Production of functional antibody fragments in a vesicle-based eukaryotic cell-free translation system

Synthesis of mono Cytochrome P450 in a modified CHO-CPR cell-free protein production platform

Cytochromes P450 (CYPs) are a group of monooxygenases that can be found in almost all kinds of organisms. For CYPs to receive electrons from co-substrate NADPH, the activity of NADPH-Cytochrome-P450-oxidoreductase (CPR) is required as well. In humans, CYPs are an integral part of liver-based phase-1 biotransformation, which is essential for the metabolization of multiple xenobiotics and drugs. Consequently, CYPs are important players during drug development and therefore these enzymes are implemented in diverse screening applications. For these applications it is usually advantageous to use mono CYP microsomes containing only the CYP of interest. The generation of mono-CYP containing mammalian cells and vesicles is difficult since endogenous CYPs are present in many cell types that contain the necessary co-factors. By obtaining translationally active lysates from a modified CHO-CPR cell line, it is now possible to generate mono CYPs in a cell-free protein synthesis process in a straightforward manner. As a proof of principle, the synthesis of active human CYPs from three different CYP450 gene families (CYP1A2, CYP2B6 and CYP3A4), which are of outstanding interest in industry and academia was demonstrated. Luciferase based activity assays confirm the activity of the produced CYPs and enable the individual adaptation of the synthesis process for efficient cell-free enzyme production. Furthermore, they allow for substrate and inhibitor screenings not only for wild-type CYPs but also for mutants and further CYP isoforms and variants. As an example, the turnover of selected CYP substrates by cell-free synthesized CYPs was demonstrated via an indirect luciferase assay-based screening setup.

State-of-the-Art Approaches to Heterologous Expression of Bispecific Antibodies Targeting Solid Tumors

Bispecific antibodies (bsAbs) are some of the most promising biotherapeutics due to the versatility provided by their structure and functional features. bsAbs simultaneously bind two antigens or two epitopes on the same antigen. Moreover, they are capable of directing immune effector cells to cancer cells and delivering various compounds (radionuclides, toxins, and immunologic agents) to the target cells, thus offering a broad spectrum of clinical applications. Current review is focused on the technologies used in bsAb engineering, current progress and prospects of these antibodies, and selection of various heterologous expression systems for bsAb production. We also discuss the platforms development of bsAbs for the therapy of solid tumors.

One to one comparison of cell-free synthesized erythropoietin conjugates modified with linear polyglycerol and polyethylene glycol

With more than 20 Food and Drug Administration (FDA)-approved poly (ethylene glycol) (PEG) modified drugs on the market, PEG is the gold standard polymer in bioconjugation. The coupling improves stability, efficiency and can prolong blood circulation time of therapeutic proteins. Even though PEGylation is described as non-toxic and non-immunogenic, reports accumulate with data showing allergic reactions to PEG. Since PEG is not only applied in therapeutics, but can also be found in foods and cosmetics, anti-PEG-antibodies can occur even without a medical treatment. Hypersensitivity to PEG thereby can lead to a reduced drug efficiency, fast blood clearance and in rare cases anaphylactic reactions. Therefore, finding alternatives for PEG is crucial. In this study, we present linear polyglycerol (LPG) for bioconjugation as an alternative polymer to PEG. We report the conjugation of LPG and PEG by click-chemistry to the glycoprotein erythropoietin (EPO), synthesized in a eukaryotic cell-free protein synthesis system. Furthermore, the influence of the polymers on EPOs stability and activity on a growth hormone dependent cell-line was evaluated. The similar characteristics of both bioconjugates show that LPGylation can be a promising alternative to PEGylation.

Vesicle-based cell-free synthesis of short and long unspecific peroxygenases

Unspecific peroxygenases (UPOs, EC 1.11.2.1) are fungal enzymes that catalyze the oxyfunctionalization of non-activated hydrocarbons, making them valuable biocatalysts. Despite the increasing interest in UPOs that has led to the identification of thousands of putative UPO genes, only a few of these have been successfully expressed and characterized. There is currently no universal expression system in place to explore their full potential. Cell-free protein synthesis has proven to be a sophisticated technique for the synthesis of difficult-to-express proteins. In this work, we aimed to establish an insect-based cell-free protein synthesis (CFPS) platform to produce UPOs. CFPS relies on translationally active cell lysates rather than living cells. The system parameters can thus be directly manipulated without having to account for cell viability, thereby making it highly adaptable. The insect-based lysate contains translocationally active, ER-derived vesicles, called microsomes. These microsomes have been shown to allow efficient translocation of proteins into their lumen, promoting post-translational modifications such as disulfide bridge formation and N-glycosylations. In this study the ability of a redox optimized, vesicle-based, eukaryotic CFPS system to synthesize functional UPOs was explored. The influence of different reaction parameters as well as the influence of translocation on enzyme activity was evaluated for a short UPO from Marasmius rotula and a long UPO from Agrocybe aegerita. The capability of the CFPS system described here was demonstrated by the successful synthesis of a novel UPO from Podospora anserina, thus qualifying CFPS as a promising tool for the identification and evaluation of novel UPOs and variants thereof.

Medicinal Plants: Guests and Hosts in the Heterologous Expression of High-Value Products

Medicinal plants play an important dual role in the context of the heterologous expression of high-value pharmaceutical products. On the one hand, the classical biochemical and modern omics approaches allowed for the discovery of various genes encoding biosynthetic pathways in medicinal plants. Recombinant DNA technology enabled introducing these genes and regulatory elements into host organisms and enhancing the heterologous production of the corresponding secondary metabolites. On the other hand, the transient expression of foreign DNA in plants facilitated the production of numerous proteins of pharmaceutical importance. This review summarizes several success stories of the engineering of plant metabolic pathways in heterologous hosts. Likewise, a few examples of recombinant protein expression in plants for therapeutic purposes are also highlighted. Therefore, the importance of medicinal plants has grown immensely as sources for valuable products of low and high molecular weight. The next step ahead for bioengineering is to achieve more success stories of industrial-scale production of secondary plant metabolites in microbial systems and to fully exploit plant cell factories' commercial potential for recombinant proteins.

