Selection of TNF-alpha binding affibody molecules using a beta-lactamase protein fragment complementation assay

Protein fragment complementation assays (PCAs) based on different reporter proteins have been described as powerful tools for monitoring dynamic protein-protein interactions in living cells. The present study describes the construction of a PCA system based on genetic splitting of TEM-1 beta-lactamase for the selection of proteins specifically interacting in the periplasm of Escherichia coli bacterial cells, and its application for the selection of affibody molecules binding human tumour necrosis factor-alpha (TNF-alpha) from a combinatorial library. Vectors encoding individual members of a naïve 10(9) affibody protein library fused to a C-terminal fragment of the beta-lactamase reporter were distributed via phage infection to a culture of cells harbouring a common construct encoding a fusion protein between a non-membrane anchored version of a human TNF-alpha target and the N-terminal segment of the reporter. An initial binding analysis of 29 library variants derived from surviving colonies using selection plates containing ampicillin and in some cases also the beta-lactamase inhibitor tazobactam, indicated a stringent selection for target binding variants. Subsequent analyses showed that the binding affinities (K(D)) for three selected variants studied in more detail were in the range 14-27 nm. The selectivity in binding to TNF-alpha for these variants was further demonstrated in both a cross-target PCA-based challenge and the specific detection of a low nm concentration of TNF-alpha spiked into a complex cell lysate sample. Further, in a biosensor-based competition assay, the binding to TNF-alpha of three investigated affibody variants could be completely blocked by premixing the target with the therapeutic monoclonal antibody adalimumab (Humira), indicating overlapping epitopes between the two classes of reagents. The data indicate that beta-lactamase PCA is a promising methodology for stringent selection of binders from complex naïve libraries to yield high affinity reagents with selective target binding characteristics.

Decreased immune reactivity towards a knobless, affibody-targeted adenovirus type 5 vector

In this study, a prototype Adenovirus type 5 (Ad5) vector deleted of the fiber knob domain and carrying an Affibody molecule as the targeting ligand showed decreased susceptibility to human pre-existing antibodies. This vector, Ad5/R7-Z(taq)Z(taq), has short fibers carrying seven shaft repeats, a non-native trimerization signal and an affibody molecule (Z(taq)) reactive to Taq polymerase. Ad5/R7-Z(taq)Z(taq) could be specifically targeted to 293 cells stably expressing membrane-bound anti-Z(taq) idiotypic affibody called Z(ztaq) (293Z(ztaq)). Sera from 50 blood donors were analyzed for neutralization activity (NA) against the parental Ad5/Fiwt vector and knobless Ad5/R7-Z(taq)Z(taq) on 293Z(ztaq) cells. Twenty-three sera had NA titers (> or =1:64) against Ad5/Fiwt (46%) and only two against Ad5/R7-Z(taq)Z(taq) (4%). Characterization of sera with NA titers showed that the knob domain is one of the targets of the antibodies. Neutralization assays using sera pre-adsorbed on knob and hexon proteins showed that the NA of the sera was carried mainly by anti-knob and anti-hexon antibodies, but in certain sera the anti-hexon antibodies represent the major population of the neutralizing antibodies (NAbs). Our results suggested that a combination of knob deletion and hexon switching could be an effective strategy for Ad vectors to better evade the anti-Ad NAbs.

Tumor cell targeted gene delivery by adenovirus 5 vectors carrying knobless fibers with antibody-binding domains

Most human carcinoma cell lines lack the high-affinity receptors for adenovirus serotype 5 (Ad5) at their surface and are nonpermissive to Ad5. We therefore tested the efficiency of retargeting Ad5 to alternative cellular receptors via immunoglobulin (Ig)-binding domains inserted at the extremity of short-shafted, knobless fibers. The two recombinant Ad5's constructed, Ad5/R7-Z(wt)-Z(wt) and Ad5/R7-C2-C2, carried tandem Ig-binding domains from Staphylococcal protein A (abbreviated Z(wt)) and from Streptococcal protein G (C2), respectively. Both viruses bound their specific Ig isotypes with the expected affinity. They transduced human carcinoma cells independently of the CAR pathway, via cell surface receptors targeted by specific monoclonal antibodies, that is, EGF-R on A549, HT29 and SW1116, HER-2/neu on SK-OV-3 and SK-BR-3, CA242 (epitope recognized by the monoclonal antibody C242) antigen on HT29 and SW1116, and PSMA (prostate-specific membrane antigen) expressed on HEK-293 cells, respectively. However, Colo201 and Colo205 cells were neither transduced by targeting CA242 or EGF-R nor were LNCaP cells transduced by targeting PSMA. Our results suggested that one given surface receptor could mediate transduction of certain cells but not others, indicating that factors and steps other than cell surface expression and virus-receptor interaction are additional determinants of Ad5-mediated transduction of tumor cells. Using penton base RGD mutants, we found that one of these limiting steps was virus endocytosis.

Inhibition of the CD28-CD80 co-stimulation signal by a CD28-binding affibody ligand developed by combinatorial protein engineering

CD28 is one of the key molecules for co-stimulatory signalling in T cells. Here, novel ligands (affibodies) showing selective binding to human CD28 (hCD28) have been selected by phage display technology from a protein library constructed through combinatorial mutagenesis of a 58-residue three-helix bundle domain derived from staphylococcal protein A. Analysis of selected affibodies showed a marked sequence homology and biosensor analyses showed that all investigated affibodies bound to hCD28 with micromolar affinities (KD). No cross-reactivity towards the related protein human CTLA-4 could be observed. This lack of cross-reactivity to hCTLA-4 suggests that the recognition site on hCD28 for the affibodies resides outside the conserved MYPPPYY motif. The apparent binding affinity for hCD28 could be improved through fusion to an Fc fragment fusion partner, resulting in a divalent presentation of the affibody ligand. For the majority of selected anti-CD28 affibodies, in co-culture experiments involving Jurkat T-cells and CHO cell lines transfected to express human CD80 (hCD80) or LFA-3 (hLFA-3) on the cell surface, respectively, pre-incubation of Jurkat cells with affibodies resulted in inhibition of IL-2 production when they were co-cultured with CHO (hCD80+) cells, but not with CHO (hLFA-3+) cells. For one affibody variant denoted Z(CD28:5) a clear concentration-dependent inhibition was seen, indicating that this affibody binds hCD28 and specifically interferes in the interaction between hCD28 and hCD80.

