An insight into fusion technology aiding efficient recombinant protein production for functional proteomics

Using peptide barcodes for simultaneous profiling of protein expression from mRNA

RATIONALE

mRNA technology has begun to play a significant role in the areas of therapeutic intervention and vaccine development. However, optimizing the mRNA sequence that influences protein expression levels is a resource-intensive and time-consuming process. This study introduces a new method to accelerate the selection of sequences of mRNA for optimal protein expression.

METHODS

We designed the mRNA sequences in such a way that a unique peptide barcode, corresponding to each mRNA sequence, is attached to the expressed protein. These barcodes, cleaved off by a protease and simultaneously quantified by mass spectrometry, reflect the protein expression, enabling a parallel analysis. We validated this method using two mRNAs, each with different untranslated regions (UTRs) but encoding enhanced green fluorescence protein (eGFP), and investigated whether the peptide barcodes could analyze the differential eGFP expression levels.

RESULTS

The fluorescence intensity of eGFP, a marker of its expression level, has shown noticeable changes between the two UTR sequences in mRNA-transfected cells when measured using flow cytometry. This suggests alterations in the expression level of eGFP due to the influence of different UTR sequences. Furthermore, the quantified amount of peptide barcodes that were released from eGFP showed consistent patterns with these changes.

CONCLUSIONS

The experimental findings suggest that peptide barcodes serve as a valuable tool for assessing protein expression levels. The process of mRNA sequence selection, aimed at maximizing protein expression, can be enhanced by the parallel analysis of peptide barcodes using mass spectrometry.

Cytotoxic Effects of a Novel tagged Apoptin on Breast Cancer Cell Lines

Backgrounds

Apoptin can induce tumor cell-specific apoptosis in a broad range of human tumor cells and is a potential anticancer therapeutic candidate to kill tumor cells.

Materials and Methods

We designed two structures of apoptin fusion protein, SUMO-PTD4-Apoptin, and PTD4-Apoptin. To express these fusion proteins, E. coli BL21(DE3) was employed. MTT assay, Flow cytometry, and cell cycle analysis were used to investigate the function of proteins on two breast cancer cell lines (MDA-MB-231 and MCF-7) and MCF 10A cell line (as normal cells).

Results

Expression of the recombinant SUMO-PTD4-Apoptin and PTD4-Apoptin in E. coli BL21(DE3) was successful. MTT assay results showed that the IC50 was 6.4 µg/ml for SUMO-PTD4-Apoptin in MDA-MB-231 and was 9.3 after 24 h of treatment in MCF-7. The specific cytotoxicity in both cell lines is significant in comparison with MCF-10A, which is used as a normal cell line (IC50 = 29.4). The IC50 for PTD4-Apoptin was 11.07 µg/ml after 24 h of treatment in MDA-MB-231, while the IC50 of PTD4-Apoptin for MCF7 cells was not significantly different from normal cells. The flow cytometry analysis displayed a significant increment in the apoptosis and late apoptosis number in the MDA-MB-231 cells after treatment with SUMO-PTD4-Apoptin and PTD4-Apoptin protein. PTD4-Apoptin and SUMO-PTD4-Apoptin treatment of MDA-MB-231 cells caused a noteworthy increase in the G0-G1 phase and a reduction in the cell population of S and M/G2.

Conclusion

This study demonstrates that the fusion of PTD4-Apoptin to SUMO-PTD4-Apoptin could provide an effective method to help enhance the expression and solubility of heterologous Apoptin in E. coli. BL21 (DE3).

Caspase-Based Fusion Protein Technology: Substrate Cleavability Described by Computational Modeling and Simulation

The Caspase-based fusion protein technology (CASPON) allows for universal cleavage of fusion tags from proteins of interest to reconstitute the native N-terminus. While the CASPON enzyme has been optimized to be promiscuous against a diversity of N-terminal peptides, the cleavage efficacy for larger proteins can be surprisingly low. We develop an efficient means to rationalize and predict the cleavage efficiency based on a structural representation of the intrinsically disordered N-terminal peptides and their putative interactions with the CASPON enzyme. The number of favorably interacting N-terminal conformations shows a very good agreement with the experimentally observed cleavage efficiency, in agreement with a conformational selection model. The method relies on computationally cheap molecular dynamics simulations to efficiently generate a diverse collection of N-terminal conformations, followed by a simple fitting procedure into the CASPON enzyme. It can be readily used to assess the CASPON cleavability a priori.

High-yield and cost-effective biosynthesis process for producing antimicrobial peptide AA139

AA139, a variant of natural antimicrobial peptide (AMP) arenicin-3, displayed potent activity against multidrug-resistant (MDR) and extensively drug-resistant (XDR) Gram-negative bacteria. Nevertheless, there were currently few reports on the bioprocess of AA139, and the yields were less than 5 mg/L. Additionally, it was difficult and expensive to prepare AA139 through chemical synthesis due to its complex structure. These factors have impeded the further research and following clinical application of AA139. Here, we reported a bioprocess for the preparation of AA139, which was expressed in Escherichia coli (E. coli) BL21 (DE3) intracellularly in a soluble form via SUMO (small ubiquitin-related modifier) fusion technology. Then, recombinant AA139 (rAA139, refer to AA139 obtained by recombinant expression in this study) was obtained through the simplified downstream process, which was rationally designed in accordance with the physicochemical characteristics. Subsequently, the expression level of the interest protein was increased by 54% after optimization of high cell density fermentation (HCDF). Finally, we obtained a yield of 56 mg of rAA139 from 1 L culture with a purity of 98%, which represented the highest reported yield of AA139 to date. Furthermore, various characterizations were conducted to confirm the molecular mass, disulfide bonds, and antimicrobial activity of rAA139.

Soluble and insoluble expression of recombinant human interleukin-2 protein using pET expression vector in Escherichia coli

Interleukin-2 has emerged as a potent protein-based drug to treat various cancers, AIDS, and autoimmune diseases. Despite its immense requirement, the production procedures are inefficient to meet the demand. Therefore, efficient production procedures must be adopted to improve protein yield and decrease procedural loss. This study analyzed cytoplasmic and periplasmic IL-2 expression for increased protein yield and significant biological activity. The study is focused on cloning IL-2 into a pET-SUMO and pET-28a vector that expresses IL-2 in soluble form and inclusion bodies, respectively. Both constructs were expressed into different E. coli expression strains, but the periplasmic and cytoplasmic expression of IL-2 was highest in overnight culture in Rosetta 2 (DE3). Therefore, E. coli Rosetta 2 (DE3) was selected for large-scale production and purification. Purified IL-2 was characterized by SDS-PAGE and western blotting, while its biological activity was determined using MTT bioassay. The results depict that the periplasmic and cytoplasmic IL-2 achieved adequate purification, yielding 0.86 and 0.51 mg/mL, respectively, with significant cytotoxic activity of periplasmic and cytoplasmic IL-2. Periplasmic IL-2 has shown better yield and significant biological activity in vitro which describes its attainment of native protein structure and function.