Cell-free production of the bifunctional glycoside hydrolase GH78 from Xylaria polymorpha

The ability to catalyze diverse reactions with relevance for chemical and pharmaceutical research and industry has led to an increasing interest in fungal enzymes. There is still an enormous potential considering the sheer amount of new enzymes from the huge diversity of fungi. Most of these fungal enzymes have not been characterized yet due to the lack of high throughput synthesis and analysis methods. This bottleneck could be overcome by means of cell-free protein synthesis. In this study, cell-free protein synthesis based on eukaryotic cell lysates was utilized to produce a functional glycoside hydrolase (GH78) from the soft-rot fungus Xylaria polymorpha (Ascomycota). The enzyme was successfully synthesized under different reaction conditions. We characterized its enzymatic activities and immobilized the protein via FLAG-Tag interaction. Alteration of several conditions including reaction temperature, template design and lysate supplementation had an influence on the activity of cell-free synthesized GH78. Consequently this led to a production of purified GH78 with a specific activity of 15.4 U mg^- 1. The results of this study may be foundational for future high throughput fungal enzyme screenings, including substrate spectra analysis and mutant screenings.

A Cell-free Expression Pipeline for the Generation and Functional Characterization of Nanobodies

Cell-free systems are well-established platforms for the rapid synthesis, screening, engineering and modification of all kinds of recombinant proteins ranging from membrane proteins to soluble proteins, enzymes and even toxins. Also within the antibody field the cell-free technology has gained considerable attention with respect to the clinical research pipeline including antibody discovery and production. Besides the classical full-length monoclonal antibodies (mAbs), so-called "nanobodies" (Nbs) have come into focus. A Nb is the smallest naturally-derived functional antibody fragment known and represents the variable domain (V_HH, ∼15 kDa) of a camelid heavy-chain-only antibody (HCAb). Based on their nanoscale and their special structure, Nbs display striking advantages concerning their production, but also their characteristics as binders, such as high stability, diversity, improved tissue penetration and reaching of cavity-like epitopes. The classical way to produce Nbs depends on the use of living cells as production host. Though cell-based production is well-established, it is still time-consuming, laborious and hardly amenable for high-throughput applications. Here, we present for the first time to our knowledge the synthesis of functional Nbs in a standardized mammalian cell-free system based on Chinese hamster ovary (CHO) cell lysates. Cell-free reactions were shown to be time-efficient and easy-to-handle allowing for the "on demand" synthesis of Nbs. Taken together, we complement available methods and demonstrate a promising new system for Nb selection and validation.

Cell-Free Systems Enable the Production of AB5 Toxins for Diagnostic Applications

Cell-free protein synthesis (CFPS) represents a versatile key technology for the production of toxic proteins. As a cell lysate, rather than viable cells, is used, the toxic effects on the host organism can be circumvented. The open nature of cell-free systems allows for the addition of supplements affecting protein concentration and folding. Here, we present the cell-free synthesis and functional characterization of two AB₅ toxins, namely the cholera toxin (Ctx) and the heat-labile enterotoxin (LT), using two eukaryotic cell-free systems based on Chinese hamster ovary (CHO) and Spodoptera frugiperda (Sf21) cells. Through an iterative optimization procedure, the synthesis of the individual AB₅ toxins was established, and the formation of multimeric structures could be shown by autoradiography. A functional analysis was performed using cell-based assays, thereby demonstrating that the LT complex induced the characteristic cell elongation of target cells after 24 h. The LT complex induced cell death at higher concentrations, starting at an initial concentration of 5 nM. The initial toxic effects of the Ctx multimer could already be detected at 4 nM. The detection and characterization of such AB₅ toxins is of utmost importance, and the monitoring of intracellular trafficking facilitates the further identification of the mechanism of action of these toxins. We showed that the B-subunit of LT (LTB) could be fluorescently labeled using an LTB-Strep fusion protein, which is a proof-of-concept for future Trojan horse applications. Further, we performed a mutational analysis of the CtxA subunit as its template was modified, and an amber stop codon was inserted into CtxA’s active site. Subsequently, a non-canonical amino acid was site-specifically incorporated using bio-orthogonal systems. Finally, a fluorescently labeled CtxA protein was produced using copper-catalyzed click reactions as well as a Staudinger ligation. As expected, the modified Ctx multimer no longer induced toxic effects. In our study, we showed that CFPS could be used to study the active centers of toxins by inserting mutations. Additionally, this methodology can be applied for the design of Trojan horses and targeted toxins, as well as enabling the intracellular trafficking of toxins as a prerequisite for the analysis of the toxin’s mechanism of action.

A broadly cross-reactive monoclonal antibody against hepatitis E virus capsid antigen

Abstract

To generate a hepatitis E virus (HEV) genotype 3 (HEV-3)–specific monoclonal antibody (mAb), the Escherichia coli–expressed carboxy-terminal part of its capsid protein was used to immunise BALB/c mice. The immunisation resulted in the induction of HEV-specific antibodies of high titre. The mAb G117-AA4 of IgG1 isotype was obtained showing a strong reactivity with the homologous E. coli, but also yeast-expressed capsid protein of HEV-3. The mAb strongly cross-reacted with ratHEV capsid protein derivatives produced in both expression systems and weaker with an E. coli–expressed batHEV capsid protein fragment. In addition, the mAb reacted with capsid protein derivatives of genotypes HEV-2 and HEV-4 and common vole hepatitis E virus (cvHEV), produced by the cell-free synthesis in Chinese hamster ovary (CHO) and Spodoptera frugiperda (Sf21) cell lysates. Western blot and line blot reactivity of the mAb with capsid protein derivatives of HEV-1 to HEV-4, cvHEV, ratHEV and batHEV suggested a linear epitope. Use of truncated derivatives of ratHEV capsid protein in ELISA, Western blot, and a Pepscan analysis allowed to map the epitope within a partially surface-exposed region with the amino acid sequence LYTSV. The mAb was also shown to bind to human patient–derived HEV-3 from infected cell culture and to hare HEV-3 and camel HEV-7 capsid proteins from transfected cells by immunofluorescence assay. The novel mAb may serve as a useful tool for further investigations on the pathogenesis of HEV infections and might be used for diagnostic purposes.

Key points

• The antibody showed cross-reactivity with capsid proteins of different hepeviruses.

• The linear epitope of the antibody was mapped in a partially surface-exposed region.