Genetic modification of adenovirus 5 tropism by a novel class of ligands based on a three-helix bundle scaffold derived from staphylococcal protein A

The use of adenovirus (Ad) as an efficient and versatile vector for in vivo tumor therapy requires the modulation of its cellular tropism. We previously developed a method to genetically alter the tropism of Ad5 fibers by replacing the fiber knob domain by an extrinsic trimerization motif and a new cellular ligand. However, fibers carrying complex ligands such as single-chain antibody fragments did not assemble into functional pentons in vitro in the presence of penton base, and failed to be rescued into infectious virions because of their inability to fold correctly within the cytoplasm of Ad-infected cells. Here we show that the coding sequence for a disulfide bond-independent three-helix bundle scaffold Z, derived from domain B of Staphylococcal protein A and capable of binding to the Fc portion of immunoglobulin (Ig) G1, could be incorporated into modified knobless Ad fiber gene constructs with seven shaft repeats. These fiber gene constructs could be rescued into viable virions that were demonstrated to enter 293 cells engineered for IgG Fc surface expression but not unmodified 293 cells, via a mechanism that could be specifically blocked with soluble Fc target protein. However, the tropism modified viruses showed a slightly impaired cellular entry and a lower infectivity than wildtype (WT) virus. In addition, we generated recombinant fibers containing an IgA binding Affibody ligand, derived from combinatorial specificity-engineering of the Z domain scaffold. Such fiber constructs also showed the expected target specific binding, indicating that the affibody protein class is ideally suited for genetic engineering of Ad tropism.

Generation of metal-binding staphylococci through surface display of combinatorially engineered cellulose-binding domains

Ni(2+)-binding staphylococci were generated through surface display of combinatorially engineered variants of a fungal cellulose-binding domain (CBD) from Trichoderma reesei cellulase Cel7A. Novel CBD variants were generated by combinatorial protein engineering through the randomization of 11 amino acid positions, and eight potentially Ni(2+)-binding CBDs were selected by phage display technology. These new variants were subsequently genetically introduced into chimeric surface proteins for surface display on Staphylococcus carnosus cells. The expressed chimeric proteins were shown to be properly targeted to the cell wall of S. carnosus cells, since full-length proteins could be extracted and affinity purified. Surface accessibility for the chimeric proteins was demonstrated, and furthermore, the engineered CBDs, now devoid of cellulose-binding capacity, were shown to be functional with regard to metal binding, since the recombinant staphylococci had gained Ni(2+)-binding capacity. Potential environmental applications for such tailor-made metal-binding bacteria as bioadsorbents in biofilters or biosensors are discussed.

Recombinant human factor VIII-specific affinity ligands selected from phage-displayed combinatorial libraries of protein A

Factor VIII-specific affibodies were selected from phage displayed libraries constructed by combinatorial mutagenesis of an alpha helical bacterial receptor domain derived from staphylococcal protein A. Bead-immobilized recombinant human factor VIII (rVIII) (80 and 90 kDa chains) protein was used during competitive biopannings in the presence of free 80-kDa chain protein, resulting in the selection of several binders that showed dissociation constants (Kd) in the range 100-200 nM as determined by biosensor analyses. One variant (Z[rVIII:3], 90-kDa chain specific) was further characterized in small-scale affinity chromatography experiments, and showed efficient and selective recovery of biologically active rVIII from Chinese hamster ovary cell supernatant-derived feed stocks. The purity of the enriched rVIII was comparable with rVIII material purified by immunoaffinity chromatography using a 90-kDa chain-specific monoclonal antibody. Interestingly, epitope mapping showed that the monoclonal antibody and the affibody ligand competed for the same or at least overlapping epitopes on rVIII. In addition, the Z[rVIII:3] variant was produced by peptide synthesis with a C-terminal cysteine to enable directed coupling to solid supports. This 59-residue protein was analyzed by circular dichroism and showed a secondary structure content similar to that of the parental Z domain used as scaffold. In biosensor studies, the synthetic affibody was immobilized recruiting the C-terminal cysteine residue, and demonstrated to bind both recombinantly produced and plasma-derived factor VIII. From a secondary library, constructed by re-randomization of relevant positions identified after alignment of the first-generation variants, a panel of affinity-improved second-generation affibodies were selected of which one clone showed a dissociation constant (Kd) for rVIII of 5 nM. Several of these variants also showed higher apparent binding efficiencies towards rVIII when analyzed as immobilized ligands in biosensor experiments. Taken together, the results suggest that affibody ligands produced by bacterial or synthetic routes could be of interest as an alternative to monoclonal antibodies in purification processes or as diagnostic or monitoring tools.

In vitro selection of enzymatically active lipase variants from phage libraries using a mechanism-based inhibitor

The 'detergent lipase' Lipolase, from Thermomyces lanuginosa was subjected to a combinatorial protein engineering/phage display approach with the aim of identifying new enzyme variants with improved characteristics in the presence of detergents. First it was demonstrated that wild-type Lipolase could be produced in Escherichia coli retaining full activity and be displayed as an active enzyme fused to coat protein 3 on E. coli phage M13. A phagemid library designed to result in approximately two to three mutations per lipase gene was then constructed. Nine amino acids located in two regions close to the active site were targeted for randomization. Selections using a mechanism-based biotinylated inhibitor showed that phages displaying Lipolase could be specifically enriched from a population of control phages. Selections on a library phage stock in the presence of inhibitor and a commercial powder detergent resulted in a step-wise increase in the proportion of active clones. Analysis of 84 active clones revealed that they all expressed lipase activity, but with lower activities than that of a wild-type Lipolase-producing clone. In six of the seven most active clones a wild-type serine at position 83 had been replaced by threonine, a substitution known to alter the substrate chain length preference of Lipolase variants. Furthermore, the selection had enriched enzyme variants with a high degree of conservatism in one of the variegated regions, suggesting that this region is important for enzymatic activity and that the designed selection procedure was relevant. The selected variants contained primarily basic amino acid residues within the other variegated region. Taken together, the described results show that selection protocols based on enzymatic activity can be designed for this enzyme class which should be of importance for future protein engineering attempts.

Alpha-amylase inhibitors selected from a combinatorial library of a cellulose binding domain scaffold

A disulfide bridge-constrained cellulose binding domain (CBD(WT)) derived from the cellobiohydrolase Cel7A from Trichoderma reesei has been investigated for use in scaffold engineering to obtain novel binding proteins. The gene encoding the wild-type 36 aa CBD(WT) domain was first inserted into a phagemid vector and shown to be functionally displayed on M13 filamentous phage as a protein III fusion protein with retained cellulose binding activity. A combinatorial library comprising 46 million variants of the CBD domain was constructed through randomization of 11 positions located at the domain surface and distributed over three separate beta-sheets of the domain. Using the enzyme porcine alpha-amylase (PPA) as target in biopannings, two CBD variants showing selective binding to the enzyme were characterized. Reduction and iodoacetamide blocking of cysteine residues in selected CBD variants resulted in a loss of binding activity, indicating a conformation dependent binding. Interestingly, further studies showed that the selected CBD variants were capable of competing with the binding of the amylase inhibitor acarbose to the enzyme. In addition, the enzyme activity could be partially inhibited by addition of soluble protein, suggesting that the selected CBD variants bind to the active site of the enzyme.