A systematic analysis of affinity tags in the haloarchaeal expression system, Haloferax volcanii for protein purification

Extremophilic proteins are valuable in various fields, but their expression can be challenging in traditional hosts like Escherichia coli due to misfolding and aggregation. Haloferax volcanii (H. volcanii), a halophilic expression system, offers a solution. This study examined cleavable and non-cleavable purification tags at both the N- and C-termini when fused with the superfolder green fluorescent protein (sfGFP) in H. volcanii. Our findings reveal that an N-terminal 8xHis-tag or Strep-tag®II significantly enhances protein production, purity, and yield in H. volcanii. Further experiments with mCherry and halophilic alcohol dehydrogenase (ADH) showed improved expression and purification yields when the 8xHis-tag or Strep-tag®II was positioned at the C-terminus for mCherry and at the N-terminus for ADH. Co-positioning 8xHis-tag and Twin-Strep-tag® at the N-terminus of sfGFP, mCherry, and ADH yielded significantly enhanced results. These findings highlight the importance of thoughtful purification tag design and selection in H. volcanii, providing valuable insights for improving protein production and purification with the potential to advance biotechnological applications.

Improvement of Carotenoids' Production by Increasing the Activity of Beta-Carotene Ketolase with Different Strategies

Canthaxanthin is an important antioxidant with wide application prospects, and β-carotene ketolase is the key enzyme involved in the biosynthesis of canthaxanthin. However, the challenge for the soluble expression of β-carotene ketolase is that it hinders the large-scale production of carotenoids such as canthaxanthin and astaxanthin. Hence, this study employed several strategies aiming to improve the soluble expression of β-carotene ketolase and its activity, including selecting optimal expression vectors, screening induction temperatures, adding soluble expression tags, and adding a molecular chaperone. Results showed that all these strategies can improve the soluble expression and activity of β-carotene ketolase in Escherichia coli. In particular, the production of soluble β-carotene ketolase was increased 8 times, with a commercial molecular chaperon of pG-KJE8, leading to a 1.16-fold enhancement in the canthaxanthin production from β-carotene. Interestingly, pG-KJE8 could also enhance the soluble expression of β-carotene ketolase derived from eukaryotic microalgae. Further research showed that the production of canthaxanthin and echinenone was significantly improved by as many as 30.77 times when the pG-KJE8 was added, indicating the molecular chaperone performed differently among different β-carotene ketolase. This study not only laid a foundation for further research on the improvement of β-carotene ketolase activity but also provided new ideas for the improvement of carotenoid production.

Development of a tag-free plant-made interferon gamma production system with improved therapeutic efficacy against viruses

Plants offer a promising platform for cost-effective production of biologically active therapeutic glycoproteins. In previous studies, we have developed a plant expression system based on Bamboo mosaic virus (BaMV) by incorporating secretory signals and an affinity tag, which resulted in notably enhanced yields of soluble and secreted fusion glycoproteins (FGs) in Nicotiana benthamiana. However, the presence of fusion tags on recombinant glycoproteins is undesirable for biomedical applications. This study aimed to develop a refined expression system that can efficiently produce tag-free glycoproteins in plants, with enhanced efficacy of mature interferon gamma (mIFNγ) against viruses. To accommodate the specific requirement of different target proteins, three enzymatically or chemically cleavable linkers were provided in this renovated BaMV-based expression system. We demonstrated that Tobacco etch virus (TEV) protease could process the specific cleavage site (LTEV) of the fusion protein, designated as SSExtHis(SP)10LTEV-mIFNγ, with optimal efficiency under biocompatible conditions to generate tag-free mIFNγ glycoproteins. The TEV protease and secretory-affinity tag could be effectively removed from the target mIFNγ glycoproteins through Ni2+-NTA chromatography. In addition, the result of an antiviral assay showed that the tag-free mIFNγ glycoproteins exhibited enhanced biological properties against Sindbis virus, with comparable antiviral activity of the commercialized HEK293-expressed hIFNγ. Thus, the improved BaMV-based expression system developed in this study may provide an alternative strategy for producing tag-free therapeutic glycoproteins intended for biomedical applications.

Use of the selected metal-dependent enzymes for exploring applicability of human annexin A1 as a purification tag

Several fusion tags have been developed for non-chromatographic fusion protein purification. Previously, we identified that human annexin A1 as a novel N-terminal purification tag was used for purifying the fusion proteins produced in Escherichia coli through precipitation in 10 mM Ca2+ buffer, and redissolution of the precipitate in 15 mM EDTA buffer. In this work, we selected four metal-dependent enzymes including E. coli 5-aminolevulinate dehydratase, yeast 3-hydroxyanthranilate 3,4-dioxygenase, maize serine racemase and copper amine oxidase for investigating the annexin A1 tag applicability. Fusion of the His6-tag or the enzyme changed the behavior of precipitation-redissolution. The relatively high recovery yields of three tagged enzymes with the improved purities were obtained through two rounds of purification, whereas low recovery yield of the annexin A1 tagged maize amine oxidase was prepared. The added EDTA displayed different abilities to redissolve the fusion proteins precipitates in two precipitation-redissolution cycles. It inactivated three enzymes and obviously inhibited the activity of the fused maize serine racemase. Based on current findings, we believe that four enzymes could be applied for evaluating applicability of the proteins or peptides as affinity tags for chromatographic purification in a calcium dependent manner.

Advancing large-scale production of TEV protease through an innovative NT* tag-based fusion construct

Tobacco etch virus Protease (TEVp), a cysteine protease, is renowned for its remarkable specific proteolysis, making it an invaluable tool for removing fusion tags from recombinant proteins. However, TEV protease's inherent insolubility limits its broad application. Fusion constructs like an N-terminal MBP fusion, known for its improved solubility, have been employed for TEVp production to address this issue. In this study, we fused the TEVp with the N-terminal domain of the spider silk protein, specifically utilizing a charge-reversed mutant (D40K/K65D) of the N-terminal domain of major ampullate spidroin-1 protein from Euprosthenops australis, referred to as NT*. This fusion construct contains a TEVp cleavage site, enabling intracellular self-processing and the release of a His7-tagged protease. The significant increase in soluble protein expression allowed us to purify approximately 90-100 mg of TEVp from a 1-L E. coli culture, surpassing previous findings by a considerable margin. The enzyme remained stable and catalytically active even after several months of storage in a deep freezer (-80 °C).

