uncomplexed serum retinol-binding protein in Escherichia coli. Ligand binding and reversible unfolding characteristics.

The serum retinol-binding protein solubilizes the lipophilic vitamin A alcohol and plays an important physiological role in the transport of this compound. The monomeric single-domain protein, the three-dimensional structure of which is known, constitutes a well-characterized member of the lipocalin family of proteins. We report here the functional expression of the apo-protein in Escherichia coli by secretion to the periplasm. The recombinant protein, purified in a single step by metal chelate affinity chromatography, exhibits the same ligand binding characteristics as described for the natural protein. Guanidinium chloride-induced unfolding and refolding experiments suggest that the recombinant retinol-binding protein adopts a stable conformation despite being expressed and purified in the absence of the large hydrophobic ligand. The expression system described here should also be useful for the recombinant production of other lipocalin proteins, thus permitting the elucidation of the structure-function relationships of ligand binding by protein engineering.

A novel type of receptor protein, based on the lipocalin scaffold, with specificity for digoxigenin.

We demonstrate that the bilin-binding protein, a member of the lipocalin family of proteins, can be structurally reshaped in order to specifically complex digoxigenin, a steroid ligand commonly used for the non-radioactive labelling of biomolecules. 16 amino acid residues, distributed across the four loops which form the binding site of the bilin-binding protein, were subjected to targeted random mutagenesis. From the resulting library the variant DigA16 was obtained by combined use of phage display and a filter-sandwich colony screening assay, followed by in vitro affinity maturation. DigA16 possesses strong binding activity and high specificity for the digoxigenin group, with a K(D) of 30.2(+/-3.6) nM. The derivative compound digitoxigenin is bound even more tightly, with a K(D) of 2.0(+/-0.52) nM, whereas the steroid glycoside ouabain is not recognized at all. Fusion proteins between DigA16 and alkaline phosphatase were constructed and shown to retain both the digoxigenin-binding function and enzymatic activity, irrespective of whether the enzyme was fused to the N or the C terminus of the bilin-binding protein variant. Our findings suggest that the lipocalin scaffold can be generally employed for the construction of specific receptor proteins, so-called "anticalins", which provide a promising alternative to recombinant antibody fragments.

The de novo design of an antibody combining site. Crystallographic analysis of the VL domain confirms the structural model.

In a protein design approach the molecular model of an artificial antibody Fv fragment was generated with predicted complementarity to part of the known crystal structure of chicken egg-white cystatin. The model of the Fv fragment was based on the three-dimensional structure of the anti-lysozyme antibody HyHEL-10, which was modified by substituting amino acid side-chains in the complementarity-determining regions (CDRs) as well as the framework without altering the backbone. In the course of crystallization experiments with the bacterially produced Fv fragment crystals of the VL domain alone were obtained. These crystals diffracted X-rays to a resolution of 2.17 A and were shown to belong to the space group P2(1)2(1)2(1) with unit cell dimensions a = 46.89 A, b = 58.05 A, c = 83.22 A containing two VL monomers in the asymmetric unit. The crystal structure was solved by molecular replacement and refined to a crystallographic R-factor of 17.5%. The two VL monomers exhibit an asymmetric mode of association, which is different from other crystallized VL domains described before and shows the peculiar feature of an isopropanol precipitant molecule buried at the interface. Both VL structures reveal a high level of similarity to the predicted three-dimensional model. With the exception of two loop segments in the framework region that are involved in crystal packing contacts, the backbone structures of the two VL monomers in the crystal and the molecular model of the VL domain are practically identical. Although six amino acid residues had been replaced in the hypervariable regions, the CDR conformations remained conserved and only minor deviations in the orientation of some side-chains and peptide planes were detected. The crystallographic analysis of the VL domain modelled as part of a complex between an artificial Fv fragment and the small protein cystatin, deliberately chosen as antigen target, confirms the concept of distinct structural classes for CDR backbones and supports our strategy for the de novo design of an antibody combining site.

Molecular interaction between the Strep-tag affinity peptide and its cognate target, streptavidin.

The Strep-tag is a selected nine-amino acid peptide (AWRHPQFGG) that displays intrinsic binding affinity towards streptavidin and has been used as an affinity tag for recombinant proteins. In order to elucidate the molecular mechanism underlying this type of artificial protein-peptide recognition, X-ray crystallographic analyses and binding measurements were carried out. The crystal structure of the complex between recombinant core streptavidin and the synthesized peptide was solved and refined at 1.7 A resolution (space group I4(1)22; unit cell dimensions a = b = 58.3 A, c = 176.9 A). The Strep-tag was bound at the same surface pocket where biotin, the natural ligand of streptavidin, gets complexed. The peptide backbone exhibited 3(10)-helical conformation, with eight of the residues involved in protein contacts. The C-terminal Gly-Gly moiety of the Strep-tag participated in a salt bridge to Arg84 of streptavidin with its free carboxylate group. This finding explained why the use of the Strep-tag in fusions with recombinant proteins was restricted to their carboxyl end. Employing a synthetic peptide spot assay, the variant Strep-tag II was screened, which did not have this limitation. The isomorphous crystal structure of its complex with streptavidin revealed that a glutamate side-chain provided the salt bridge in this case, with an otherwise almost unchanged mode of binding. Affinity constants between the peptides and streptavidin were measured by isothermal titration calorimetry. A value of 2.7 x 10(4) M-1 was determined for the Strep-tag peptide, and slightly tighter binding was seen when the Strep-tag was applied as part of a bacterially produced fusion protein. This affinity is significantly higher, compared with values previously reported for shorter streptavidin-binding peptides, and agrees well with the remarkable selectivity observed in recombinant protein purification applications.