• The antibody detected native HEV-3 antigen in infected mammalian cells.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-021-11342-7.

Design, Development and Optimization of a Functional Mammalian Cell-Free Protein Synthesis Platform

In this paper, we describe the stepwise development of a cell-free protein synthesis (CFPS) platform derived from cultured Chinese hamster ovary (CHO) cells. We provide a retrospective summary of the design challenges we faced, and the optimized methods developed for the cultivation of cells and the preparation of translationally active lysates. To overcome low yields, we developed procedures to supplement two accessory proteins, GADD34 and K3L, into the reaction to prevent deactivation of the translational machinery by phosphorylation. We compared different strategies for implementing these accessory proteins including two variants of the GADD34 protein to understand the potential trade-offs between yield and ease of implementation. Addition of the accessory proteins increased yield of turbo Green Fluorescent Protein (tGFP) by up to 100-fold depending on which workflow was used. Using our optimized protocols as a guideline, users can successfully develop their own functional CHO CFPS system, allowing for broader application of mammalian CFPS.

Cell-free synthesis of the hirudin variant 1 of the blood-sucking leech Hirudo medicinalis

Synthesis and purification of peptide drugs for medical applications is a challenging task. The leech-derived factor hirudin is in clinical use as an alternative to heparin in anticoagulatory therapies. So far, recombinant hirudin is mainly produced in bacterial or yeast expression systems. We describe the successful development and application of an alternative protocol for the synthesis of active hirudin based on a cell-free protein synthesis approach. Three different cell lysates were compared, and the effects of two different signal peptide sequences on the synthesis of mature hirudin were determined. The combination of K562 cell lysates and the endogenous wild-type signal peptide sequence was most effective. Cell-free synthesized hirudin showed a considerably higher anti-thrombin activity compared to recombinant hirudin produced in bacterial cells.

Cell-Free Protein Synthesis: A Promising Option for Future Drug Development

Proteins are the main source of drug targets and some of them possess therapeutic potential themselves. Among them, membrane proteins constitute approximately 50% of the major drug targets. In the drug discovery pipeline, rapid methods for producing different classes of proteins in a simple manner with high quality are important for structural and functional analysis. Cell-free systems are emerging as an attractive alternative for the production of proteins due to their flexible nature without any cell membrane constraints. In a bioproduction context, open systems based on cell lysates derived from different sources, and with batch-to-batch consistency, have acted as a catalyst for cell-free synthesis of target proteins. Most importantly, proteins can be processed for downstream applications like purification and functional analysis without the necessity of transfection, selection, and expansion of clones. In the last 5 years, there has been an increased availability of new cell-free lysates derived from multiple organisms, and their use for the synthesis of a diverse range of proteins. Despite this progress, major challenges still exist in terms of scalability, cost effectiveness, protein folding, and functionality. In this review, we present an overview of different cell-free systems derived from diverse sources and their application in the production of a wide spectrum of proteins. Further, this article discusses some recent progress in cell-free systems derived from Chinese hamster ovary and Sf21 lysates containing endogenous translocationally active microsomes for the synthesis of membrane proteins. We particularly highlight the usage of internal ribosomal entry site sequences for more efficient protein production, and also the significance of site-specific incorporation of non-canonical amino acids for labeling applications and creation of antibody drug conjugates using cell-free systems. We also discuss strategies to overcome the major challenges involved in commercializing cell-free platforms from a laboratory level for future drug development.

Automated Cell-Free Multiprotein Synthesis Facilitates the Identification of a Secretory, Oligopeptide Elicitor-Like, Immunoreactive Protein of the Oomycete Pythium insidiosum

Technical limitations of conventional biotechnological methods (i.e., genetic engineering and protein synthesis) prevent extensive functional studies of the massive amounts of genetic information available today. We employed a cell-free protein synthesis system to rapidly and simultaneously generate multiple proteins from genetic codes of the oomycete Pythium insidiosum, which causes the life-threatening disease called pythiosis, in humans and animals worldwide. We aimed to screen for potential diagnostic and therapeutic protein targets of this pathogen. Eighteen proteins were synthesized. Of the 18 proteins, one was a secreted immunoreactive protein, called I06, that triggered host immunity and was recognized explicitly by all tested sera from pythiosis patients. It is one of the OPEL proteins; these proteins are present only in the unique group of microorganisms called oomycetes. Here, we demonstrated that cell-free protein synthesis was useful for the production of multiple proteins to facilitate functional studies and identify a potential target for diagnosis and treatment of pythiosis.

Protein production relies on time-consuming genetic engineering and in vivo expression, which is a bottleneck for functional studies in the postgenomic era. Cell-free protein synthesis (CFPS) overcomes the limitation of in vivo protein biosynthesis by processing in vitro transcription and translation of multiple genes to proteins within hours. We employed an automated CFPS to simultaneously synthesize proteins from 24 genes of the oomycete Pythium insidiosum (which causes the life-threatening disease pythiosis) and screen for a diagnostic and therapeutic target. CFPS successfully synthesized 18 proteins (∼75% success rate). One protein, namely, I06, was explicitly recognized by all pythiosis sera, but not control sera, tested. Py. insidiosum secreted a significant amount of I06. The protein architecture of I06 is compatible with the oligopeptide elicitor (OPEL) of the phylogenetically related plant-pathogenic oomycete Phytophthora parasitica. The OPEL-like I06 protein of Py. insidiosum can stimulate host antibody responses, similar to the P. parasitica OPEL that triggers plant defense mechanisms. OPEL-like I06 homologs are present only in the oomycetes. Py. insidiosum contains two OPEL-like I06 homologs, but only one of the two homologs was expressed during hyphal growth. Twenty-nine homologs derived from 15 oomycetes can be phylogenetically divided into two groups. The OPEL-like genes might occur in the common ancestor, before independently undergoing gene gain and loss during the oomycete speciation. In conclusion, CFPS offers a fast in vitro protein synthesis. CFPS simultaneously generated multiple proteins of Py. insidiosum and facilitated the identification of the secretory OPEL-like I06 protein, a potential target for the development of a control measure against the pathogen.