Design and Evolution of Enhanced Peptide-Peptide Ligation for Modular Transglutaminase Assembly

Robust and precise tools are needed to enhance the functionality and resilience of synthetic nanoarchitectures. Here, we have employed directed evolution and rational design to build a fast-acting molecular superglue from a bacterial adhesion protein. We have generated the SnoopLigase2 coupling system, a genetically encoded route for efficient transamidation between SnoopTag2 and DogTag2 peptides. Each peptide was selected for rapid reaction by phage display screening. The optimized set allows more than 99% completion and is compatible with diverse buffers, pH values, and temperatures, accelerating the reaction over 1000-fold. SnoopLigase2 directs a specific reaction in the mammalian secretory pathway, allowing covalent display on the plasma membrane. Transglutaminase 2 (TG2) has a network of interactions and substrates amidst the mammalian cell surface and extracellular matrix. We expressed a modified TG2 with resistance to oxidative inactivation and minimal self-reactivity. SnoopLigase2 enables TG2 functionalization with transforming growth factor alpha (TGFα) in routes that would be impossible through genetic fusion. The TG2:TGFα conjugate retained transamidase activity, stably anchored TGFα for signal activation in the extracellular environment, and reprogrammed cell behavior. This modular toolbox should create new opportunities for molecular assembly, both for novel biomaterials and complex cellular environments.

Non-covalent binding tags for batch and flow biocatalysis

Enzyme immobilization offers considerable advantage for biocatalysis in batch and continuous flow reactions. However, many currently available immobilization methods require that the surface of the carrier is chemically modified to allow site specific interactions with their cognate enzymes, which requires specific processing steps and incurs associated costs. Two carriers (cellulose and silica) were investigated here, initially using fluorescent proteins as models to study binding, followed by assessment of industrially relevant enzyme performance (transaminases and an imine reductase/glucose oxidoreductase fusion). Two previously described binding tags, the 17 amino acid long silica-binding peptide from the Bacillus cereus CotB protein and the cellulose binding domain from the Clostridium thermocellum, were fused to a range of proteins without impairing their heterologous expression. When fused to a fluorescent protein both tags conferred high avidity specific binding with their respective carriers (low nanomolar K_d values). The CotB peptide (CotB1p) induced protein aggregation in the transaminase and imine reductase/glucose oxidoreductase fusions when incubated with the silica carrier. The Clostridium thermocellum cellulose binding domain (CBDclos) allowed immobilization of all the proteins tested, but immobilization led to loss of enzymatic activity in the transaminases (< 2-fold) and imine reductase/glucose oxidoreductase fusion (> 80%). A transaminase-CBDclos fusion was then successfully used to demonstrate the application of the binding tag in repetitive batch and a continuous-flow reactor.

Engineering acyl-ACP reductase with fusion tags enhances alka(e)ne synthesis in Escherichia coli

Alka(e)nes are high-value chemicals with a potentially broad range of industrial applications because of their following advantages: (1) chemical and structural resemblance to petroleum hydrocarbons and (2) higher energy density and hydrophobicity than those of other biofuels. The low yield of bio-alka(e)nes, however, hinders their commercial application. The activity and solubility of acyl carrier protein (ACP) reductase (AAR) affect alka(e)ne biosynthesis in cyanobacteria. The enhancement of the activity and concentration of soluble AAR through genetic and process engineering can improve bio-alka(e)ne yield. Although fusion tags are used to enhance the expression or solubility of recombinant proteins, their effectiveness in improving the production of bio-alka(e)nes has not yet been reported. Fusion tags can be used to improve the amount or activity of soluble AAR in Escherichia coli and to increase the yield of alka(e)nes in E. coli cells co-expressing aldehyde deformylating oxygenase (ADO). Hence, in the present study, histidine (His₆/His₁₂), thioredoxin (Trx), maltose-binding protein (MBP), and N-utilization substance (NusA) were used as AAR fusion tags. The strain expressing SeAAR with His₁₂ tag and NpADO showed a 7.2-fold higher yield of alka(e)nes than the strain expressing AAR without fusion tag and NpADO. The highest titer of alka(e)nes (194.78 mg/L) was achieved with the His₁₂ tag.

A histidine-rich fusion tag enables real-time monitoring of recombinant protein expression by Pauly reaction-based colorimetric assay

Commercially available recombinant expression systems always use fusion tags to facilitate target protein purification and SDS-PAGE analysis followed by Coomassie Brilliant Blue (CBB) staining is the classical method to validate the expression level of target protein, which is time-consuming, although not very laborious. Previously, we found that a histidine-rich elastin-like polypeptide (HRELP) tag could make its fusion proteins being quickly and specifically stained with Pauly's reagent. In this study, we designed a Pauly reaction-based colorimetric assay to real-time monitoring of the expression level of recombinant protein tagged HRELP and found that the absorption value of post-induction E. coli cells stained with Pauly's reagent correlated well with both the band intensity of the target protein from Pauly's reagent-stained and CBB-stained gels. Moreover, we found the colorimetric assay could also be helpful to roughly estimate the expression efficiency by using a poly-histidine-tagged protein, which has only 1.17% histidine residue. In our opinion, Pauly reaction-based colorimetric assay could significantly shorten the time to validate the over-expression of recombinant protein tagged with either HRELP or poly-histidine. And HRELP seemed to be an ideal fusion tag for it can not only facilitate protein purification but also simplify protein detection.