Charge engineering of a protein domain to allow efficient ion-exchange recovery

We have created protein domains with extreme surface charge. These mutated domains allow for ion-exchange chromatography under conditions favourable for selective and efficient capture, using Escherichia coli as a host organism. The staphylococcal protein A-derived domain Z (Zwt) was used as a scaffold when constructing two mutants, Zbasic1 and Zbasic2, with high positive surface charge. Far-ultraviolet circular dichroism measurements showed that they have a secondary structure content comparable to the parental molecule Zwt. Although melting temperatures (Tm) of the engineered domains were lower than that of the wild-type Z domain, both mutants could be produced successfully as intracellular full-length products in E. coli and purified to homogeneity by ion-exchange chromatography. Further studies performed on Zbasic1 and Zbasic2 showed that they were able to bind to a cation exchanger even at pH values in the 9 to 11 range. A gene fusion between Zbasic2 and the acidic human serum albumin binding domain (ABD), derived from streptococcal protein G, was also constructed. The gene product Zbasic2-ABD could be purified using cation-exchange chromatography from a whole cell lysate to more than 90% purity.

Design and production of recombinant subunit vaccines

The development of subunit vaccines is presently the main strategy being evaluated for prevention of infectious diseases. The use of recombinant-DNA techniques has facilitated the development of new principles for design and production of subunit vaccines. First of all, the properties of a target protein immunogen can be improved by the use of gene-fusion technology or by the creation of specific changes, to generate 'second-generation protein vaccines'. Properties that can be modified include protein solubility, protein stability, in vivo half-lives, etc. In addition, for subunit protein vaccine candidates, the immunogenic properties can be significantly augmented by the addition of immunopotentiating tags or by means of targeting to immunoreactive sites. The recombinant subunit vaccine can furthermore be adapted by gene-fusion technology, to be efficiently incorporated into immunopotentiating adjuvant systems. Also in passive vaccination strategies, i.e. the use of antibodies or antibody fragments for prevention of infectious diseases, the recombinant strategies have become increasingly important. Humanized antibodies and antibody fusion proteins represent common present anti-infectious-disease agents. The selected examples will indicate that recombinant strategies will indeed have an impact on the design, selection and production of recombinant proteins to be used in the prevention of infectious diseases.

Partitioning of peptides and recombinant protein-peptide fusions in thermoseparating aqueous two-phase systems: effect of peptide primary structure

Genetic engineering has been used for fusion of peptides, with different length and composition, on a protein to study the effect on partitioning in an aqueous two-phase system. The system was composed of dextran and the thermoseparating ethylene oxide-propylene oxide random copolymer, EO30P070. Peptides containing tryptophan, proline, arginine or aspartate residues were fused at the C-terminus of the recombinant protein ZZ-cutinase. The aim was to find effective tags for the lipolytic enzyme cutinase for large-scale extraction. The target protein and peptide tags were partitioned separately and then together in the fusion proteins in order to gain increased understanding of the influence of certain amino acid residues on the partitioning. The salt K2SO4 was used to reduce the charge dependent salt effects on partitioning and to evaluate the contribution to the partition coefficient from the hydrophobic-hydrophilic properties of the amino acid residues. The effect of Trp on peptide partitioning was independent of the difference in primary structure for (Trp)n, (Trp-Pro)n, (Ala-Trp-Trp-Pro)n and was only determined by the number of Trp. The effect of the charged residues, Arg and Asp, was dependent on the surrounding residues, i.e. if they were situated next to Trp or not. The partitioning behaviour observed for the peptides was qualitatively and in some cases also quantitatively the same as for the fusion proteins. The effect of the salts sodium perchlorate and triethylammonium phosphate on the partitioning was also studied. The salt effects observed for the peptides were qualitatively similar to the effects observed for the fusion proteins.

Ligands selected from combinatorial libraries of protein A for use in affinity capture of apolipoprotein A-1M and taq DNA polymerase

Here we show that robust and small protein ligands can be used for affinity capture of recombinant proteins from crude cell lysates. Two ligands selectively binding to bacterial Taq DNA polymerase and human apolipoprotein A-1(M), respectively, were used in the study. The ligands were selected from libraries of a randomized alpha-helical bacterial receptor domain derived from staphylococcal protein A and have dissociation constants in the micromolar range, which is typical after primary selection from these libraries consisting of approximately 40 million different members each. Using these ligands in affinity chromatography, both target proteins were efficiently recovered from crude cell lysates with high selectivities. No loss of column capacity or selectivity was observed for repeated cycles of sample loading, washing and low pH elution. Interestingly, column sanitation could be performed using 0. 5 M sodium hydroxide without significant loss of ligand performance. The results suggest that combinatorial approaches using robust protein domains as scaffolds can be a general tool in the process of designing purification strategies for biomolecules.

Genetic engineering of the Fusarium solani pisi lipase cutinase for enhanced partitioning in PEG-phosphate aqueous two-phase systems

The Fusarium solani pisi lipase cutinase has been genetically engineered to investigate the influence of C-terminal peptide extensions on the partitioning of the enzyme in PEG-salt based aqueous two-phase bioseparation systems. Seven different cutinase lipase variants were constructed containing various C-terminal peptide extensions including tryptophan rich peptide tags ((WP)(2) and (WP)(4)), positively ((RP)(4)) and negatively ((DP)(4)) charged tags as well as combined tags with tryptophan together with either positively ((WPR)(4)) or negatively ((WPD)(4)) charged amino acids. The modified cutinase variants were stably produced in Escherichia coli as secreted to the periplasm from which they were efficiently purified by IgG-affinity chromatography employing an introduced N-terminal IgG-binding ZZ affinity fusion partner present in all variants. Partitioning experiments performed in a PEG 4000/sodium phosphate aqueous two-phase system showed that for variants containing either (WP)(2) or (WP)(4) peptide extensions, 10- to 70-fold increases in the partitioning to the PEG rich top-phase were obtained, when compared to the wild type enzyme. An increased partitioning was also seen for cutinase variants tagged with both tryptophans and charged amino acids, whereas the effect of solely charged peptide extensions was relatively small. In addition, when performing partitioning experiments from cell disintegrates, the (WP)(4)-tagged cutinase showed a similarly high PEG-phase partitioning, indicating that the effect from the peptide tag was unaffected by the background of the host proteins. Taken together, the results show that the partitioning of the recombinantly produced cutinase model enzyme could be significantly improved by relatively minor genetic engineering and that the effects observed for purified proteins are retained also in an authentic whole cell disintegrate system. The results presented should be of general interest also for the improvement of the partitioning properties of other industrially interesting proteins including bulk enzymes.