IMPORTANCE Technical limitations of conventional biotechnological methods (i.e., genetic engineering and protein synthesis) prevent extensive functional studies of the massive amounts of genetic information available today. We employed a cell-free protein synthesis system to rapidly and simultaneously generate multiple proteins from genetic codes of the oomycete Pythium insidiosum, which causes the life-threatening disease called pythiosis, in humans and animals worldwide. We aimed to screen for potential diagnostic and therapeutic protein targets of this pathogen. Eighteen proteins were synthesized. Of the 18 proteins, one was a secreted immunoreactive protein, called I06, that triggered host immunity and was recognized explicitly by all tested sera from pythiosis patients. It is one of the OPEL proteins; these proteins are present only in the unique group of microorganisms called oomycetes. Here, we demonstrated that cell-free protein synthesis was useful for the production of multiple proteins to facilitate functional studies and identify a potential target for diagnosis and treatment of pythiosis.

Synthetic Biology Goes Cell-Free

Cell-free systems (CFS) have recently evolved into key platforms for synthetic biology applications. Many synthetic biology tools have traditionally relied on cell-based systems, and while their adoption has shown great progress, the constraints inherent to the use of cellular hosts have limited their reach and scope. Cell-free systems, which can be thought of as programmable liquids, have removed many of these complexities and have brought about exciting opportunities for rational design and manipulation of biological systems. Here we review how these simple and accessible enzymatic systems are poised to accelerate the rate of advancement in synthetic biology and, more broadly, biotechnology.

Cell-free synthesis of human toll-like receptor 9 (TLR9): Optimization of synthesis conditions and functional analysis

The Toll-like receptor family belongs to the group of pathogen recognition receptors which is responsible for the discrimination of self and non-self pathogen-associated molecular patterns (PAMP's). Toll-like receptors play an important role in the innate immunity and defects in protein expression or polymorphism is linked to various diseases such as Systemic Lupus Erythematosus (SLE). The elucidation of the underlying mechanism is crucial for future treatment and therapeutics of toll-like receptor linked diseases. Herein, we report the cell-free synthesis of human Toll-like receptor 9 (hTLR9) using CHO lysate and the continuous exchange cell-free (CECF) synthesis platform. The functionality of this protein was demonstrated by an ELISA binding assay using the ectodomain of TLR9 (TLR9-ECD).

Accelerating the Production of Druggable Targets: Eukaryotic Cell-Free Systems Come into Focus

In the biopharmaceutical pipeline, protein expression systems are of high importance not only for the production of biotherapeutics but also for the discovery of novel drugs. The vast majority of drug targets are proteins, which need to be characterized and validated prior to the screening of potential hit components and molecules. A broad range of protein expression systems is currently available, mostly based on cellular organisms of prokaryotic and eukaryotic origin. Prokaryotic cell-free systems are often the system of choice for drug target protein production due to the simple generation of expression hosts and low cost of preparation. Limitations in the production of complex mammalian proteins appear due to inefficient protein folding and posttranslational modifications. Alternative protein production systems, so-called eukaryotic cell-free protein synthesis systems based on eukaryotic cell-lysates, close the gap between a fast protein generation system and a high quality of complex mammalian proteins. In this study, we show the production of druggable target proteins in eukaryotic cell-free systems. Functional characterization studies demonstrate the bioactivity of the proteins and underline the potential for eukaryotic cell-free systems to significantly improve drug development pipelines.

A User's Guide to Cell-Free Protein Synthesis

Cell-free protein synthesis (CFPS) is a platform technology that provides new opportunities for protein expression, metabolic engineering, therapeutic development, education, and more. The advantages of CFPS over in vivo protein expression include its open system, the elimination of reliance on living cells, and the ability to focus all system energy on production of the protein of interest. Over the last 60 years, the CFPS platform has grown and diversified greatly, and it continues to evolve today. Both new applications and new types of extracts based on a variety of organisms are current areas of development. However, new users interested in CFPS may find it challenging to implement a cell-free platform in their laboratory due to the technical and functional considerations involved in choosing and executing a platform that best suits their needs. Here we hope to reduce this barrier to implementing CFPS by clarifying the similarities and differences amongst cell-free platforms, highlighting the various applications that have been accomplished in each of them, and detailing the main methodological and instrumental requirement for their preparation. Additionally, this review will help to contextualize the landscape of work that has been done using CFPS and showcase the diversity of applications that it enables.

MaxSynBio: Avenues Towards Creating Cells from the Bottom Up

A large German research consortium mainly within the Max Planck Society ("MaxSynBio") was formed to investigate living systems from a fundamental perspective. The research program of MaxSynBio relies solely on the bottom-up approach to synthetic biology. MaxSynBio focuses on the detailed analysis and understanding of essential processes of life through modular reconstitution in minimal synthetic systems. The ultimate goal is to construct a basic living unit entirely from non-living components. The fundamental insights gained from the activities in MaxSynBio could eventually be utilized for establishing a new generation of biotechnological processes, which would be based on synthetic cell constructs that replace the natural cells currently used in conventional biotechnology.

Unexpected instabilities explain batch-to-batch variability in cell-free protein expression systems

Cell-free methods of protein synthesis offer rapid access to expressed proteins. Though the amounts produced are generally only at a small scale, these are sufficient to perform protein-protein interaction assays and tests of enzymatic activity. As such they are valuable tools for the biochemistry and bioengineering community. However the most complex, eukaryotic cell-free systems are difficult to manufacture in house and can be prohibitively expensive to obtain from commercial sources. The Leishmania tarentolae system offers a relatively cheap alternative which is capable of producing difficult to express proteins, but which is simpler to produce in large scale. However, this system suffers from batch-to-batch variability, which has been accepted as a consequence of the complexity of the extracts. Here we show an unexpected origin for the variability observed and demonstrate that small variations in a single parameter can dramatically affect expression, such that minor pipetting errors can have major effects on yields. L. tarentolae cell-free lysate activity is shown to be more stable to changes in Mg²⁺ concentration at a lower ratio of feed solution to lysate in the reaction than typically used, and a higher Mg²⁺ optimum. These changes essentially eliminate batch-to-batch variability of L. tarentolae lysate activity and permit their full potential to be realized.