Tuning an efficient Escherichia coli whole-cell catalyst expressing l-pantolactone dehydrogenase for the biosynthesis of d-(-)-pantolactone

d-(-)-Pantolactone (DPL) is a key intermediate for the production of d-(+)-pantothenate (vitamin B5). Deracemization of d,l-pantolactone (D,L-PL) through oxidizing l-(+)-pantolactone (LPL) to ketopantoyl lactone (KPL) and subsequently reducing KPL to DPL is a promising route for synthesizing DPL. Herein, a newly mined l-pantolactone dehydrogenase from Rhodococcus hoagie (RhoLPLDH) was used for the oxidative dehydrogenation of LPL. To alleviate inclusion bodies formed by membrane-bound RhoLPLDH intracellular expression in E. coli, strategies involving chaperone assistance and decreasing induction temperature were used to achieve RhoLPLDH soluble expression. To enhance its activity, directed evolution and hydrophilicity-based engineering yielded increased catalytic activity and thermostability. 1M LPL was efficiently converted to KPL by engineering strain CM5 co-expressing RhoLPLDH^{L254I/V241I/I156L/F224Q/N164K} and chaperone. A "two stages in one-pot" method was employed in deracemization of 1M D,L-PL with 91.2% yield. These results demonstrated that CM5 catalyst exhibits great potential in enzyme cascade deracemization for the production of DPL.

Detection of human annexin A1 as the novel N-terminal tag for separation and purification handle

BACKGROUND

Several fusion tags for separation handle have been developed, but the fused tag for simply and cheaply separating the target protein is still lacking.

RESULTS

Separation conditions for the human annexin A1 (hanA1) tagged emerald green fluorescent protein (EmGFP) in Escherichia coli were optimized via precipitation with calcium chloride (CaCl₂) and resolubilization with ethylenediamine tetraacetic acid disodium salt (EDTA-Na₂). The HanA1-EmGFP absorbing with other three affinity matrix was detected, only it was strongly bound to heparin Sepharose. The separation efficiency of the HanA1-EmGFP was comparable with purification efficiency of the His6-tagged HanA1-EmGFP via metal ion affinity chromatography. Three fluorescent proteins for the EmGFP, mCherry red fluorescent protein and flavin-binding cyan-green fluorescent protein LOV from Chlamydomonas reinhardtii were used for naked-eye detection of the separation and purification processes, and two colored proteins including a red protein for a Vitreoscilla hemoglobin (Vhb), and a brown protein for maize sirohydrochlorin ferrochelatase (mSF) were used for visualizing the separation process. The added EDTA-Na₂ disrupted the Fe-S cluster in the mSF, but it showed little impact on heme in Vhb.

CONCLUSIONS

The selected five colored proteins were efficient for detecting the applicability of the highly selective hanA1 for fusion separation and purification handle. The fused hanA1 tag will be potentially used for simple and cheap affinity separation of the target proteins in industry and diagnosis.

Engineering sigma factors and chaperones for enhanced heterologous lipoxygenase production in Escherichia coli

BACKGROUND

Lipoxygenase (EC. 1.13.11.12, LOX) can catalyze the addition of oxygen into polyunsaturated fatty acids to produce hydroperoxides, which are widely used in the food, chemical, and pharmaceutical industries. In recent years, the heterologous production of LOX by Escherichia coli has attracted extensive attention. However, overexpressed recombinant LOX in E. coli aggregates and forms insoluble inclusion bodies owing to protein misfolding.

RESULTS

In this study, a split green fluorescent protein-based screening method was developed to screen sigma (σ) factors and molecular chaperones for soluble LOX expression. Three mutant libraries of Skp, GroES, and RpoH was analyzed using the high-throughput screening method developed herein, and a series of mutants with significantly higher yield of soluble heterologous LOX were obtained. The soluble expression level of LOX in the isolated mutants increased by 4.2- to 5.3-fold. Further, the highest LOX activity (up to 6240 ± 269 U·g-DCW^-1) was observed in E. coli REopt, with the regulatory factor mutants, RpoH and GroES. Based on RNA-Seq analysis of the selected strains, E. coli Eopt, E. coli Sopt, E. coli Ropt, and wild type, amino acid substitutions in σ factors and molecular chaperones regulated the expression level of genes related to gene replication, recombination, and repair. Furthermore, the regulatory factor mutants were identified to be beneficial to the soluble expression of two other heterologous proteins, amylase and bone morphological protein 12.

CONCLUSION

In this study, a high-throughput screening method was developed for improved soluble LOX expression. The obtained positive mutants of the regulatory factor were analyzed and employed for the expression of other heterologous proteins, thus providing a potential solution for the inclusion-body protein.

Fusion Tag Design Influences Soluble Recombinant Protein Production in Escherichia coli

Fusion protein technologies to facilitate soluble expression, detection, or subsequent affinity purification in Escherichia coli are widely used but may also be associated with negative consequences. Although commonly employed solubility tags have a positive influence on titers, their large molecular mass inherently results in stochiometric losses of product yield. Furthermore, the introduction of affinity tags, especially the polyhistidine tag, has been associated with undesirable changes in expression levels. Fusion tags are also known to influence the functionality of the protein of interest due to conformational changes. Therefore, particularly for biopharmaceutical applications, the removal of the fusion tag is a requirement to ensure the safety and efficacy of the therapeutic protein. The design of suitable fusion tags enabling the efficient manufacturing of the recombinant protein remains a challenge. Here, we evaluated several N-terminal fusion tag combinations and their influence on product titer and cell growth to find an ideal design for a generic fusion tag. For enhancing soluble expression, a negatively charged peptide tag derived from the T7 bacteriophage was combined with affinity tags and a caspase-2 cleavage site applicable for CASPase-based fusiON (CASPON) platform technology. The effects of each combinatorial tag element were investigated in an integrated manner using human fibroblast growth factor 2 as a model protein in fed-batch lab-scale bioreactor cultivations. To confirm the generic applicability for manufacturing, seven additional pharmaceutically relevant proteins were produced using the best performing tag of this study, named CASPON-tag, and tag removal was demonstrated.

Plant Antimicrobial Peptides (PAMPs): Features, Applications, Production, Expression, and Challenges

The quest for an extraordinary array of defense strategies is imperative to reduce the challenges of microbial attacks on plants and animals. Plant antimicrobial peptides (PAMPs) are a subset of antimicrobial peptides (AMPs). PAMPs elicit defense against microbial attacks and prevent drug resistance of pathogens given their wide spectrum activity, excellent structural stability, and diverse mechanism of action. This review aimed to identify the applications, features, production, expression, and challenges of PAMPs using its structure-activity relationship. The discovery techniques used to identify these peptides were also explored to provide insight into their significance in genomics, transcriptomics, proteomics, and their expression against disease-causing pathogens. This review creates awareness for PAMPs as potential therapeutic agents in the medical and pharmaceutical fields, such as the sensitive treatment of bacterial and fungal diseases and others and their utilization in preserving crops using available transgenic methods in the agronomical field. PAMPs are also safe to handle and are easy to recycle with the use of proteases to convert them into more potent antimicrobial agents for sustainable development.

