Challenges and opportunities in the purification of recombinant tagged proteins

Carboxymethylated polyethylenimine modified magnetic nanoparticles specifically for purification of His-tagged protein

Employing immobilized metal-ion affinity chromatography and magnetic separation could ideally provide a useful analytical strategy for purifying His-tagged protein. In the current study, a facile route was designed to prepare CMPEI-Ni²⁺ @SiO₂ @Fe₃ O₄ (CMPEI=carboxymethylated polyethyleneimine) magnetic nanoparticles composed of a strong magnetic core of Fe₃ O₄ and a Ni²⁺ -immobilized carboxymethylated polyethyleneimine coated outside shell, which was formed by electrostatic interactions between polyanionic electrolyte of carboxymethylated polyethyleneimine and positively charged surface of 3-(trimethoxysilyl)propylamin modified SiO₂ @Fe₃ O₄ . The resulting CMPEI-Ni²⁺ @SiO₂ @Fe₃ O₄ composite nanoparticles displayed well-uniform structure and high magnetic responsiveness. Hexa His-tagged peptides and purified His-tagged recombinant retinoid X receptor alpha were chosen as the model samples to evaluate the adsorption, capacity, and reusability of the composite nanoparticles. The results demonstrated the CMPEI-Ni²⁺ @SiO₂ @Fe₃ O₄ nanoparticles possessed rapid adsorption, large capacity, and good recyclability. The obtained nanoparticles were further used to purify His-tagged protein in practical environment. It was found that the nanoparticles could selectively capture His-tagged recombinant retinoid X receptor protein from complex cell lysate. Owing to its easy synthesis, large binding capacity, and good reusability, the prepared CMPEI-Ni²⁺ @SiO₂ @Fe₃ O₄ magnetic nanoparticles have great potential for application in biotechnological fields.

Non-chromatographic purification of Teriparatide with a pH-responsive CspB tag

Cell surface protein B (CspB) from Corynebacterium glutamicum is used as a pH-responsive peptide tag to enable a simple solid-liquid separation method for isolating a CspB fusion protein. Here we demonstrate the first application of a CspB tag for the purification of Teriparatide, which is a biologic drug that is prescribed for osteoporosis. The Teriparatide was constructed as CspB50TEV-Teriparatide, comprising 50 amino acid residues of CspB, the cleavage site of TEV protease, and Teriparatide. CspB50TEV-Teriparatide was expressed in a culture supernatant by C. glutamicum secretion system at 3.0 g/L (equivalent to approximately 1.2 g/L Teriparatide). The CspB50TEV-Teriparatide was precipitated by reducing the pH of the culture supernatant, and the precipitate was then dissolved in a neutral buffer. A TEV protease treatment was applied to cleave the Teriparatide from the CspB50TEV-Teriparatide. Then, the remaining digested CspB50TEV, undigested CspB50TEV-Teriparatide, and TEV protease were precipitated in an acidic pH, whereas the soluble Teriparatide remained in the supernatant. The process had a yield of 96.5% and resulted in Teriparatide with a purity of 98.0% and productivity of 1.1 g/L of C. glutamicum culture. Thus, tag-free Teriparatide was successfully purified from the CspB fusion protein using only pH changes, centrifugation, and protease digestion without the need for chromatography. This versatile purification protocol is expected to be applicable to various proteins from laboratory to industrial scales.

Expression and purification of human phosphatase and actin regulator 1 (PHACTR1) in plant-based systems

Cardiovascular diseases are a prevalent cause of morbidity and mortality especially in industrialized countries. The human phosphatase and actin regulator 1 (PHACTR1) may be involved in such diseases, but its precise regulatory function remains unclear due to the large number of potential interaction partners. The same phenomenon makes this protein difficult to express in mammalian cells, but it is also an intrinsically disordered protein that likely aggregates when expressed in bacteria due to the absence of chaperones. We therefore used a design of experiments approach to test the suitability of three plant-based systems for the expression of satisfactory quantities of recombinant PHACTR1, namely transient expression in tobacco (Nicotiana tabacum) BY-2 plant cell packs (PCPs), whole N. benthamiana leaves and BY-2 cell lysate (BYL). The highest yield was achieved using the BYL: up to 120 mg product kg-1 biomass equivalent within 48 h of translation. This was 1.3-fold higher than transient expression in N. benthamiana together with the silencing inhibitor p19, and 6-fold higher than the PCP system. The presence of Triton X-100 in the extraction buffer increased the recovery of PHACTR1 by 2-200-fold depending on the conditions. PHACTR1 was incompatible with biomass blanching and was stable for less than 16 h in raw plant extracts. Purification using a DDK-tag proved inefficient whereas 15% purity was achieved by immobilized metal affinity chromatography.