Protein labeling strategies for liquid-state NMR spectroscopy using cell-free synthesis

Preparation of a protein sample for liquid-state nuclear magnetic resonance (NMR) spectroscopy analysis requires optimization of many parameters. This review describes labeling strategies for obtaining assignments of protein resonances. Particular emphasis is placed on the advantages of cell-free protein production, which enables exclusive labeling of the protein of interest, thereby simplifying downstream processing steps and increasing the availability of different labeling strategies for a target protein. Furthermore, proteins can be synthesized in milligram yields, and the open nature of the cell-free system allows the addition of stabilizers, scrambling inhibitors or hydrophobic solubilization environments directly during the protein synthesis, which is especially beneficial for membrane proteins. Selective amino acid labeling of the protein of interest, the possibility of addressing scrambling issues and avoiding the need for labile amino acid precursors have been key factors in enabling the introduction of new assignment strategies based on different labeling schemes as well as on new pulse sequences. Combinatorial selective labeling methods have been developed to reduce the number of protein samples necessary to achieve a complete backbone assignment. Furthermore, selective labeling helps to decrease spectral overlap and overcome size limitations for solution NMR analysis of larger complexes, oligomers, intrinsically disordered proteins and membrane proteins.

Cell-free production of pore forming toxins: Functional analysis of thermostable direct hemolysin from Vibrio parahaemolyticus

The pore forming characteristic of TDH1 and TDH2 variants of thermostable direct hemolysin (TDH), a major toxin involved in the pathogenesis of Vibrio parahaemolyticus, was studied on a planar lipid bilayer painted over individual picoliter cavities containing microelectrodes assembled in a multiarray. Both proteins formed pores upon insertion into the lipid bilayer which was shown as a shift in the conductance from the baseline current. TDH2 protein was able to produce stable currents and the currents were influenced by external factors like concentration, type of salt and voltage. The pore currents were influenced and showed a detectable response in the presence of polymers which makes them suitable for biotechnology applications.

Cell-free synthesis of functional antibodies using a coupled in vitro transcription-translation system based on CHO cell lysates

Antibodies are indispensable tools for basic research as well as diagnostic and therapeutic applications. Consequently, the development of alternative manufacturing strategies which circumvent the hurdles connected to conventional antibody production technologies is of enormous interest. To address this issue, we demonstrate the synthesis of complex antibody formats, in particular immunoglobulin G (IgG) and single-chain variable fragment Fc fusion (scFv-Fc), in a microsome-containing cell-free system based on translationally active chinese hamster ovary (CHO) cell lysates. To mimic the environment for antibody folding and assembly present in living cells, antibody genes were fused to an endoplasmic reticulum (ER)-specific signal sequence. Signal-peptide induced translocation of antibody polypeptide chains into the lumen of ER microsomes was found to be the prerequisite for antibody chain assembly and functionality. In this context, we show the rapid synthesis of antibody molecules in different reaction formats, including batch and continuous-exchange cell-free (CECF) reactions, depending on the amount of protein needed for further analysis. In addition, we demonstrate site-specific and residue-specific labeling of antibodies with fluorescent non-canonical amino acids. In summary, our study describes a novel antibody production platform which combines the highly efficient mammalian protein folding machinery of CHO cells with the benefits of cell-free protein synthesis.

High-yield production of "difficult-to-express" proteins in a continuous exchange cell-free system based on CHO cell lysates

Cell-free protein synthesis (CFPS) represents a promising technology for efficient protein production targeting especially so called "difficult-to-express" proteins whose synthesis is challenging in conventional in vivo protein production platforms. Chinese hamster ovary (CHO) cells are one of the most prominent and safety approved cell lines for industrial protein production. In this study we demonstrated the ability to produce high yields of various protein types including membrane proteins and single chain variable fragments (scFv) in a continuous exchange cell-free (CECF) system based on CHO cell lysate that contains endogenous microsomal structures. We showed significant improvement of protein yield compared to batch formatted reactions and proved biological activity of synthesized proteins using various analysis technologies. Optimized CECF reaction conditions led to membrane protein yields up to 980 µg/ml, which is the highest protein yield reached in a microsome containing eukaryotic cell-free system presented so far.

High-yield production of "difficult-to-express" proteins in a continuous exchange cell-free system based on CHO cell lysates

Cell-free protein synthesis (CFPS) represents a promising technology for efficient protein production targeting especially so called "difficult-to-express" proteins whose synthesis is challenging in conventional in vivo protein production platforms. Chinese hamster ovary (CHO) cells are one of the most prominent and safety approved cell lines for industrial protein production. In this study we demonstrated the ability to produce high yields of various protein types including membrane proteins and single chain variable fragments (scFv) in a continuous exchange cell-free (CECF) system based on CHO cell lysate that contains endogenous microsomal structures. We showed significant improvement of protein yield compared to batch formatted reactions and proved biological activity of synthesized proteins using various analysis technologies. Optimized CECF reaction conditions led to membrane protein yields up to 980 µg/ml, which is the highest protein yield reached in a microsome containing eukaryotic cell-free system presented so far.

Development of a CHO-Based Cell-Free Platform for Synthesis of Active Monoclonal Antibodies

Chinese Hamster Ovary (CHO) cells are routinely optimized to stably express monoclonal antibodies (mAbs) at high titers. At the early stages of lead isolation and optimization, hundreds of sequences for the target protein of interest are screened. Typically, cell-based transient expression technology platforms are used for expression screening, but these can be time- and resource-intensive. Here, we have developed a cell-free protein synthesis (CFPS) platform utilizing a commercially available CHO extract for the rapid in vitro synthesis of active, aglycosylated mAbs. Specifically, we optimized reaction conditions to maximize protein yields, established an oxidizing environment to enable disulfide bond formation, and demonstrated the importance of temporal addition of heavy chain and light chain plasmids for intact mAb production. Using our optimized platform, we demonstrate for the first time to our knowledge the cell-free synthesis of biologically active, intact mAb at >100 mg/L using a eukaryotic-based extract. We then explored the utility of our system as a tool for ranking yields of candidate antibodies. Unlike stable or transient transfection-based screening, which requires a minimum of 7 days for setup and execution, results using our CHO-based CFPS platform are attained within 2 days and it is well-suited for automation. Further development would provide a tool for rapid, high-throughput prediction of mAb expression ranking to accelerate design-build-test cycles required for antibody expression and engineering. Looking forward, the CHO-based CFPS platform could facilitate the synthesis of toxic proteins as well.