Display of active subtilisin 309 on phage: analysis of parameters influencing the selection of subtilisin variants with changed substrate specificity from libraries using phosphonylating inhibitors

Many attempts have been made to endow enzymes with new catalytic activities. One general strategy involves the creation of random combinatorial libraries of mutants associated with an efficient screening or selection scheme. Phage display has been shown to greatly facilitate the selection of polypeptides with desired properties by establishing a close link between the polypeptide and the gene that encodes it. Selection of phage displayed enzymes for new catalytic activities remains a challenge. The aim of this study was to display the serine protease subtilisin 309 (savinase) from Bacillus lentus on the surface of filamentous fd phage and to develop selection schemes that allow the extraction of subtilisin variants with a changed substrate specificity from libraries. Subtilisins are produced as secreted preproenzyme that mature in active enzyme autocatalytically. They have a broad substrate specificity but exhibit a significant preference for hydrophobic residues and very limited reactivity toward charged residues at the P4 site in the substrate. Here, we show that savinase can be functionally displayed on phage in the presence of the proteic inhibitor CI2. The free enzyme is released from its complex with CI2 upon addition of the anionic detergent LAS. The phage-enzyme can be panned on streptavidin beads after labelling by reaction with (biotin-N-epsilon-aminocaproyl-cystamine-N'-glutaryl)-l-Ala-l-Ala-l-P ro-Phe(P)-diphenyl ester. Reactions of libraries, in which residues 104 and 107 forming part of the S4 pocket have been randomised, with (biotin-N-epsilon-aminocaproyl-cystamine-N'-glutaryl)-alpha-l-Lys-l-A la-l-Pro-Phe(P)-diphenylester allowed us to select enzymes with increased specific activity for a substrate containing a lysine in P4. Parameters influencing the selection as for instance the efficiency of maturation of mutant enzymes in libraries have been investigated.

Integrated bioprocess for production of human proinsulin C-peptide via heat release of an intracellular heptameric fusion protein

An integrated bioprocess has been developed suitable for production of recombinant peptides using a gene multimerization strategy and site-specific cleavage of the resulting gene product. The process has been used for production in E. coli of the human proinsulin C-peptide via a fusion protein BB-C7 containing seven copies of the 31-residues C-peptide monomer. The fusion protein BB-C7 was expressed at high level, 1.8 g l(-1), as a soluble gene product in the cytoplasm. A heat treatment procedure efficiently released the BB-C7 fusion protein into the culture medium. This step also served as an initial purification step by precipitating the majority of the host cell proteins, resulting in a 70% purity of the BB-C7 fusion protein. Following cationic polyelectrolyte precipitation of the nucleic acids and anion exchange chromatography, native C-peptide monomers were obtained by enzymatic cleavage at flanking arginine residues. The released C-peptide material was further purified by reversed-phase chromatography and size exclusion chromatography. The overall yield of native C-peptide at a purity exceeding 99% was 400 mg l(-1) culture, corresponding to an overall recovery of 56%. The suitability of this process also for the production of other recombinant proteins is discussed.

Kinetic characterization of the interaction of the Z-fragment of protein A with mouse-IgG3 in a volume in chemical space

The kinetic rate parameters for the interaction between a single domain analogue of staphylococcal protein A (Z) and a mouse-IgG3 monoclonal antibody (MAb) were measured in Hepes buffer with different chemical additives. Five buffer ingredients (pH, NaCl, DMSO, EDTA, and KSCN) were varied simultaneously in 16 experiments following a statistical experimental plan. The 16 buffers thus spanned a volume in chemical space. A mathematical model, using data from the buffer composition, was developed and used to predict apparent kinetic parameters in five new buffers within the spanned volume. Association and dissociation parameters were measured in the new buffers, and these agreed with the predicted values, indicating that the model was valid within the spanned volume. The pattern of variation of the kinetic parameters in relation to buffer composition was different for association and dissociation, such that pH influenced both association and dissociation and NaCl influenced only dissociation. This indicated that the recognition mechanism (association) and the stability of the formed complex (dissociation) involve different binding forces, which can be further investigated by kinetic studies in systematically varied buffers.

Staphylococcal surface display of immunoglobulin A (IgA)- and IgE-specific in vitro-selected binding proteins (affibodies) based on Staphylococcus aureus protein A

An expression system designed for cell surface display of hybrid proteins on Staphylococcus carnosus has been evaluated for the display of Staphylococcus aureus protein A (SpA) domains, normally binding to immunoglobulin G (IgG) Fc but here engineered by combinatorial protein chemistry to yield SpA domains, denoted affibodies, with new binding specificities. Such affibodies, with human IgA or IgE binding activity, have previously been selected from a phage library, based on an SpA domain. In this study, these affibodies have been genetically introduced in monomeric or dimeric forms into chimeric proteins expressed on the surface of S. carnosus by using translocation signals from a Staphylococcus hyicus lipase construct together with surface-anchoring regions of SpA. The recombinant surface proteins, containing the IgA- or IgE-specific affibodies, were demonstrated to be expressed as full-length proteins, localized and properly exposed at the cell surface of S. carnosus. Furthermore, these chimeric receptors were found to be functional, since recombinant S. carnosus cells were shown to have gained IgA and IgE binding capacity, respectively. In addition, a positive effect in terms of IgA and IgE reactivity was observed when dimeric versions of the affibodies were present. Potential applications for recombinant bacteria with redirected binding specificity in their surface proteins are discussed.

An in vitro selected binding protein (affibody) shows conformation-dependent recognition of the respiratory syncytial virus (RSV) G protein

Using phage-display technology, a novel binding protein (Z-affibody) showing selective binding to the RSV (Long strain) G protein was selected from a combinatorial library of a small alpha-helical protein domain (Z), derived from staphylococcal protein A (SPA). Biopanning of the Z-library against a recombinant fusion protein comprising amino acids 130-230 of the G protein from RSV-subgroup A, resulted in the selection of a Z-affibody (Z(RSV1)) which showed G protein specific binding. Using biosensor technology, the affinity (K(D)) between Z(RSV1) and the recombinant protein was determined to be in the micromolar range (10(-6) M). Interestingly, the Z(RSV1) affibody was demonstrated to also recognize the partially (54%) homologous G protein of RSV subgroup B with similar affinity. Using different recombinant RSV G protein derived fragments, the binding was found to be dependent on the presence of the cysteinyl residues proposed to be involved in the formation of an intramolecular disulfide-constrained loop structure, indicating a conformation-dependent binding. Results from epitope mapping studies, employing a panel of monoclonal antibodies directed to different RSV G protein subfragments, suggest that the Z(RSV1) affibody binding site is located within the region of amino acids 164-186 of the G protein. This region contains a 13 amino acid residue sequence which is totally conserved between subgroups A and B of RSV and extends into the cystein loop region (amino acids 173-186). The potential use of the RSV G protein-specific Z(RSV1) affibody in diagnostic and therapeutic applications is discussed.