Materials Functionalization with Multicomponent Reactions: State of the Art

The emergence of neoteric synthetic routes for materials functionalization is an interesting phenomenon in materials chemistry. In particular, the union of materials chemistry with multicomponent reactions (MCRs) opens a new avenue leading to the realm of highly innovative functionalized architectures with unique features. MCRs have recently been recognized as considerable part of the synthetic chemist's toolbox due to their great efficiency, inherent molecular diversity, atom and pot economy along with operational simplicity. Also, MCRs can improve E-factor and mass intensity as important green chemistry metrics. By rational tuning of the materials, as well as the MCRs, wide ranges of functionalized materials can be produced with tailorable properties that can play important roles in the plethora of applications. To date, there has not reported any exclusive review of a materials functionalization with MCRs. This critical review highlights the state-of-the-art on the one-pot functionalization of carbonaceous and siliceous materials, polysaccharides, proteins, enzymes, synthetic polymers, etc., via diverse kind of MCRs like Ugi, Passerini, Petasis, Khabachnik-Fields, Biginelli, and MALI reactions through covalent or noncovalent manners. Besides the complementary discussion of synthetic routes, superior properties and detailed applicability of each functionalized material in modern technologies are discussed. Our outlook also emphasizes future strategies for this unprecedented area and their use as materials for industrial implementation. With no doubt, MCRs-functionalization of materials bridges the gap between materials science domain and applied chemistry.

Overexpression and purification of human myosins from transiently and stably transfected suspension adapted HEK293SF-3F6 cells

The myosin family of motor proteins is an attractive target of therapeutic small-molecule protein inhibitors and modulators. Milligrams of protein quantities are required to conduct proper biophysical and biochemical studies to understand myosin functions. Myosin protein expression and purification represent a critical starting point towards this goal. Established utilization of Dictyostelium discoideum, Drosophila melanogaster, insect and mouse cells for myosin expression and purification is limited, cost, labor and time inefficient particularly for (full-length) human myosins. Here we are presenting detailed protocols for production of several difficult-to-purify recombinant human myosins in efficient quantities up to 1 mg of protein per liter of cell culture. This is the first time that myosins have been purified in large scales from suspension adapted transiently and stably expressing human cells. The method is also useful for expressing other human proteins in quantities sufficient to perform extensive biochemical and biophysical characterization.

A multicolour HLA-specific B-cell FluoroSpot assay to functionally track circulating HLA-specific memory B cells

Emerging evidence suggests that donor-reactive memory B cells (mBC) play a key role inducing antibody-mediated rejection (ABMR) after solid organ transplantation and show a broader antigen repertoire than plasma cells thus, being potentially present even in absence of donor-specific antibodies. Therefore, the development of novel immune assays capable of quantifying circulating donor-reactive mBC in organ transplantation is highly warranted. We developed a novel HLA-specific B-cell FluoroSpot assay capable of enumerating multiple HLA-specific Antibody Secreting Cells (ASC) originated from circulating mBC after in-vitro polyclonal activation. We performed a thorough characterization of distinct selective in-vitro mBC activation methods based on either the TLR7,8 agonist R848 plus Interleukin-2 or an anti-CD40 agonist monoclonal antibody, assessed optimal activation culture conditions, cell sources, activation time-frame as well as the advantage of measuring HLA-specific IgG-ASC as compared to HLA-IgG Ab detected in supernatants of in-vitro stimulated B-cell to characterize anti-HLA alloreactivity. Notably, using fluorescently-labeled multimerized HLA molecules as detection matrix, we show the ability of this assay to precisely quantify multiple anti-HLA mBC specificities. In conclusion, evaluating circulating donor-reactive mBC using new technology may provide novel insight of the pathogenesis of humoral rejection and may help identifying transplant recipients at high risk of allograft rejection.