Insect Cells-Baculovirus System for the Production of Difficult to Express Proteins: From Expression Screening for Soluble Constructs to Protein Quality Control

Rapid preparation of proteins for functional and structural analysis is a major challenge both in academia and industry. The number potential targets continuously increases and many are difficult to express proteins which, when produced in bacteria, result in insoluble and/or misfolded recombinant proteins, protein aggregates, or unusable low protein yield. We focus here on the baculovirus expression vector system which is now commonly used for heterologous production of human targets. This chapter describes simple and cost-effective protocols that enable iterative cycles of construct design, expression screening and optimization of protein production. We detail time- and cost-effective methods for generation of baculoviruses by homologous recombination and titer evaluation. Handling of insect cell cultures and preparation of bacmid for cotransfection are also presented.

Improving Protein Quantity and Quality—The Next Level of Plant Molecular Farming

Plants offer several unique advantages in the production of recombinant pharmaceuticals for humans and animals. Although numerous recombinant proteins have been expressed in plants, only a small fraction have been successfully put into use. The hugely distinct expression systems between plant and animal cells frequently cause insufficient yield of the recombinant proteins with poor or undesired activity. To overcome the issues that greatly constrain the development of plant-produced pharmaceuticals, great efforts have been made to improve expression systems and develop alternative strategies to increase both the quantity and quality of the recombinant proteins. Recent technological revolutions, such as targeted genome editing, deconstructed vectors, virus-like particles, and humanized glycosylation, have led to great advances in plant molecular farming to meet the industrial manufacturing and clinical application standards. In this review, we discuss the technological advances made in various plant expression platforms, with special focus on the upstream designs and milestone achievements in improving the yield and glycosylation of the plant-produced pharmaceutical proteins.

Peptide Tags and Domains for Expression and Detection of Mammalian Membrane Proteins at the Cell Surface

Normal functions of cell-surface proteins are dependent on their proper trafficking from the site of synthesis to the cell surface. Transport proteins mediating solute transfer across the plasma membrane constitute an important group of cell-surface proteins. There are several diseases resulting from mutations in these proteins that interfere with their transport function or trafficking, depending on the impact of the mutations on protein folding and structure. Recent advances in successful treatment of some of these diseases with small molecules which correct the mutations-induced folding and structural changes underline the need for detailed structural and biophysical characterization of membrane proteins. This requires methods to express and purify these proteins using heterologous expression systems. Here, using the solute carrier (SLC) transporter NaCT (Na⁺-coupled citrate transporter) as an example, we describe experimental strategies for this approach. We chose this example because several mutations in NaCT, distributed throughout the protein, cause a severe neurologic disease known as early infantile epileptic encephalopathy-25 (EIEE-25). NaCT was modified with various peptide tags, including a RGS-His₁₀, a Twin-Strep, the SUMOstar domain, and an enhanced green fluorescent protein (EGFP), each alone or in various combinations. When transiently expressed in HEK293 cells, recombinant NaCT proteins underwent complex glycosylation, compartmentalized with the plasma membrane, and exhibited citrate transport activity similar to the nontagged protein. Surface NaCT expression was enhanced by the presence of SUMOstar on the N-terminus. The dual-purpose peptide epitopes RGS-His₁₀ and Twin-Strep facilitated detection of NaCT by immunohistochemistry and western blot and may serve useful tags for affinity purification. This approach sets the stage for future analyses of mutant NaCT proteins that may alter protein folding and trafficking. It also demonstrates the capability of a transient mammalian cell expression system to produce human NaCT of sufficient quality and quantity to augment future biophysical and structural studies and drug discovery efforts.

Overcoming the Solubility Problem in E. coli: Available Approaches for Recombinant Protein Production

Despite the importance of recombinant protein production in the academy and industrial fields, many issues concerning the expression of soluble and homogeneous products are still unsolved. Several strategies were developed to overcome these obstacles; however, at present, there is no magic bullet that can be applied for all cases. Indeed, several key expression parameters need to be evaluated for each protein. Among the different hosts for protein expression, Escherichia coli is by far the most widely used. In this chapter, we review many of the different tools employed to circumvent protein insolubility problems.

Construction, Investigation and Application of TEV Protease Variants with Improved Oxidative Stability

Tobacco etch virus protease (TEVp) is a useful tool for removing fusion tags, but wild-type TEVp is less stable under oxidized redox state. In this work, we introduced and combined C19S, C110S and C130S into TEVp variants containing T17S, L56V, N68D, I77V and S135G to improve protein solubility, and S219V to inhibit self-proteolysis. The solubility and cleavage activity of the constructed variants in Escherichia coli strains including BL21(DE3), BL21(DE3)pLys, Rossetta(DE3) and Origami(DE3) under the same induction conditions were analyzed and compared. The desirable soluble amounts, activity, and oxidative stability were identified to be reluctantly favored in the TEVp. Unlike C19S, C110S and C130S hardly impacted on decreasing protein solubility in the BL21(DE3), but they contributed to improved tolerance to the oxidative redox state in vivo and in vitro. After two fusion proteins were cleaved by purified TEVp protein containing double mutations under the oxidized redox state, the refolded disulfide-rich bovine enterokinase catalytic domain or maize peroxidase with enhanced yields were released from the regenerated amorphous cellulose via affinity absorption of the cellulose-binding module as the affinity tag.

Cytotoxicity of Frutalin on Distinct Cancer Cells Is Independent of Its Glycosylation

Frutalin is a plant lectin with beneficial immunobiological action, although the access to its active form is still restricted. Moreover, there is a knowledge gap on isoform activity and glycosylation impact on its bioactivity, and recombinant production protocols were seen as ineffective. Here, a simpler and faster production and purification protocol was developed, attaining a yield of purified frutalin 3.3-fold higher than that obtained previously. Hemagglutination assays confirmed that this frutalin isoform could not agglutinate rabbit erythrocytes, while maintaining the native tetrameric structure, as indicated by DLS analysis, and strong interaction with methyl-alpha-galactose, in fluorescence spectroscopy studies. The cytotoxicity of the recombinant frutalin isoform was shown in a broad panel of human cancer cells: colon (HCT116), melanoma (A375), triple-negative breast cancer (MDA-MB-231), and ovarian (IGROV-1). Treatment with 8.5–11.8 μM TrxFTL reduced proliferation of all cancer cells to half in 48 h. This anti-proliferative effect encompasses the p53 pathway since it was significantly reduced in p53-null colon cancer cells (HCT116 p53^−/−; GI₅₀ of 25.0 ± 3.0 μM), when compared to the isogenic p53-positive cells (HCT116 p53^+/+; GI₅₀ of 8.7 ± 1.8 μM; p < 0.002). This recombinantly produced frutalin isoform has relevant cytotoxic effect and its biological activity is not dependent on glycosylation. The developed E. coli production and purification protocol generates high yield of non-glycosylated frutalin isoform with potent cytotoxic activity, enabling the development of novel anticancer p53-targeting therapies.