Mutational scanning of PCR products by subtractive oligonucleotide hybridization analysis

Here, we describe a new approach for mutational scanning of PCR products through hybridization analysis between complementary oligonucleotides. Sets of overlapping probe oligonucleotides complementary to wild-type (WT) sequence are hybridized to microbead-immobilized PCR products under solution-like conditions. Mismatch-hybridization situations between a mutant sample and probe oligonucleotides result in higher remaining concentrations in solution of involved probe oligonucleotides. Post-hybridization supernatants are subsequently analyzed for their probe oligonucleotide compositions using surface plasmon resonance-based biosensor technology. Relative remaining probe oligonucleotide concentrations are monitored in real-time through hybridization analysis between probe oligonucleotides and their corresponding sensor-chip immobilized complementary counterparts. This allows for the construction of composition diagrams revealing the existence and approximate location of a mutation within an investigated sample DNA sequence. Applied on PCR products derived from clinical samples of microdissected tumor biopsies, single mutations in exons 6 and 7 of the human p53 tumor-suppressor gene were successfully detected and approximately localized.

Genetic engineering of protein-peptide fusions for control of protein partitioning in thermoseparating aqueous two-phase systems

Genetic engineering has been used for the fusion of peptides, with different length and composition, on a protein to study the effect on partitioning in aqueous two-phase systems containing thermoseparating polymers. Peptides containing 2-6 tryptophan residues or tryptophan plus 1-3 lysine or aspartate residues, were fused near the C-terminus of the recombinant protein ZZT0, where Z is a synthetic IgG-binding domain derived from domain B in staphylococcal protein A. The partitioning behavior of the peptides and fusion proteins were studied in an aqueous two-phase system composed of dextran and the thermoseparating ethylene oxide-propylene oxide random copolymer, EO30PO70. The zwitterionic compound beta-alanine was used to reduce the charge-dependent salt effects on partitioning, and to evaluate the contribution to the partition coefficient from the amino acid residues, Trp, Lys, and Asp, respectively. Trp was found to direct the fusion proteins to the EO-PO copolymer phase, while Asp and Lys directed them to the dextran phase. The effect of sodium perchlorate and triethylammonium phosphate on the partitioning of the fusion proteins was also studied. Salt effects were directly proportional to the net charge of the fusion proteins. Sodium perchlorate was found to be 3.5 times more effective in directing positively charged proteins to the EO-PO copolymer phase compared to the effect of triethyl ammonium phosphate on negatively charged proteins. An empirical correlation has been tested where the fusion protein partitioning is a result of independent contributions from unmodified protein, fused peptide, and salt effects. A good agreement with experimental data was obtained which indicates the possibility, by independent measurements of partitioning of target protein and fusion peptide, to approximately predict the fusion protein partitioning.

Gene fragment polymerization gives increased yields of recombinant human proinsulin C-peptide

A multimerization strategy to improve yields upon recombinant production of the 31-aa human proinsulin C-peptide is presented. Gene fragments encoding the C-peptide were assembled using specific head-to-tail multimerization. DNA constructs encoding one, three or seven copies of the C-peptide gene, fused to a serum albumin binding affinity tag, were expressed intracellularly in Escherichia coli. The three fusion proteins were produced at similar levels (approximately 50 mg/l) and were proteolytically stable during production. Enzymatic digestion by trypsin-carboxypeptidase B treatment of the fusion proteins was shown to efficiently release native C-peptide, as determined by mass spectrometry, reverse-phase chromatography and a radioimmunoassay. The quantitative yields of C-peptide obtained from the three different fusion proteins suggest that this multimerization strategy could provide a cost-efficient production scheme for the C-peptide, and that this strategy could be useful also for production of other recombinant peptides.

Capture of single-stranded DNA assisted by oligonucleotide modules

Real-time biospecific interaction analysis was employed to monitor direct capture of a hepatitis C virus (HCV) derived polymerase chain reaction (PCR) product by nucleic acid hybridization. Different formats for hybridization were used to study the interaction between a single-stranded HCV PCR product and capture oligonucleotides immobilized on a sensor chip via streptavidin-biotin chemistry. By employing a prehybridization step in solution with nonbiotin oligonucleotides complementary to the single-stranded target and adjacent to the immobilized probe, a significant capture was achieved in comparison to the low capture efficiency obtained using single immobilized probes (9-36 mer). High capture efficiencies were also observed when shorter immobilized probes were used in combination with strings of adjacently positioned prehybridized probes (i.e., modules). Interestingly, the introduction of single nucleotide gaps between prehybridized and/or immobilized probes dramatically reduced the capture efficiency. These results suggest that flexible systems for capture could be designed from libraries of short oligonucleotides (9 mers) used in module fashion, taking advantage of stacking interactions between the oligonucleotides. The potential applications of such oligonucleotide-assisted capture systems are discussed.

All individual domains of staphylococcal protein A show Fab binding

The interactions between the individual domains (E, D, A, B and C) of staphylococcal protein A (SPA) and Fc and Fab regions of human immunoglobulins were studied using real-time biospecific interaction analysis. An engineered domain Z, similar to fragment B but with a single glycine to alanine amino acid substitution, was also included in the study. The domains were expressed in Escherichia coli, affinity purified and immobilised onto sensor chip surfaces in a directed manner using a unique C-terminal cysteine residue engineered into the recombinant proteins. All domains bound to a recombinant human IgG1 Fc fragment with similar strength. For the first time, binding to human Fab was demonstrated for all native SPA domains, using both polyclonal F(ab')2 and a recombinant scFv fragment as reagents. Interestingly, the engineered Z domain showed a considerably lower affinity for Fab as compared to the native domains.

Binding proteins selected from combinatorial libraries of an alpha-helical bacterial receptor domain

Small protein domains, capable of specific binding to different target proteins have been selected using combinatorial approaches. These binding proteins, called affibodies, were designed by randomization of 13 solvent-accessible surface residues of a stable alpha-helical bacterial receptor domain Z, derived from staphylococcal protein A. Repertoires of mutant Z domain genes were assembled and inserted into a phagemid vector adapted for monovalent phage display. Two libraries, each comprising approximately 4 x 10(7) transformants, were constructed using either an NN(G/T) or an alternative (C/A/G)NN degeneracy. Biopanning against the target proteins Taq DNA polymerase, human insulin, and a human apolipoprotein A-1 variant, showed that in all cases significant enrichments were obtained by the selection procedures. Selected clones were subsequently expressed in Escherichia coli and analyzed by SDS-PAGE, circular dichroism spectroscopy, and binding studies to their respective targets by biospecific interaction analysis. The affibodies have a secondary structure similar to the native Z domain and have micromolar dissociation constants (KD) for their respective targets.