Production of G protein-coupled receptors in an insect-based cell-free system

The biochemical analysis of human cell membrane proteins remains a challenging task due to the difficulties in producing sufficient quantities of functional protein. G protein‐coupled receptors (GPCRs) represent a main class of membrane proteins and drug targets, which are responsible for a huge number of signaling processes regulating various physiological functions in living cells. To circumvent the current bottlenecks in GPCR studies, we propose the synthesis of GPCRs in eukaryotic cell‐free systems based on extracts generated from insect (Sf21) cells. Insect cell lysates harbor the fully active translational and translocational machinery allowing posttranslational modifications, such as glycosylation and phosphorylation of de novo synthesized proteins. Here, we demonstrate the production of several GPCRs in a eukaryotic cell‐free system, performed within a short time and in a cost‐effective manner. We were able to synthesize a variety of GPCRs ranging from 40 to 133 kDa in an insect‐based cell‐free system. Moreover, we have chosen the μ opioid receptor (MOR) as a model protein to analyze the ligand binding affinities of cell‐free synthesized MOR in comparison to MOR expressed in a human cell line by “one‐point” radioligand binding experiments. Biotechnol. Bioeng. 2017;114: 2328–2338. © 2017 The Authors. Biotechnology and Bioengineering Published by Wiley Periodicals, Inc.

Qualifying a eukaryotic cell-free system for fluorescence based GPCR analyses

Membrane proteins are key elements in cell-mediated processes. In particular, G protein-coupled receptors (GPCRs) have attracted increasing interest since they affect cellular signaling. Furthermore, mutations in GPCRs can cause acquired and inheritable diseases. Up to date, there still exist a number of GPCRs that has not been structurally and functionally analyzed due to difficulties in cell-based membrane protein production. A promising approach for membrane protein synthesis and analysis has emerged during the last years and is known as cell-free protein synthesis (CFPS). Here, we describe a simply portable method to synthesize GPCRs and analyze their ligand-binding properties without the requirement of additional supplements such as liposomes or nanodiscs. This method is based on eukaryotic cell lysates containing translocationally active endogenous endoplasmic reticulum-derived microsomes where the insertion of GPCRs into biologically active membranes is supported. In this study we present CFPS in combination with fast fluorescence-based screening methods to determine the localization, orientation and ligand-binding properties of the endothelin B (ET-B) receptor upon expression in an insect-based cell-free system. To determine the functionality of the cell-free synthesized ET-B receptor, we analyzed the binding of its ligand endothelin-1 (ET-1) in a qualitative fluorescence-based assay and in a quantitative radioligand binding assay.

Engineering genetic circuit interactions within and between synthetic minimal cells

Genetic circuits and reaction cascades are of great importance for synthetic biology, biochemistry and bioengineering. An open question is how to maximize the modularity of their design to enable the integration of different reaction networks and to optimize their scalability and flexibility. One option is encapsulation within liposomes, which enables chemical reactions to proceed in well-isolated environments. Here we adapt liposome encapsulation to enable the modular, controlled compartmentalization of genetic circuits and cascades. We demonstrate that it is possible to engineer genetic circuit-containing synthetic minimal cells (synells) to contain multiple-part genetic cascades, and that these cascades can be controlled by external signals as well as inter-liposomal communication without crosstalk. We also show that liposomes that contain different cascades can be fused in a controlled way so that the products of incompatible reactions can be brought together. Synells thus enable a more modular creation of synthetic biology cascades, an essential step towards their ultimate programmability.

Cell-Free Synthetic Biology: Engineering Beyond the Cell

Cell-free protein synthesis (CFPS) technologies have enabled inexpensive and rapid recombinant protein expression. Numerous highly active CFPS platforms are now available and have recently been used for synthetic biology applications. In this review, we focus on the ability of CFPS to expand our understanding of biological systems and its applications in the synthetic biology field. First, we outline a variety of CFPS platforms that provide alternative and complementary methods for expressing proteins from different organisms, compared with in vivo approaches. Next, we review the types of proteins, protein complexes, and protein modifications that have been achieved using CFPS systems. Finally, we introduce recent work on genetic networks in cell-free systems and the use of cell-free systems for rapid prototyping of in vivo networks. Given the flexibility of cell-free systems, CFPS holds promise to be a powerful tool for synthetic biology as well as a protein production technology in years to come.

Cell-Free Systems Based on CHO Cell Lysates: Optimization Strategies, Synthesis of "Difficult-to-Express" Proteins and Future Perspectives

Nowadays, biotechnological processes play a pivotal role in target protein production. In this context, Chinese Hamster Ovary (CHO) cells are one of the most prominent cell lines for the expression of recombinant proteins and revealed as a safe host for nearly 40 years. Nevertheless, the major bottleneck of common in vivo protein expression platforms becomes obvious when looking at the production of so called “difficult-to-express” proteins. This class of proteins comprises in particular several ion channels and multipass membrane proteins as well as cytotoxic proteins. To enhance the production of “difficult-to-express” proteins, alternative technologies were developed, mainly based on translationally active cell lysates. These so called “cell-free” protein synthesis systems enable an efficient production of different classes of proteins. Eukaryotic cell-free systems harboring endogenous microsomal structures for the synthesis of functional membrane proteins and posttranslationally modified proteins are of particular interest for future applications. Therefore, we present current developments in cell-free protein synthesis based on translationally active CHO cell extracts, underlining the high potential of this platform. We present novel results highlighting the optimization of protein yields, the synthesis of various “difficult-to-express” proteins and the cotranslational incorporation of non-standard amino acids, which was exemplarily demonstrated by residue specific labeling of the glycoprotein Erythropoietin and the multimeric membrane protein KCSA.

Protein synthesis directly from PCR: progress and applications of cell-free protein synthesis with linear DNA

A rapid, versatile method of protein expression and screening can greatly facilitate the future development of therapeutic biologics, proteomic drug targets and biocatalysts. An attractive candidate is cell-free protein synthesis (CFPS), a cell-lysate-based in vitro expression system, which can utilize linear DNA as expression templates, bypassing time-consuming cloning steps of plasmid-based methods. Traditionally, such linear DNA expression templates (LET) have been vulnerable to degradation by nucleases present in the cell lysate, leading to lower yields. This challenge has been significantly addressed in the recent past, propelling LET-based CFPS as a useful tool for studying, screening and engineering proteins in a high-throughput manner. Currently, LET-based CFPS has promise in fields such as functional proteomics, protein microarrays, and the optimization of complex biological systems.