Development of a Fast Organic Extraction-Precipitation Method for Improved Purification of Elastin-Like Polypeptides That Is Independent of Sequence and Molecular Weight

Elastin-like polypeptides (ELP), an increasingly popular tag for protein purification, commonly rely upon inverse transition cycling (ITC) to exploit their lower critical solution temperature characteristics for purification. While considerably faster than chromatography, ITC is still time consuming and often fails to remove host cell contaminants to an acceptable level for in vivo experiments. Here, we present a rapid purification workflow for ELP of broadly varying molecular weight and sequence using a polar organic solvent extraction and precipitation strategy. Four different ELP purification methods were directly compared for their ability to remove host cell protein, nucleic acids, and lipopolysaccharide (LPS) contaminants using a model ELP. On the basis of these findings, an optimized extraction-precipitation method was developed that gave highly pure ELP from bacterial pellets in approximately 2.5 h while removing major host cell contaminants, including LPS to levels below 1 EU/mL, to produce highly pure material that is suitable for in vivo applications. Application of this method to the rapid purification of an ELP-epidermal growth factor fusion gave an isolate that retained its capacity to bind to epidermal growth factor receptor positive cells, thereby demonstrating that this method is capable of producing a functional construct after purification by organic extraction-precipitation.

Easy Synthesis of Complex Biomolecular Assemblies: Wheat Germ Cell-Free Protein Expression in Structural Biology

Cell-free protein synthesis (CFPS) systems are gaining more importance as universal tools for basic research, applied sciences, and product development with new technologies emerging for their application. Huge progress was made in the field of synthetic biology using CFPS to develop new proteins for technical applications and therapy. Out of the available CFPS systems, wheat germ cell-free protein synthesis (WG-CFPS) merges the highest yields with the use of a eukaryotic ribosome, making it an excellent approach for the synthesis of complex eukaryotic proteins including, for example, protein complexes and membrane proteins. Separating the translation reaction from other cellular processes, CFPS offers a flexible means to adapt translation reactions to protein needs. There is a large demand for such potent, easy-to-use, rapid protein expression systems, which are optimally serving protein requirements to drive biochemical and structural biology research. We summarize here a general workflow for a wheat germ system providing examples from the literature, as well as applications used for our own studies in structural biology. With this review, we want to highlight the tremendous potential of the rapidly evolving and highly versatile CFPS systems, making them more widely used as common tools to recombinantly prepare particularly challenging recombinant eukaryotic proteins.

Biological drug therapy for ocular angiogenesis: Anti-VEGF agents and novel strategies based on nanotechnology

Currently, biological drug therapy for ocular angiogenesis treatment is based on the administration of anti‐VEGF agents via intravitreal route. The molecules approved with this purpose for ocular use include pegaptanib, ranibizumab, and aflibercept, whereas bevacizumab is commonly off‐label used in the clinical practice. The schedule dosage involves repeated intravitreal injections of anti‐VEGF agents to achieve and maintain effective concentrations in retina and choroids, which are administrated as solutions form. In this review article, we describe the features of different anti‐VEGF agents, major challenges for their ocular delivery and the nanoparticles in development as delivery system of them. In this way, several polymeric and lipid nanoparticles are explored to load anti‐VEGF agents with the aim of achieving sustained drug release and thus, minimize the number of intravitreal injections required. The main challenges were focused in the loading the molecules that maintain their bioactivity after their release from nanoparticulate system, followed the evaluation of them through studies of formulation stability, pharmacokinetic, and efficacy in in vitro and in vivo models. The analysis was based on the information published in peer‐reviewed published papers relevant to anti‐VEGF treatments and nanoparticles developed as ocular anti‐VEGF delivery system.

Site-specific epitope insertion into recombinant proteins using the MAP tag system

The MAP tag system comprises a 14-residue peptide derived from mouse podoplanin and its high-affinity monoclonal antibody PMab-1. We determined the crystal structure of PMab-1 complexed with the MAP tag peptide and found that the recognition required only the N-terminal 8 residues of MAP tag sequence, enabling the shortening of the tag length without losing the affinity for PMab-1. Furthermore, the structure illustrated that the MAP tag adopts a U-shaped conformation when bound by PMab-1, suggesting that loop-inserted MAP tag would assume conformation compatible with the PMab-1 binding. We inserted the 8-residue MAP tag into multiple loop regions in various proteins including fibronectin type III domain and G-protein-coupled receptors and tested if they maintain PMab-1 reactivity. Despite the conformational restraints forced by the insertion position, all MAP-inserted mutants were expressed well in mammalian cells at levels comparable to the non-tagged proteins. Furthermore, the binding by PMab-1 was fully maintained even for the mutant where MAP tag was inserted at a structurally restricted β-hairpin, indicating that the MAP tag system has unique feature that allows placement in the middle of protein domain at desired locations. Our results indicate the versatile utility of the MAP tag system in 'site-specific epitope insertion' application.

Recombinant expression of HIV-1 protease using soluble fusion tags in Escherichia coli: A vital tool for functional characterization of HIV-1 protease

HIV-1 protease expression in the laboratory is demanding because of its high cytotoxicity, making it difficult to express in bacterial expression systems such as Escherichia coli. To overcome these challenges, HIV-1 protease fusion with solubility enhancing tags helps to mitigate its cytotoxic effect and drive its expression as a soluble protein. Therefore, this review focuses on the expression of bioactive HIV-1 protease using solubility-enhancing fusion tags in Escherichia coli and summarises the characteristic features of the different common fusion tags that have been used in the expression of HIV-1 protease. This review will assist researchers with their choice of protein fusion tag for HIV-1 protease expression.