Scaffolds for engineering novel binding sites in proteins

The combination of combinational protein chemistry and powerful selection techniques has resulted in the development of novel protein ligands based on the randomization of surface residues of a parental protein which is used as a scaffold. Such binding proteins, selected from libraries via specific binding towards a given target ligand, have the potential to replace natural antibodies in various biotechnological applications.

Heat-mediated activation of affinity-immobilized Taq DNA polymerase

A novel strategy for heat-mediated activation of recombinant Taq DNA polymerase is described. A serum albumin binding protein tag is used to affinity-immobilize an E. coli-expressed Taq DNA polymerase fusion protein onto a solid support coated with human serum albumin (HSA). Analysis of heat-mediated elution showed that elevated temperatures (> 70 degrees C) were required to significantly release the fusion protein from the solid support. A primer-extension assay showed that immobilization of the fusion protein resulted in little or no extension product. In contrast, fusion protein released from the HSA ligand by heat showed high polymerase activity. Thus, a heat-mediated release and reactivation of the Taq DNA polymerase fusion protein from the solid support can be obtained to allow for hot-start PCR with improved amplification performance.

Predominance of H-2d- and H-2k-restricted T-cell epitopes in the highly repetitive Plasmodium falciparum antigen Pf332

Genetic restriction of immune responses to malaria antigens is an important issue for a better comprehension of malaria immunity as well as for development of subunit vaccines. To experimentally define the major histocompatibility complex restriction of immune responses to the highly repetitive Plasmodium falciparum high-molecular-weight antigen Pf332, H-2-congenic mice were immunized with EB200, a recombinant fragment of Pf332 consisting of degenerate repeat motifs. Strong B- and T-cell responses were elicited in H-2d and H-2k mice whereas responses in H-2b, H-2q and H-2s mice were of lower magnitude. The T-cell specificity elicited by EB200 was defined by in vitro proliferative responses to a panel of overlapping peptides spanning EB200. Dominant epitopes were identified for H-2d and H-2k mice, respectively, and an additional epitope was recognized by all five mouse strains. Selected EB200-derived peptides were further investigated for their ability to elicit T-cell help when injected as multiple antigen peptides. Defined H-2d- and H-2k-restricted T-cell epitopes generated high antibody levels in the respective mouse strains, as did several peptides lacking defined epitopes indicating the presence of additional H-2d- and H-2k-restricted, cryptic or subdominant T-cell epitopes in EB200. The biased H-2 restriction pattern of T-cell epitopes in Pf332 and, as previously reported, in structurally related repeats in the malaria antigens Pf11.1 and Pf155/RESA may be explained by a shared motif for H-2d and H-2k class II-restricted T-cell epitopes, as revealed by alignment of these sequences.

Engineering of Fc(1) and Fc(3) from human immunoglobulin G to analyse subclass specificity for staphylococcal protein A

A system for production of recombinant Fc fragments of human IgG in Escherichia coli has been developed to allow for structural and functional studies of human Fc. The genes for the Fc fragments of human IgG subclasses 1 and 3, designated Fc(1) and Fc(3), were cloned from a human spleen cDNA library. The interactions to Staphylococcal protein A (SpA), a bacterial Fc receptor, that interacts with human IgG-Fc(1), but not with human IgG-Fc(3), were analyzed. To corroborate the involvement of amino acid residues in Fc, responsible for these differences in binding, two Fc variants were constructed; Fc(1(3)) and Fc(3(1)), each containing an isotypic dipeptide substitution. Production levels in E. coli of 1-10 mg/l of secreted Fc proteins, covalently linked as dimers, were routinely obtained. SpA-binding analyses of all four Fc variants using biosensor technology, showed that Fc(1) and Fc(3(1)) interact with SpA, while Fc(3) and Fc(1(3)) lack detectable SpA binding. The rendered SpA binding of the Fc variant Fc(3(1)), is concluded to result from the introduced dipeptide substitution (R435H, F436Y). The results demonstrate that the Fc expression system efficiently can be used in Fc engineering.

The serum albumin-binding region of streptococcal protein G: a bacterial fusion partner with carrier-related properties

In this study, we have explored the use of the serum albumin-binding region (BB) from streptococcal protein G (SpG) as a bacterial fusion partner for production of peptide immunogens. The fusion protein BB-M3, containing BB and repeated structures from the Plasmodium falciparum malaria antigen Pf155/RESA, was efficiently purified from Escherichia coli culture supernatants by affinity chromatography using BB as an affinity tag. Rabbits immunized with BB-M3 in Freund's adjuvant produced high levels of antibodies which reacted with both M3 and BB in ELISA and stained intact Pf155/RESA in the membrane of infected erythrocytes. These antibody levels were sustained for more than 30 weeks. BB-M3 also induced antibody responses to M3, BB and intact Pf155/RESA in a number of mouse strains, including several strains which are non-responders to the malaria sequences. In the latter mice, however, BB-M3 only activated BB-specific T cells, suggesting that BB has ability to provide carrier-related T cell help for antibody production. Moreover, the minimal albumin-binding motif of SpG, containing only 46 amino acids, was immunogenic in both B10.BR, B10.D2 and C57BL/6 mice (H-2k, H-2d and H-2b, respectively). These results indicate that BB has both affinity tag and carrier-related properties and suggest that fusion proteins containing BB can be efficient tools for the generation of antibody responses to peptides which are weak immunogens.

Production of a thermostable DNA polymerase by site-specific cleavage of a heat-eluted affinity fusion protein

A novel strategy is described for bacterial expression and affinity purification of a recombinant truncated version of the heat-stable DNA polymerase I from Thermus aquaticus. The DNA polymerase (deltaTaq) was produced as a fusion to a serum albumin binding affinity handle (ABP) derived from streptococcal protein G. Based on the thermostability of the deltaTaq DNA polymerase, affinity-purified ABP-deltaTaq could be heat-eluted from HSA columns by incubation at 85 degrees C. To produce free deltaTaq DNA polymerase, efficient site-specific cleavage of the affinity tag was performed using a recombinant coxsackievirus 3C protease (3Cpro), also produced as an ABP affinity fusion. Thus, an integrated strategy could be devised where both the cleaved ABP affinity tag and the protease fusion could be recovered after site-specific cleavage using HSA-affinity chromatography. The flow-through fraction contained essentially pure deltaTaq DNA polymerase with full enzymatic activity.

Detection of mutations in PCR products from clinical samples by surface plasmon resonance

Two different strategies for scanning and screening of mutations in polymerase chain reaction (PCR) products by hybridization analysis are described, employing real-time biospecific interaction analysis (BIA) for detection. Real-time BIA was used to detect differences in hybridization responses between PCR products and different 17-mer oligonucleotide probes. For the analysis using a biosensor instrument, two different experimental formats were investigated based on immobilization of either biotinylated PCR products or oligonucleotide probes onto a sensor chip. Applied on the human tumour suppressor p53 gene, differences in hybridization levels for full-match and mismatch situations employing both formats allowed the detection of point mutations in exon 6 PCR products, derived from a breast tumour biopsy sample. In addition, a mutant sample sequence could be detected in a 50/50 background of wild type exon 6 sequence. The suitability of the different formats for obtaining a regenerable system and a high throughput of samples is discussed.