On-chip automation of cell-free protein synthesis: new opportunities due to a novel reaction mode

Many pharmaceuticals are proteins or their development is based on proteins. Cell-free protein synthesis (CFPS) is an innovative alternative to conventional cell based systems which enables the production of proteins with complex and even new characteristics. However, the short lifetime, low protein production and expensive reagent costs are still limitations of CFPS. Novel automated microfluidic systems might allow continuous, controllable and resource conserving CFPS. The presented microfluidic TRITT platform (TRITT for Transcription - RNA Immobilization & Transfer - Translation) addresses the individual biochemical requirements of the transcription and the translation step of CFPS in separate compartments, and combines the reaction steps by quasi-continuous transfer of RNA templates to enable automated CFPS. In detail, specific RNA templates with 5' and 3' hairpin structures for stabilization against nucleases were immobilized during in vitro transcription by newly designed and optimized hybridization oligonucleotides coupled to magnetizable particles. Transcription compatibility and reusability for immobilization of these functionalized particles was successfully proven. mRNA transfer was realized on-chip by magnetic actuated particle transfer, RNA elution and fluid flow to the in vitro translation compartment. The applicability of the microfluidic TRITT platform for the production of the cytotoxic protein Pierisin with simultaneous incorporation of a non-canonical amino acid for fluorescence labeling was demonstrated. The new reaction mode (TRITT mode) is a modified linked mode that fulfills the precondition for an automated modular reactor system. By continual transfer of new mRNA, the novel procedure overcomes problems caused by nuclease digestion and hydrolysis of mRNA during TL in standard CFPS reactions.

Cell-Free Protein Synthesis: Pros and Cons of Prokaryotic and Eukaryotic Systems

From its start as a small-scale in vitro system to study fundamental translation processes, cell-free protein synthesis quickly rose to become a potent platform for the high-yield production of proteins. In contrast to classical in vivo protein expression, cell-free systems do not need time-consuming cloning steps, and the open nature provides easy manipulation of reaction conditions as well as high-throughput potential. Especially for the synthesis of difficult to express proteins, such as toxic and transmembrane proteins, cell-free systems are of enormous interest. The modification of the genetic code to incorporate non-canonical amino acids into the target protein in particular provides enormous potential in biotechnology and pharmaceutical research and is in the focus of many cell-free projects. Many sophisticated cell-free systems for manifold applications have been established. This review describes the recent advances in cell-free protein synthesis and details the expanding applications in this field.

A versatile coupled cell-free transcription-translation system based on tobacco BY-2 cell lysates

Cell-free protein synthesis is a powerful method for the high-throughput production of recombinant proteins, especially proteins that are difficult to express in living cells. Here we describe a coupled cell-free transcription-translation system based on tobacco BY-2 cell lysates (BYLs). Using a combination of fractional factorial designs and response surface models, we developed a cap-independent system that produces more than 250 μg/mL of functional enhanced yellow fluorescent protein (eYFP) and about 270 μg/mL of firefly luciferase using plasmid templates, and up to 180 μg/mL eYFP using linear templates (PCR products) in 18 h batch reactions. The BYL contains actively-translocating microsomal vesicles derived from the endoplasmic reticulum, promoting the formation of disulfide bonds, glycosylation and the cotranslational integration of membrane proteins. This was demonstrated by expressing a functional full-size antibody (∼ 150 μg/mL), the model enzyme glucose oxidase (GOx) (∼ 7.3 U/mL), and a transmembrane growth factor (∼ 25 μg/mL). Subsequent in vitro treatment of GOx with peptide-N-glycosidase F confirmed the presence of N-glycans. Our results show that the BYL can be used as a high-throughput expression and screening platform that is particularly suitable for complex and cytotoxic proteins.

Automated production of functional membrane proteins using eukaryotic cell-free translation systems

Due to their high abundance and pharmacological relevance there is a growing demand for the efficient production of functional membrane proteins. In this context, cell-free protein synthesis represents a valuable alternative that allows for the high-throughput synthesis of functional membrane proteins. Here, we demonstrate the potential of our cell-free protein synthesis system, based on lysates from cultured Spodoptera frugiperda 21 cells, to produce pro- and eukaryotic membrane proteins with individual topological characteristics in an automated fashion. Analytical techniques, including confocal laser scanning microscopy, fluorescence detection of eYFP fusion proteins in a microplate reader and in-gel fluorescence of statistically incorporated fluorescent amino acid derivatives were employed. The reproducibility of our automated synthesis approach is underlined by coefficients of variation below 7.2%. Moreover, the functionality of the cell-free synthesized potassium channel KcsA was analyzed electrophysiologically. Finally, we expanded our cell-free membrane protein synthesis system by an orthogonal tRNA/synthetase pair for the site-directed incorporation of p-Azido-l-phenylalanine based on stop codon suppression. Incorporation was optimized by performance of a two-dimensional screening with different Mg(2+) and lysate concentrations. Subsequently, the selective modification of membrane proteins with incorporated p-Azido-l-phenylalanine was exemplified by Staudinger ligation with a phosphine-based fluorescence dye.

Characterization of trh2 harbouring Vibrio parahaemolyticus strains isolated in Germany

BACKGROUND

Vibrio parahaemolyticus is a recognized human enteropathogen. Thermostable direct hemolysin (TDH) and TDH-related hemolysin (TRH) as well as the type III secretion system 2 (T3SS2) are considered as major virulence factors. As tdh positive strains are not detected in coastal waters of Germany, we focused on the characterization of trh positive strains, which were isolated from mussels, seawater and patients in Germany.

RESULTS

Ten trh harbouring V. parahaemolyticus strains from Germany were compared to twenty-one trh positive strains from other countries. The complete trh sequences revealed clustering into three different types: trh1 and trh2 genes and a pseudogene Ψtrh. All German isolates possessed alleles of the trh2 gene. MLST analysis indicated a close relationship to Norwegian isolates suggesting that these strains belong to the autochthonous microflora of Northern Europe seawaters. Strains carrying the pseudogene Ψtrh were negative for T3SS2β effector vopC. Transcription of trh and vopC genes was analyzed under different growth conditions. Trh2 gene expression was not altered by bile while trh1 genes were inducible. VopC could be induced by urea in trh2 bearing strains. Most trh1 carrying strains were hemolytic against sheep erythrocytes while all trh2 positive strains did not show any hemolytic activity. TRH variants were synthesized in a prokaryotic cell-free system and their hemolytic activity was analyzed. TRH1 was active against sheep erythrocytes while TRH2 variants were not active at all.