A multi-column plate adapter provides an economical and versatile high-throughput protein purification system

Protein purification is essential in the study of protein structure and function, and the development of novel therapeutics. Many studies require purifying multiple proteins at once, increasing the demand for improved purification methods. We hypothesized that multiple chromatography columns could be interfaced with a multi-well collection plate for rapid and convenient protein purification without the need of expensive instrumentation. As such, we developed a multi-column plate adapter (MCPA), which provides an economical yet versatile and time efficient, high-throughput protein purification system. The MCPA system simultaneously purified milligrams of different proteins under gravity or under vacuum for faster purification. The MCPA handles up to twenty-four 12 mL columns and multiple MCPA's in sequence allow milligram-scale purification of 96 different samples with relative ease. We also used the MCPA system for large scale affinity purification of four proteins, providing sufficient yields and purity for protein crystallization and biophysical characterization. The MCPA system is ideal for optimizing resin type and volume or any other purification parameter by customizing individual columns during the same purification. The high-throughput and versatile nature of this system should prove to be useful in obtaining adequate amounts of protein for subsequent analyses in any laboratory setting.

New trends in aggregating tags for therapeutic protein purification

The rapid growth of the therapeutic protein market calls for more efficient purification methods. Various aggregating tags have recently emerged as simple, fast, cost-effective and column-free technologies for protein (and peptide) purification. In general, these column-free protein purification technologies involve the use of aggregating tags that induce the target protein into insoluble aggregates. These aggregates can be easily separated from soluble impurities and the target protein or peptide is then liberated by a cleavage process. This review summarizes the current state-of-the-art in using aggregating tags for protein purification. The methods are here categorized as follows: (1) tags that allow soluble expression of target protein in vivo and induce aggregation in vitro; (2) tags that induce soluble expression and self-assembling of target protein on insoluble biological polyester beads in vivo; (3) tags that induce formation of inactive aggregates in vivo; (4) tags that induce formation of active aggregates in vivo.

Versatile approach for functional analysis of human proteins and efficient stable cell line generation using FLP-mediated recombination system

Deciphering a function of a given protein requires investigating various biological aspects. Usually, the protein of interest is expressed with a fusion tag that aids or allows subsequent analyses. Additionally, downregulation or inactivation of the studied gene enables functional studies. Development of the CRISPR/Cas9 methodology opened many possibilities but in many cases it is restricted to non-essential genes. Recombinase-dependent gene integration methods, like the Flp-In system, are very good alternatives. The system is widely used in different research areas, which calls for the existence of compatible vectors and efficient protocols that ensure straightforward DNA cloning and generation of stable cell lines. We have created and validated a robust series of 52 vectors for streamlined generation of stable mammalian cell lines using the FLP recombinase-based methodology. Using the sequence-independent DNA cloning method all constructs for a given coding-sequence can be made with just three universal PCR primers. Our collection allows tetracycline-inducible expression of proteins with various tags suitable for protein localization, FRET, bimolecular fluorescence complementation (BiFC), protein dynamics studies (FRAP), co-immunoprecipitation, the RNA tethering assay and cell sorting. Some of the vectors contain a bidirectional promoter for concomitant expression of miRNA and mRNA, so that a gene can be silenced and its product replaced by a mutated miRNA-insensitive version. Our toolkit and protocols have allowed us to create more than 500 constructs with ease. We demonstrate the efficacy of our vectors by creating stable cell lines with various tagged proteins (numatrin, fibrillarin, coilin, centrin, THOC5, PCNA). We have analysed transgene expression over time to provide a guideline for future experiments and compared the effectiveness of commonly used inducers for tetracycline-responsive promoters. As proof of concept we examined the role of the exoribonuclease XRN2 in transcription termination by RNAseq.

Simple and Efficient Purification of Recombinant Proteins Using the Heparin-Binding Affinity Tag

Heparin, a member of the glycosaminoglycan family, is known to interact with more than 400 different types of proteins. For the past few decades, significant progress has been made to understand the molecular details involved in heparin-protein interactions. Based on the structural knowledge available from the FGF1-heparin interaction studies, we have designed a novel heparin-binding peptide (HBP) affinity tag that can be used for the simple, efficient, and cost-effective purification of recombinant proteins of interest. HBP-tagged fusion proteins can be purified by heparin Sepharose affinity chromatography using a simple sodium chloride gradient to elute the bound fusion protein. In addition, owing to the high density of positive charges on the HBP tag, recombinant target proteins are preferably expressed in their soluble forms. The purification of HBP-fusion proteins can also be achieved in the presence of chemical denaturants, including urea. Additionally, polyclonal antibodies raised against the affinity tag can be used to detect HBP-fused target proteins with high sensitivity. © 2017 by John Wiley & Sons, Inc.