A Zn(II)-binding site engineered into retinol-binding protein exhibits metal-ion specificity and allows highly efficient affinity purification with a newly designed metal ligand.

BACKGROUND: The Zn(II)-binding site from the active center of human carbonic anhydrase II, formed by three His side chains, can be grafted onto the recombinant serum retinol-binding protein (RBP). The artificial binding site in the resulting variant RBP/H3(A) has high affinity for Zn(II) and stabilizes the protein against denaturation. RESULTS: The metal-ion specificity of the grafted Zn(II) binding site in RBP/H3(A) was investigated. Both Cu(II) and Ni(II) bound with high affinity, although the Kd values were not as low as for Zn(II) binding. Competition experiments with the chelate ligands iminodiacetic acid (IDA) and nitrilotriacetic acid (NTA) suggested that both Ni(II) and Cu(II) bound to the protein in an octahedral manner with three vacant coordination sites, as previously observed for Zn(II). A substituted pyrrolidine-dicarboxylic acid was designed as a structurally rigid IDA compound and coupled to a matrix. Using this support in an immobilized metal affinity chromatography (IMAC), RBP/H3(A) was purified from the bacterial cell extract in one step with unprecedented efficiency. CONCLUSIONS: Although the His3 metal-binding site used here had been removed from the substrate pocket of an enzyme and exposed to solvent on a protein surface, it showed clear selectivity for Zn(II) compared to Cu(II) and Ni(II). Thus the properties of this structurally defined metal-binding site (which are not shared by isolated His residues or flexible oligo-His tags) can be preserved when it is added to proteins. An IMAC matrix with improved behaviour was designed, allowing highly selective purification of RBP/H3(A) and of His6-tagged RBP as well. Such rational design of supramolecular recognition may be generally useful in the fields of protein engineering and drug design.

Glutathione S-transferase can be used as a C-terminal, enzymatically active dimerization module for a recombinant protease inhibitor, and functionally secreted into the periplasm of Escherichia coli.

Glutathione S-transferase (GST) from Schistosoma japonicum, which is widely used for the production of fusion proteins in the cytoplasm of Escherichia coli, was employed as a functional fusion module that effects dimer formation of a recombinant protein and confers enzymatic reporter activity at the same time. For this purpose GST was linked via a flexible spacer to the C-terminus of the thiol-protease inhibitor cystatin, whose binding properties for papain were to be studied. The fusion protein was secreted into the bacterial periplasm by means of the OmpA signal peptide to ensure formation of the two disulfide bonds in cystatin. The formation of wrong crosslinks in the oxidizing milieu was prevented by replacing three of the four exposed cysteine residues in GST. Using the tetracycline promoter for tightly controlled gene expression the soluble fusion protein could be isolated from the periplasmic protein fraction. Purification to homogeneity was achieved in one step by means of an affinity column with glutathione agarose. Alternatively, the protein was isolated via streptavidin affinity chromatography after the Strep-tag had been appended to its C terminus. The GST moiety of the fusion protein was enzymatically active and the kinetic parameters were determined using glutathione and 1-chloro-2,4-dinitrobenzene as substrates. Furthermore, strong binding activity for papain was detected in an ELISA. The signal with the cystatin-GST fusion protein was much higher than with cystatin itself, demonstrating an avidity effect due to the dimer formation of GST. The quaternary structure was further confirmed by chemical crosslinking, which resulted in a specific reaction product with twice the molecular size. Thus, engineered GST is suitable as a moderately sized, secretion-competent fusion partner that can confer bivalency to a protein of interest and promote detection of binding interactions even in cases of low affinity.

Use of the tetracycline promoter for the tightly regulated production of a murine antibody fragment in Escherichia coli.

A generic vector, pASK75, was developed for the synthesis of foreign proteins in Escherichia coli under transcriptional control of the tetA promoter/operator. Tight regulation was achieved by placing the structural gene for the tet repressor, as a transcriptional fusion, downstream from the beta-lactamase-encoding gene (bla) on the same plasmid. Strong expression of the foreign gene was conveniently induced by adding anhydrotetracycline at a low concentration. Using the production of a recombinant murine immunoglobulin F(ab) fragment as an example, the system was shown to function independently of the host-strain background and to be extremely well repressed in the absence of the inducer. Thus, it represents an economic and independent alternative to IPTG-inducible promoter constructs. Additional features of pASK75 include a signal sequence and a multiple cloning site followed by a region encoding the Strep tag affinity peptide to facilitate purification of a bacterially produced protein.

A general vector, pASK84, for cloning, bacterial production, and single-step purification of antibody Fab fragments.

The expression vector pASK84 was designed for the convenient cloning of immunoglobulin variable domain genes, as well as periplasmic secretion of the corresponding F(ab) fragment in Escherichia coli. The plasmid provides the constant domain genes of mouse IgG1/kappa with a hexa-histidine tag fused to the C terminus of the heavy chain. This strategy enables the rapid and efficient purification of the functional recombinant F(ab) fragment via immobilized metal affinity chromatography. The versatility of this expression and purification system is demonstrated using the variable domains of the well-characterized anti-lysozyme antibody D1.3.