The use of gene fusions to protein A and protein G in immunology and biotechnology

This review describes the use of fusion proteins containing the immunoglobulin-binding domains of staphylococcal protein A (SpA) or the serum albumin-binding regions of streptococcal protein G (SpG), respectively, for various applications in immunology and biotechnology. The review will not cover the use of SpA and SpG for the purpose of immunoglobulin purification, but instead focus on other applications. Hundreds of SpA/SpG fusion proteins have been described in publications in the context of recombinant protein production, in a wide variety of host cells, with subsequent affinity purification of the gene product. However, this still constitutes just one area of their use. We will thus cover also other aspects of using SpA and SpG, including strategies to: (i) improve in vitro renaturation schemes for expressed gene products, (ii) enable affinity-assisted folding in vivo of target proteins, (iii) improve the stability to proteolysis of produced recombinant proteins, (iv) prolong the in vivo half-life of therapeutic proteins, (v) facilitate subunit vaccine development and functional cDNA analysis, (vi) select novel receptor variants with new specificities by the use of phage display technology.

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.

Extended in vivo half-life of human soluble complement receptor type 1 fused to a serum albumin-binding receptor

A new approach has been used to extend the T(1/2) of human soluble complement receptor type 1 (sCR1) in rats. The albumin-binding domains B2A3 (BA) and B1A2B2A3 (BABA) from Streptococcal protein G were fused to the carboxyl terminus of sCR1, and the recombinant genes were expressed and amplified in Chinese hamster ovary cells. Western blot analysis and surface plasmon resonance measurements demonstrated the binding of rat serum albumin to both sCR1-BA and sCR1-BABA but not to sCR1. The in vitro complement inhibitory activity of the fusion proteins was shown to be similar to that of sCR1, indicating that neither the albumin-binding domains nor the presence of bovine serum albumin interfere with sCR1 function. Pharmacokinetic analysis showed that the T(1/2) of the distribution phase (T(1/2alpha)) was 3.3, 20.0 and 6.0 min for sCR1, sCR1-BA and sCR1-BABA, respectively. The T(1/2) of the elimination phase (T(1/2beta)) was 103, 297 and 170 min for sCR1, sCR1-BA and sCR1-BABA, respectively. The plasma elimination of sCR1-BA and sCR1-BABA was significantly (P < .05) prolonged as compared to sCR1. The proteins showed similar tissue distribution; at 4-hr postdosing, the highest levels of 125I-radioactivity per gram of tissue were localized in the urine, blood, liver, stomach, and small intestine.

Surface display of a functional single-chain Fv antibody on staphylococci

Two different host-vector expression systems designed for cell surface display of chimeric receptors on Staphylococcus xylosus and Staphylococcus carnosus have been evaluated for surface display of a mouse immunoglobulin G1(kappa) [IgG1(kappa)] anti-human IgE single-chain Fv (scFv) antibody fragment. To achieve surface anchoring of the chimeric receptors containing the scFv, the cell surface attachment regions from Staphylococcus aureus protein A were used in both expression systems. The different chimeric receptors could be recovered from cell wall extracts of both S. xylosus and S. carnosus, and surface localization was demonstrated by taking advantage of a serum albumin-binding reporter region present within the two types of receptors. In addition, the two different recombinant staphylococci carrying hybrid receptors containing the scFv were demonstrated to react with the antigen, which was human IgE, in whole-cell enzyme-linked immunosorbent assays. This is the first report of an antibody fragment expressed in a functional form anchored to the surface of gram-positive bacteria. The potential use of recombinant gram-positive bacteria as whole-cell diagnostic devices or alternatives to filamentous phages for surface display of scFv libraries is discussed.

The serum albumin-binding domain of streptococcal protein G is a three-helical bundle: a heteronuclear NMR study

Streptococcal protein G (SPG) is a cell surface receptor protein with a multiple domain structure containing tandem repeats of serum albumin-binding domains (ABD) and immunoglobulin-binding domains (IgBD). In this paper, we have analysed the fold of ABD. Far-UV circular dichroism analysis of ABD indicates high helical content (56%). Based on an analysis of nuclear magnetic resonance 13C secondary chemical shifts, sequential and short-range NOEs, and a few key nuclear Overhauser effects, we conclude that the ABD is a three-helix bundle. The structure of the ABD is, thus, quite different from the IgBD of protein G [Gronenborn, A.M. et al. (1991) Science 253, 657-661]. This strongly suggests that the ABD and the IgBD of SPG have evolved independently from each other. However, the fold of ABD is similar to that of the IgBD of staphylococcal protein A, possibly indicating a common evolutionary ancestor, despite the lack of sequence homology.

Hydrophobicity engineering to facilitate surface display of heterologous gene products on Staphylococcus xylosus

Protein engineering has been employed to investigate the effect of specific amino acid changes on the targeting of heterologous proteins to the outer cell surface of the Gram-positive bacterium Staphylococcus xylosus. Three different variants, corresponding to a 101 amino acid region of the major glycoprotein (G protein) of human respiratory syncytial virus (RSV), were generated in which multiple hydrophobic phenylalanine residues were either substituted or deleted. The different G protein fragments were expressed as one part of recombinant receptors designed for surface display on S. xylosus cells. The engineered variants of the RSV G protein hybrid receptors were, in contrast to a non-engineered fragment, efficiently targeted to the outer cell surface of recombinant S. xylosus cells as determined by different methods, including fluorescence-activated cell sorting. In addition, immunization of mice with live recombinant S. xylosus demonstrated that surface exposure was required to generate receptor-specific antibodies. The present strategy of hydrophobic engineering should be of general interest in surface-display applications and for secretion of proteins otherwise difficult to translocate through host cell membranes.

Direct and competitive kinetic analysis of the interaction between human IgG1 and a one domain analogue of protein A

Direct and competitive kinetic analysis of the binding between a one domain analogue of protein A, and mutants thereof, to immobilised hIgG1 was compared using a biosensor system based on surface plasmon resonance detection. Rate constants determined from both assays were almost identical. The experiments demonstrate that competitive kinetic analysis can be used in combination with biosensor technology, and indicate that competitive kinetic analysis may extend the use of the technology to include low molecular weight analytes.