Production of monoclonal antibodies and development of a quantitative immuno-polymerase chain reaction assay to detect and quantify recombinant Glutathione S-transferase

GST-tagged proteins are important tools for the production of recombinant proteins. Removal of GST tag from its fusion protein, frequently by harsh chemical treatments or proteolytic methods, is often required. Thus, the monitoring of the proteins in tag-free form requires a significant effort to determine the remnants of GST during purification process. In the present study, we developed both a conventional enzyme-linked immunosorbent assay (ELISA) and an immuno-polymerase chain reaction (IPCR) assay, both specific for detection of recombinant GST (rGST). rGST was expressed in Escherichia coli JM109, using a pGEX4T-3 vector, and several anti-rGST monoclonal antibodies were generated using hybridoma technology. Two of these were rationally selected as capture and detection antibodies, allowing the development of a sandwich ELISA with a limit of detection (LOD) of 0.01 μg/ml. To develop the rGST-IPCR assay, we selected "Universal-IPCR" format, comprising the biotin-avidin binding as the coupling system. In addition, the rGST-IPCR was developed in standard PCR tubes, and the surface adsorption of antibodies on PCR tubes, the optimal neutravidin concentrations, the generation of a reporter DNA and the concentration effect were studied and determined. Under optimized assay conditions, the rGST-IPCR assay provided a 100-fold increase in the LOD as well as an expanded working range, in comparison with rGST-ELISA. The proposed method exhibited great potentiality for application in several fields in which measurement of very low levels of GST is necessary, and might provide a model for other IPCR assays.

Expression, purification and characterization of human cytosolic sulfotransferase (SULT) 1C4

Human cytosolic sulfotransferase 1C4 (hSULT1C4) is a dimeric Phase II drug-metabolizing enzyme primarily expressed in the developing fetus. SULTs facilitate the transfer of a hydrophilic sulfonate moiety from 3'-phosphoadenosine-5'-phosphosulfate (PAPS) onto an acceptor substrate altering the substrate's biological activity and increasing the compound's water solubility. While several of the hSULTs' endogenous and xenobiotic substrates have been identified, the physiological function of hSULT1C4 remains unknown. The fetal expression of hSULT1C4 leads to the hypothesis that the function of this enzyme may be to regulate metabolic and hormonal signaling molecules, such as estrogenic compounds, that may be generated or consumed by the mother during fetal development. Human SULT1C4 has previously been shown to sulfonate estrogenic compounds, such as catechol estrogens; therefore, this study focused on the expression and purification of hSULT1C4 in order to further characterize this enzyme's sulfonation of estrogenic compounds. Molecular modeling of the enzyme's native properties helped to establish a novel purification protocol for hSULT1C4. The optimal activity assay conditions for hSULT1C4 were determined to be pH 7.4 at 37°C for up to 10 min. Kinetic analysis revealed the enzyme's reduced affinity for PAPS compared to PAP. Human SULT1C4 sulfonated all the estrogenic compounds tested, including dietary flavonoids and environmental estrogens; however, the enzyme has a higher affinity for sulfonation of flavonoids. These results suggest hSULT1C4 could be metabolizing and regulating hormone signaling pathways during human fetal development.

Step-By-Step In Vitro Mutagenesis: Lessons From Fucose-Binding Lectin PA-IIL

Site-directed mutagenesis is a powerful technique which is used to understand the basis of interactions between proteins and their binding partners, as well as to modify these interactions. Methods of rational design that are based on detailed knowledge of the structure of a protein of interest are often used for preliminary investigations of the possible outcomes which can result from the practical application of site-directed mutagenesis. Also, random mutagenesis can be used in tandem with site-directed mutagenesis for an examination of amino acid "hotspots."Lectins are sugar-binding proteins which, among other functions, mediate the recognition of host cells by a pathogen and its adhesion to the host cell surface. Hence, lectins and their binding properties are studied and engineered using site-directed mutagenesis.In this chapter, we describe a site-directed mutagenesis method used for investigating the sugar binding pattern of the PA-IIL lectin from the pathogenic bacterium Pseudomonas aeruginosa. Moreover, procedures for the production and purification of PA-IIL mutants are described, and several basic methods for characterizing the mutants are discussed.