High-Throughput Optimization of Recombinant Protein Production in Microfluidic Gel Beads

Efficient production hosts are a key requirement for bringing biopharmaceutical and biotechnological innovations to the market. In this work, a truly universal high-throughput platform for optimization of microbial protein production is described. Using droplet microfluidics, large genetic libraries of strains are encapsulated into biocompatible gel beads that are engineered to selectively retain any protein of interest. Bead-retained products are then fluorescently labeled and strains with superior production titers are isolated using flow cytometry. The broad applicability of the platform is demonstrated by successfully culturing several industrially relevant bacterial and yeast strains and detecting peptides or proteins of interest that are secreted or released from the cell via autolysis. Lastly, the platform is applied to optimize cutinase secretion in Komagataella phaffii (Pichia pastoris) and a strain with 5.7-fold improvement is isolated. The platform permits the analysis of >10⁶ genotypes per day and is readily applicable to any protein that can be equipped with a His₆ -tag. It is envisioned that the platform will be useful for large screening campaigns that aim to identify improved hosts for large-scale production of biotechnologically relevant proteins, thereby accelerating the costly and time-consuming process of strain engineering.

Production of Circularly Permuted Caspase-2 for Affinity Fusion-Tag Removal: Cloning, Expression in Escherichia coli, Purification, and Characterization

Caspase-2 is the most specific protease of all caspases and therefore highly suitable as tag removal enzyme creating an authentic N-terminus of overexpressed tagged proteins of interest. The wild type human caspase-2 is a dimer of heterodimers generated by autocatalytic processing which is required for its enzymatic activity. We designed a circularly permuted caspase-2 (cpCasp2) to overcome the drawback of complex recombinant expression, purification and activation, cpCasp2 was constitutively active and expressed as a single chain protein. A 22 amino acid solubility tag and an optimized fermentation strategy realized with a model-based control algorithm further improved expression in Escherichia coli and 5.3 g/L of cpCasp2 in soluble form were obtained. The generated protease cleaved peptide and protein substrates, regardless of N-terminal amino acid with high activity and specificity. Edman degradation confirmed the correct N-terminal amino acid after tag removal, using Ubiquitin-conjugating enzyme E2 L3 as model substrate. Moreover, the generated enzyme is highly stable at -20 °C for one year and can undergo 25 freeze/thaw cycles without loss of enzyme activity. The generated cpCasp2 possesses all biophysical and biochemical properties required for efficient and economic tag removal and is ready for a platform fusion protein process.

Advances in Recombinant Lipases: Production, Engineering, Immobilization and Application in the Pharmaceutical Industry

Lipases are one of the most used enzymes in the pharmaceutical industry due to their efficiency in organic syntheses, mainly in the production of enantiopure drugs. From an industrial viewpoint, the selection of an efficient expression system and host for recombinant lipase production is highly important. The most used hosts are Escherichia coli and Komagataella phaffii (previously known as Pichia pastoris) and less often reported Bacillus and Aspergillus strains. The use of efficient expression systems to overproduce homologous or heterologous lipases often require the use of strong promoters and the co-expression of chaperones. Protein engineering techniques, including rational design and directed evolution, are the most reported strategies for improving lipase characteristics. Additionally, lipases can be immobilized in different supports that enable improved properties and enzyme reuse. Here, we review approaches for strain and protein engineering, immobilization and the application of lipases in the pharmaceutical industry.

RIEDL tag: A novel pentapeptide tagging system for transmembrane protein purification

Affinity tag systems are an essential tool in biochemistry, biophysics, and molecular biology. Although several different tag systems have been developed, the epitope tag system, composed of a polypeptide “tag” and an anti-tag antibody, is especially useful for protein purification. However, almost all tag sequences, such as the FLAG tag, are added to the N- or C-termini of target proteins, as tags inserted in loops tend to disrupt the functional structure of multi-pass transmembrane proteins. In this study, we developed a novel “RIEDL tag system,” which is composed of a peptide with only five amino acids (RIEDL) and an anti-RIEDL monoclonal antibody (mAb), LpMab-7. To investigate whether the RIEDL tag system is applicable for protein purification, we conducted the purification of two kinds of RIEDL-tagged proteins using affinity column chromatography: whale podoplanin (wPDPN) with an N-terminal RIEDL tag (RIEDL-wPDPN) and human CD20 with an internal RIEDL tag insertion (CD20-₁₆₉RIEDL₁₇₀). Using an LpMab-7-Sepharose column, RIEDL-wPDPN and CD20-₁₆₉RIEDL₁₇₀ were efficiently purified in one-step purification procedures, and were strongly detected by LpMab-7 using Western blot and flow cytometry. These results show that the RIEDL tag system can be useful for the detection and one-step purification of membrane proteins when inserted at either the N-terminus or inserted in an internal loop structure of multi-pass transmembrane proteins.

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We established a novel RIEDL tag system, composed of RIEDL peptide and LpMab-7 mAb.

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The RIEDL tag system is applicable for protein purification, as well as FCM and WB.

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The RIEDL tag, inserted into a loop structure of CD20, was detected by LpMab-7.

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RIEDL-tagged proteins were efficiently purified using 2 × RIEDL peptide.

High-throughput screening of enzyme mutants by comparison of their activity ratios to an enzyme tag

With Escherichia coli alkaline phosphatase (ECAP) as the tag fused to the N-terminus of Pseudomonas Aeruginosa arylsulfatase (PAAS) and its mutants via a flexible linker, the comparison of the activity ratios of an applicable enzyme and its mutants to a suitable enzyme tag in cell lysates of their fused forms was tested for high-throughput (HTP) screening of mutants. After both the induced expression of a fused form and alkaline lysis of the transformed cells in microplate wells, HTP assay of the activities of ECAP and PAAS/mutant was realized via spectrophotometric-dual-enzyme-simultaneous-assay to derive their activity ratio. The successful induced expression of fused forms required ECAP activities higher than 5.3 U/L in cell lysates. Of three representative fused PAAS/mutants in cell lysates, there were similar proteolytic fragments and the comparison of their activity ratios greatly enhanced the recognition of weakly positive mutants. After saturation mutagenesis at M72 of the fused PAAS, the activity ratios of PAAS/mutants to ECAP in cell lysates of their fused forms were proportional to specific activities of their non-fused counterparts in cell lysates by an immunoturbidimetric assay. Therefore, the proposed strategy was absorbing for both HTP screening of mutants and HTP elucidation of sequence-activity relationship of applicable enzymes.