A combinatorial library of an alpha-helical bacterial receptor domain

The construction and characterization of a combinatorial library of a solvent-exposed surface of an alpha-helical domain derived from a bacterial receptor is described. Using a novel solid-phase approach, the library was assembled in a directed and successive manner utilizing single-stranded oligonucleotides containing multiple random substitutions for the variegated segments of the gene fragment. The simultaneous substitution of 13 residues to all 20 possible amino acids was carried out in a region spanning 81 nucleotides. The randomization was made in codons for amino acids that were modelled to be solvent accessible at a surface made up from two of the three alpha-helices of a monovalent Fc-binding domain of staphylococcal protein A. After cloning of the PCR-amplified library into a phagemid vector adapted for phage display of the mutants, DNA sequencing analysis suggested a random distribution of codons in the mutagenized positions. Four members of the library with multiple substitutions were produced in Escherichia coli as fusions to an albumin-binding affinity tag derived from streptococcal protein G. The fusion proteins were purified by human serum albumin affinity chromatography and subsequently characterized by SDS-electrophoresis, CD spectroscopy and biosensor analysis. The analyses showed that the mutant protein A derivatives could all be secreted as soluble full-length proteins. Furthermore, the CD analysis showed that all mutants, except one with a proline introduced into helix 2, have secondary structures in close agreement with the wild-type domain. These results proved that members of this alpha-helical receptor library with multiple substitutions in the solvent-exposed surface remain stable and soluble in E. coli.(ABSTRACT TRUNCATED AT 250 WORDS)

Real-time monitoring of DNA manipulations using biosensor technology

The potential of real-time biospecific interaction analysis technology for applications in molecular biology is described. DNA fragments are immobilized onto a biosensor surface using the high-affinity streptavidin-biotin system and subsequently used to monitor different unit operations in molecular biology, e.g., DNA strand separation, DNA hybridization kinetics, and enzymatic modifications. A model system comprising six oligonucleotides was used, which can be assembled into a 69-bp double-stranded DNA fragment. Using this system, the biosensor approach was employed to analyze multistep solid-phase gene assembly and the performance of different enzymes routinely used for the synthesis and manipulation of DNA. In addition, a concept for the determination of single-point mutations in DNA samples is described.

Competitive elution of protein A fusion proteins allows specific recovery under mild conditions

A novel system is described for mild elution of fusion proteins by competitive elution. The approach is based on displacement of immobilized fusions containing a monovalent IgG-binding staphylococcal protein A fragment (Z) from an IgG-affinity matrix by a divalent fragment fused to a serum-albumin-binding region derived from streptococcal protein G. Using real-time interaction analysis, the binding (K(aff)) to polyclonal human IgG was found to be 3.3 (+/- 0.4) x 10(8) M-1 for divalent ZZ and 2.0 (+/- 0.1) x 10(7) M-1 for monovalent Z. This more than tenfold difference in binding strength ensures a high efficiency in the elution step. The competitor protein can specifically be removed and recovered from the elution mixture by subsequent passage through a human serum albumin(HSA)-affinity column, leaving only the target fusion protein in the flow-through fraction. Here, we show that a recombinant Klenow fragment of DNA polymerase I expressed in Escherichia coli can be recovered with high yield, and retained activity, from a crude bacterial lysate by IgG-affinity chromatography using mild conditions during both binding and elution.

Engineering proteins to facilitate bioprocessing

Genetic engineering is now being applied to aid the purification of recombinant proteins. The addition of specifically designed tags or the modification of sequences within the target-gene product has enabled the development of novel strategies for downstream processing that can be employed for efficient recovery of both native or modified proteins. This article discusses novel trends in genetic engineering that aid the bioprocessing of recombinant proteins.

Engineered bacterial receptors in immunology

Gram-positive surface receptors, such as staphylococcal protein A ans streptococcal protein G, have been genetically engineered for many applications in the field of immunology, including detection antigens, affinity purification of fusion proteins and display of heterologous epitopes on the surface of bacterial cells.

Cyclin A-cdc2 kinase does not trigger but delays cyclin degradation in interphase extracts of amphibian eggs

Purified cyclin B-cdc2 kinase has been shown previously to trigger cyclin degradation in interphase frog extracts by initiating a cascade of reactions that includes cyclin ubiquitinylation and ends with proteolysis. However, cyclin A-cdc2 kinase was not assayed in these early experiments. Here we have shown that full-length recombinant human cyclin A failed to induce cyclin degradation when it was added to frog extracts free of cyclin B, although it formed an active kinase complex with Xenopus cdc2. A highly purified kinase complex containing a truncated human cyclin A and starfish cdc2 also failed to switch on the cyclin degradation pathway. In contrast, both recombinant cyclin B and highly purified cyclin B-cdc2 kinase readily triggered degradation of both cyclins B and A in frog extracts. Whilst free cyclin A had no inhibitory effect, cyclin A-cdc2 kinase delayed degradation of both cyclins A and B induced by cyclin B-cdc2 kinase. The finding that cyclin A-cdc2 kinase cannot turn on, and even delays, cyclin destruction may be essential to prevent premature inactivation of MPF (maturation-promoting factor) before complete condensation of chromosomes and formation of the metaphase spindle.

Stabilization of recombinant proteins from proteolytic degradation in Escherichia coli using a dual affinity fusion strategy

A dual affinity fusion approach has been used to study the expression and secretion of labile recombinant proteins in Escherichia coli. Here we show that three small eukaryotic proteins (human proinsulin, a thioredoxin homologous domain of rat protein disulfide isomerase, and the extracellular domain of the alpha 1.2-chain of a human T-cell receptor) are stabilized in vivo using a dual affinity fusion strategy, where the gene encoding the desired product is fused between two genes encoding two different affinity domains. Relatively high yields of full-length product were obtained for all three proteins as compared to when fused to a single fusion partner. Despite the use of a signal peptide, significant amounts of the disulfide protein isomerase and T-cell receptor gene products were maintained in the cytoplasm, while the proinsulin fusion was efficiently secreted to the periplasm. Interestingly, the E. coli heat shock proteins DnaK and GroEL were associated with the fusion proteins isolated from the cytoplasm.

Structural and functional analysis of the human IgG-Fab receptor activity of streptococcal protein G

Streptococcal protein G (SPG) shows specific binding activity to IgGs and serum albumins from various species. In order to investigate the structural domains of SPG responsible for the specific interaction with human IgG-Fab, the binding characteristics of a collection of recombinant receptors were analysed. The study includes receptors comprising different parts of the SPG molecule as well as chimeric receptors containing IgG-binding domains of staphylococcal protein A (SPA) fused to the N-terminal AB-region of SPG, which has been claimed to interact with human IgG-Fab. Purified defined gene products were allowed to compete for the binding to human IgG, human IgG-F(ab')2 fragments and human serum albumin (HSA) in several sets of competitive binding experiments. The results demonstrate that the C-terminal C domains have both IgG-Fc- and IgG-Fab-binding capacities, whereas the N-terminal AB region is responsible for the HSA-binding only. These results, which are in conflict with previous work, demonstrate that the binding to both the IgG-Fc and the IgG-Fab region is mediated by the same structurally distinct receptor region of SPG.