Encapsulation into Stealth Liposomes Enhances the Antitumor Action of Recombinant Cratylia mollis Lectin Expressed in Escherichia coli

This study evaluated the in vivo antitumor potential of the recombinant lectin from seeds of Cratylia mollis (rCramoll) expressed in Escherichia coli, free or encapsulated in stealth liposomes, using mice transplanted with sarcoma 180. rCramoll-loaded stealth liposomes (rCramoll-lipo) were formulated by hydration of the lipid film followed by cycles of freezing and thawing, and about 60% of rCramoll was encapsulated. This novel preparation showed particle size, polydispersity index, and pH suitable for the evaluation of antitumor activity in vivo. Tumor growth inhibition rates were 59% for rCramoll and 75% for rCramoll-lipo. Histopathological analysis of the experimental groups showed that both free and encapsulated lectin caused no changes in the kidneys of animals. Hematological analysis revealed that treatment with rCramoll-lipo significantly increased leukocyte concentration when compared with the untreated and rCramoll group. In conclusion, the encapsulation of rCramoll in stealth liposomes improves its antitumor activity without substantial toxicity; this approach was more successful than the previous results reported for pCramoll loaded into conventional liposomes. At this point, a crucial difference between the antitumor action of free and encapsulated rCramoll was found along with their effects on immune cells. Further investigations are required to elucidate the mechanism(s) of the antitumor effect induced by rCramoll.

Application of volcanic ash particles for protein affinity purification with a minimized silica-binding tag

We recently reported that the spore coat protein, CotB1 (171 amino acids), from Bacillus cereus mediates silica biomineralization and that the polycationic C-terminal sequence of CotB1 (14 amino acids), designated CotB1p, serves as a silica-binding tag when fused to other proteins. Here, we reduced the length of this silica-binding tag to only seven amino acids (SB7 tag: RQSSRGR) while retaining its affinity for silica. Alanine scanning mutagenesis indicated that the three arginine residues in the SB7 tag play important roles in binding to a silica surface. Monomeric l-arginine, at concentrations of 0.3-0.5 M, was found to serve as a competitive eluent to release bound SB7-tagged proteins from silica surfaces. To develop a low-cost, silica-based affinity purification procedure, we used natural volcanic ash particles with a silica content of ∼70%, rather than pure synthetic silica particles, as an adsorbent for SB7-tagged proteins. Using green fluorescent protein, mCherry, and mKate2 as model proteins, our purification method achieved 75-90% recovery with ∼90% purity. These values are comparable to or even higher than that of the commonly used His-tag affinity purification. In addition to low cost, another advantage of our method is the use of l-arginine as the eluent because its protein-stabilizing effect would help minimize alteration of the intrinsic properties of the purified proteins. Our approach paves the way for the use of naturally occurring materials as adsorbents for simple, low-cost affinity purification.

Differential secretion pathways of proteins fused to the Escherichia coli maltose binding protein (MBP) in Pichia pastoris

The Escherichia coli maltose binding protein (MBP) is an N-terminal fusion partner that was shown to enhance the secretion of some heterologous proteins from the yeast Pichia pastoris, a popular host for recombinant protein expression. The amount of increase in secretion was dependent on the identity of the cargo protein, and the fusions were proteolyzed prior to secretion, limiting its use as a purification tag. In order to overcome these obstacles, we used the MBP as C-terminal partner for several cargo peptides. While the Cargo-MBP proteins were no longer proteolyzed in between these two moieties when the MBP was in this relative position, the secretion efficiency of several fusions was lower than when MBP was located at the opposite end of the cargo protein (MBP-Cargo). Furthermore, fluorescence analysis suggested that the MBP-EGFP and EGFP-MBP proteins followed different routes within the cell. The effect of several Pichia pastoris beta-galactosidase supersecretion (bgs) strains, mutants showing enhanced secretion of select reporters, was also investigated on both MBP-EGFP and EGFP-MBP. While the secretion efficiency, proteolysis and localization of the MBP-EGFP was influenced by the modified function of Bgs13, EGFP-MBP behavior was not affected in the bgs strain. Taken together, these results indicate that the location of the MBP in a fusion affects the pathway and trans-acting factors regulating secretion in P. pastoris.