Development of INSOL-tag for proteome-wide protein handling and its application in protein array analysis

Proteomic analysis requires protein tags that enable high-throughput handling; however, versatile tags that can be used in in vitro expression systems are currently lacking. In this study, we developed an insoluble protein tag, INSOL-tag, derived from human transcription factor MafG. The INSOL-tagged target protein is expressed in a eukaryotic in vitro expression system and recovered as a pellet following centrifugation at 19,000 × g for 20 min. Comparisons of the target protein recovery rates of GST-tag and INSOL-tag using 111 cytoplasmic proteins revealed a fourfold increase in the yield of INSOL-tagged proteins. Using 267 cancer antigens purified with INSOL-tag, we subsequently developed an INSOL-CTA array method, for profiling autoantibodies in sera of cancer patients. The detection limit of the array was approximately 11.1 pg IgG, and the correlation with ELISA was high (R²  = .993, .955). Moreover, when autoantibody profiling of digestive cancer patient sera was performed, antigen spreading was observed. These data suggest that INSOL-tag is a versatile tag that can insolubilize a wide range of target proteins. It is therefore expected to become a powerful tool in comprehensive protein preparation for protein arrays, antibody production, and mass spectrometry.

N-terminal fusion of the N-terminal domain of bacterial enzyme I facilitates recombinant expression and purification of the human RNA demethylases FTO and Alkbh5

Various fusion tags are commonly employed to increase the heterologous expression and solubility of aggregation-prone proteins within Escherichia coli. Herein, we present a protocol for efficient recombinant expression and purification of the human RNA demethylases Alkbh5 and FTO. Our method incorporates a novel fusion tag (the N-terminal domain of bacterial enzyme I, EIN) that dramatically increases the solubility of its fusion partner and is promptly removed upon digestion with a protease. The presented protocol allows for the production of mg amounts of Alkbh5 and FTO in 1L of both rich and minimal media. We developed a liquid chromatography-mass spectrometry (LC-MS)-based assay to confirm that both proteins are enzymatically active. Furthermore, the LC-MS method developed here is applicable to other members of the AlkB family of Fe(II)/α-ketoglutarate-dependent dioxygenases. The superior protein yield, afforded by our expression and purification method, will facilitate biochemical investigations into the biological function of the human RNA demethylases and endorse employment of EIN as a broadly applicable fusion tag for recombinant expression projects.

The Pol β variant containing exon α is deficient in DNA polymerase but has full dRP lyase activity

DNA polymerase (Pol) β is a key enzyme in base excision repair (BER), an important repair system for maintaining genomic integrity. We previously reported the presence of a Pol β transcript containing exon α (105-nucleotide) in normal and colon cancer cell lines. The transcript carried an insertion between exons VI and VII and was predicted to encode a ~42 kDa variant of the wild-type 39 kDa enzyme. However, little is known about the biochemical properties of the exon α-containing Pol β (exon α Pol β) variant. Here, we first obtained evidence indicating expression of the 42 kDa exon α Pol β variant in mouse embryonic fibroblasts. The exon α Pol β variant was then overexpressed in E. coli, purified, and characterized for its biochemical properties. Kinetic studies of exon α Pol β revealed that it is deficient in DNA binding to gapped DNA, has strongly reduced polymerase activity and higher Km for dNTP during gap-filling. On the other hand, the 5'-dRP lyase activity of the exon α Pol β variant is similar to that of wild-type Pol β. These results indicate the exon α Pol β variant is base excision repair deficient, but does conduct 5'-trimming of a dRP group at the gap margin. Understanding the biological implications of this Pol β variant warrants further investigation.

An efficient expression tag library based on self-assembling amphipathic peptides

Background

Self-assembling amphipathic peptides (SAPs) may improve protein production or induce the formation of inclusion bodies by fusing them to the N-terminus of proteins. However, they do not function uniformly well with all target enzymes and systematic research on how the composition of SAPs influence the production of fusion protein is still limited.

Results

To improve the efficiency of SAPs, we studied factors that might be involved in SAP-mediated protein production using S1 (AEAEAKAK)₂ as the original SAP and green fluorescent protein (GFP) as the reporter. The results indicate that hydrophobicity and net charges of SAPs play a key role in protein expression. As hydrophobicity regulation tend to cause the formation of insoluble inclusion bodies of protein, an expression tag library composed of SAPs, which varied in net charge (from + 1 to + 20), was constructed based on the random amplification of S1nv1 (ANANARAR)₁₀. The efficiency of the library was validated by polygalacturonate lyase (PGL), lipoxygenase (LOX), l-asparaginase (ASN) and transglutaminase (MTG). To accelerate preliminary screening, each enzyme was fused at the C-terminus with GFP. Among the four enzyme fusions, the SAPs with + 2 – + 6 net charges were optimal for protein expression. Finally, application of the library improved the expression of PGL, LOX, ASN, and MTG by 8.3, 3.5, 2.64, and 3.68-fold relative to that of the corresponding wild-type enzyme, respectively.

Conclusions

This is the first report to study key factors of SAPs as an expression tag to enhance recombinant enzyme production. The SAP library could be used as a novel plug-and-play protein-engineering method to screen for enzymes or proteins with enhanced production.

Electronic supplementary material

The online version of this article (10.1186/s12934-019-1142-9) contains supplementary material, which is available to authorized users.

Purification of proteins with native terminal sequences using a Ni(II)-cleavable C-terminal hexahistidine affinity tag

The role of the termini of protein sequences is often perturbed by remnant amino acids after the specific protease cleavage of the affinity tags and/or by the amino acids encoded by the plasmid at/around the restriction enzyme sites used to insert the genes. Here we describe a method for affinity purification of a metallonuclease with its precisely determined native termini. First, the gene encoding the target protein is inserted into a newly designed cloning site, which contains two self-eliminating BsmBI restriction enzyme sites. As a consequence, the engineered DNA code of Ni(II)-sensitive Ser-X-His-X motif is fused to the 3'-end of the inserted gene followed by the gene of an affinity tag for protein purification purpose. The C-terminal segment starting from Ser mentioned above is cleaved off from purified protein by a Ni(II)-induced protease-like action. The success of the purification and cleavage was confirmed by gel electrophoresis and mass spectrometry, while structural integrity of the purified protein was checked by circular dichroism spectroscopy. Our new protein expression DNA construct is an advantageous tool for protein purification, when the complete removal of affinity or other tags, without any remaining amino acid residue is essential. The described procedure can easily be generalized and combined with various affinity tags at the C-terminus for chromatographic applications.