An antigen-specific, four-color, B-cell FluoroSpot assay utilizing tagged antigens for detection

The FluoroSpot assay, a variant of ELISpot utilizing fluorescent detection, has so far been used primarily for assessment of T cells, where simultaneous detection of several cytokines has allowed a more qualitative analysis of functionally distinct T cells. The potential to measure multiple analytes also presents several advantages when analyzing B cells. Our aim was to develop a B-cell FluoroSpot assay adaptable to studies of a variety of antigens. The assay utilizes anti-IgG antibodies immobilized in 96-well filter membrane plates. During cell culture, IgG antibodies secreted by antibody-secreting cells (ASCs) are captured in the vicinity of each of these cells and the specificity of single ASCs is defined using antigens for detection. The antigens were labeled with biotin or peptide tags enabling secondary detection with fluorophore-conjugated streptavidin or tag-specific antibodies. The assay, utilizing up to four different tag systems and fluorophores simultaneously, was evaluated using hybridomas and immunized splenocytes as ASCs. Assay variants were developed that could: i) identify multiple ASCs with different antigen specificities; ii) detect ASCs showing cross-reactivity with different but related antigens; and iii) define the antigen-specificity and, by including anti-IgG subclass detection reagents, simultaneously determine the IgG subclass of antibodies secreted by ASCs. As demonstrated here, the B-cell FluoroSpot assay using tag-based detection systems provides a versatile and powerful tool to investigate antibody responses by individual cells that can be readily adapted to studies of a variety of antigen-specific ASCs.

A Chimeric Affinity Tag for Efficient Expression and Chromatographic Purification of Heterologous Proteins from Plants

The use of plants as expression hosts for recombinant proteins is an increasingly attractive option for the production of complex and challenging biopharmaceuticals. Tools are needed at present to marry recent developments in high-yielding gene vectors for heterologous expression with routine protein purification techniques. In this study, we designed the Cysta-tag, a new purification tag for immobilized metal affinity chromatography (IMAC) of plant-made proteins based on the protein-stabilizing fusion partner SlCYS8. We show that the Cysta-tag may be used to readily purify proteins under native conditions, and then be removed enzymatically to isolate the protein of interest. We also show that commonly used protease recognition sites for linking purification tags are differentially stable in leaves of the commonly used expression host Nicotiana benthamiana, with those linkers susceptible to cysteine proteases being less stable then serine protease-cleavable linkers. As an example, we describe a Cysta-tag experimental scheme for the one-step purification of a clinically useful protein, human α1-antitrypsin, transiently expressed in N. benthamiana. With potential applicability to the variety of chromatography formats commercially available for IMAC-based protein purification, the Cysta-tag provides a convenient means for the efficient and cost-effective purification of recombinant proteins from plant tissues.

Extraction and purification methods in downstream processing of plant-based recombinant proteins

During the last two decades, the production of recombinant proteins in plant systems has been receiving increased attention. Currently, proteins are considered as the most important biopharmaceuticals. However, high costs and problems with scaling up the purification and isolation processes make the production of plant-based recombinant proteins a challenging task. This paper presents a summary of the information regarding the downstream processing in plant systems and provides a comprehensible overview of its key steps, such as extraction and purification. To highlight the recent progress, mainly new developments in the downstream technology have been chosen. Furthermore, besides most popular techniques, alternative methods have been described.

Evaluation of employing poly-lysine tags versus poly-histidine tags for purification and characterization of recombinant copper-binding proteins

Quantitative characterization of metalloproteins at molecular and atomic levels generally requires tens of milligrams of highly purified samples, a situation frequently challenged by problems in generating unmodified native forms. A variety of affinity tags, such as the popular poly-histidine tag, have been developed to facilitate purification but they generally rely on expensive affinity resins and their presence may interfere with protein characterization. This paper documents that addition of a poly-lysine tag to the C-terminus enables, for the copper-binding proteins examined, ready purification in large scale via cost-effective cation-exchange chromatography. The tag may be removed readily by the enzyme carboxypeptidase B to generate the native protein with no extra residues. However, this cleavage step is normally not necessary since the poly-lysine tag is shown to have no detectable affinity for either Cu(I) or Cu(II) and imposes no interference to the copper binding properties of the target proteins. In contrast, the poly-histidine tag possesses a sub-picomolar affinity for Cu(I) and -nanomolar affinity for Cu(II) and may need to be removed for reliable characterization of the target proteins. These conclusions may be extended to the study of other metallo-proteins and metallo-enzymes.

HaloTag technology: a versatile platform for biomedical applications

Exploration of protein function and interaction is critical for discovering links among genomics, proteomics, and disease state; yet, the immense complexity of proteomics found in biological systems currently limits our investigational capacity. Although affinity and autofluorescent tags are widely employed for protein analysis, these methods have been met with limited success because they lack specificity and require multiple fusion tags and genetic constructs. As an alternative approach, the innovative HaloTag protein fusion platform allows protein function and interaction to be comprehensively analyzed using a single genetic construct with multiple capabilities. This is accomplished using a simplified process, in which a variable HaloTag ligand binds rapidly to the HaloTag protein (usually linked to the protein of interest) with high affinity and specificity. In this review, we examine all current applications of the HaloTag technology platform for biomedical applications, such as the study of protein isolation and purification, protein function, protein–protein and protein–DNA interactions, biological assays, in vitro cellular imaging, and in vivo molecular imaging. In addition, novel uses of the HaloTag platform are briefly discussed along with potential future applications.

Extraction and downstream processing of plant-derived recombinant proteins

Plants offer the tantalizing prospect of low-cost automated manufacturing processes for biopharmaceutical proteins, but several challenges must be addressed before such goals are realized and the most significant hurdles are found during downstream processing (DSP). In contrast to the standardized microbial and mammalian cell platforms embraced by the biopharmaceutical industry, there are many different plant-based expression systems vying for attention, and those with the greatest potential to provide inexpensive biopharmaceuticals are also the ones with the most significant drawbacks in terms of DSP. This is because the most scalable plant systems are based on the expression of intracellular proteins in whole plants. The plant tissue must therefore be disrupted to extract the product, challenging the initial DSP steps with an unusually high load of both particulate and soluble contaminants. DSP platform technologies can accelerate and simplify process development, including centrifugation, filtration, flocculation, and integrated methods that combine solid-liquid separation, purification and concentration, such as aqueous two-phase separation systems. Protein tags can also facilitate these DSP steps, but they are difficult to transfer to a commercial environment and more generic, flexible and scalable strategies to separate target and host cell proteins are preferable, such as membrane technologies and heat/pH precipitation. In this context, clarified plant extracts behave similarly to the feed stream from microbes or mammalian cells and the corresponding purification methods can be applied, as long as they are adapted for plant-specific soluble contaminants such as the superabundant protein RuBisCO. Plant-derived pharmaceutical proteins cannot yet compete directly with established platforms but they are beginning to penetrate niche markets that allow the beneficial properties of plants to be exploited, such as the ability to produce 'biobetters' with tailored glycans, the ability to scale up production rapidly for emergency responses and the ability to produce commodity recombinant proteins on an agricultural scale.

Latest approaches for efficient protein production in drug discovery

INTRODUCTION

Pharmaceutical research looks to discover and develop new compounds which influence the function of disease-associated proteins or respective protein-protein interactions. Various scientific methods are available to discover those compounds, such as high-throughput screening of a library comprising chemical or natural compounds and computational rational drug design. The goal of these methods is to identify the seed compounds of future pharmaceuticals through the use of these technologies and laborious experiments. For every drug discovery effort made, the possession of accurate functional and structural information of the disease-associated proteins helps to assist drug development. Therefore, the investigation of the tertiary structure of disease-associated proteins and respective protein-protein interactions at the atomic level are of crucial importance for successful drug discovery.

AREAS COVERED

In this review article, the authors broadly outline current techniques utilized for recombinant protein production. In particular, the authors focus on bacterial expression systems using Escherichia coli as the living bioreactor.

EXPERT OPINION

The recently developed pCold-glutathione S-transferase (GST) system is one of the best systems for soluble protein expression in E. coli. Where the pCold-GST system does not succeed, it is preferable to change the host from E. coli to higher organisms such as yeast expression systems like Pichia pastoris and Kluyveromyces lactis. The selection of an appropriate expression system for each desired protein and the optimization of experimental conditions significantly contribute toward the successful outcome of any drug discovery study.