CONCLUSION

Our study reveals a high diversity among trh positive V. parahaemolyticus strains. The function of TRH2 hemolysins and the role of the pseudogene Ψtrh as pathogenicity factors are questionable. To assess the pathogenic potential of V. parahaemolyticus strains a differentiation of trh variants and the detection of T3SS2β components like vopC would improve the V. parahaemolyticus diagnostics and could lead to a refinement of the risk assessment in food analyses and clinical diagnostics.

Bright or dark immune complexes of anti-TAMRA antibodies for adapted fluorescence-based bioanalysis

Fluorescence labels, for example fluorescein or rhodamin derivatives, are widely used in bioanalysis applications including lateral-flow assays, PCR, and fluorescence microscopy. Depending on the layout of the particular application, fluorescence quenching or enhancement may be desired as the detection principle. Especially for multiplexed applications or high-brightness requirements, a tunable fluorescence probe can be beneficial. The alterations in the photophysics of rhodamine derivatives upon binding to two different anti-TAMRA antibodies were investigated by absorption and fluorescence-spectroscopy techniques, especially determining the fluorescence decay time and steady-state and time-resolved fluorescence anisotropy. Two monoclonal anti-TAMRA antibodies were generated by the hybridoma technique. Although surface-plasmon-resonance measurements clearly proved the high affinity of both antibodies towards 5-TAMRA, the observed effects on the fluorescence of rhodamine derivatives were very different. Depending on the anti-TAMRA antibody either a strong fluorescence quenching (G71-DC7) or a distinct fluorescence enhancement (G71-BE11) upon formation of the immune complex was observed. Additional rhodamine derivatives were used to gain further information on the binding interaction. The data reveal that such haptens as 5-TAMRA could generate different paratopes with equal binding affinities but different binding interactions, which provide the opportunity to adapt bioanalysis methods including immunoassays for optimized detection principles for the same hapten depending on the specific requirements.

Cell-free protein synthesis systems derived from cultured mammalian cells

We present a technology for the production of target proteins using novel cell-free systems derived from cultured human K562 cells and Chinese hamster ovary (CHO) cells. The protocol includes the cultivation of cells, the preparation of translationally active lysates, and the cell-free synthesis of desired proteins. An efficient expression vector based on the internal ribosome entry site (IRES) from the intergenic region (IGR) of the cricket paralysis virus (CrPV) was constructed for both systems. The coupled batch-based platforms enable the synthesis of a broad range of target proteins such as cytosolic proteins, secreted proteins, membrane proteins embedded into endogenous microsomes, and glycoproteins. The glycosylation of erythropoietin demonstrates the successful performance of posttranslational modifications in the novel cell-free systems. Protein yields of approximately 20 μg/ml (K562-based cell-free system) and 50 μg/ml (CHO-based cell-free system) of active firefly luciferase are obtained in the coupled transcription-translation systems within 3 h. As a result, both cell-free protein synthesis systems serve as powerful tools for high-throughput proteomics.

Characterization of a new maleimido functionalization of gold for surface plasmon resonance spectroscopy

Para-maleimidophenyl (p-MP) modified gold surfaces have been prepared by one-step electrochemical deposition and used in surface plasmon resonance (SPR) studies. Therefore, a FITC mimotope peptide (MP1, 12 aa), a human mucin 1 epitope peptide (MUC, 9 aa) and a protein with their specific antibodies were used as model systems. The peptides were modified with an N-terminal cysteine for covalent and directed coupling to the maleimido functionalized surface by means of Michael addition. The coupling yield of the peptide, the binding characteristics of antibody and the unspecific adsorption of the analytes were investigated. The results expand the spectrum of biosensors usable with p-MP by widely used SPR and support its potential to be versatile for several electrochemical and optical biosensors. This allows the combination of an electrochemical and optical read-out for a broad variety of biomolecular interactions on the same chip.

Cell-free eukaryotic systems for the production, engineering, and modification of scFv antibody fragments

Open cell-free translation systems based on Escherichia coli cell lysates have successfully been used to produce antibodies and antibody fragments. In this study, we demonstrate the cell-free expression of functional single-chain antibody variable fragments (scFvs) in a eukaryotic and endotoxin-free in vitro translation system based on Spodoptera frugiperda (Sf21) insect cell extracts. Three scFv candidates with different specificities were chosen as models. The first scFv candidate SH527-IIA4 specifically discriminates between its phosphorylated (SMAD2-P) and nonphosphorylated antigens (SMAD2) (where SMAD is mothers against decapentaplegic homolog 2), whereas the second scFv candidate SH527-IIC10 recognizes both, SMAD2-P and SMAD2. The third scFv candidate SH855-C11 binds specifically to a linear epitope of the CXC chemokine receptor type 5. The translocation of antibody fragments into the lumen of endogenous microsomal vesicles, which are contained in the lysate, was facilitated by fusion of scFv genes to the insect cell specific signal sequence of honeybee melittin. We compared the binding capabilities of scFv fragments with and without melittin signal peptide and detected that translocated scFv fragments were highly functional, whereas scFvs synthesized in the cytosol of the cell extract showed strongly decreased binding capabilities. Additionally, we describe a cell-free protein synthesis method for the incorporation of noncanonical amino acids into scFv molecules in eukaryotic cell lysates. We demonstrate the successful cotranslational labeling of de novo synthesized scFv molecules with fluorescent amino acids, using residue-specific as well as site-specific labeling.

The emerging age of cell-free synthetic biology

The engineering of and mastery over biological parts has catalyzed the emergence of synthetic biology. This field has grown exponentially in the past decade. As increasingly more applications of synthetic biology are pursued, more challenges are encountered, such as delivering genetic material into cells and optimizing genetic circuits in vivo. An in vitro or cell-free approach to synthetic biology simplifies and avoids many of the pitfalls of in vivo synthetic biology. In this review, we describe some of the innate features that make cell-free systems compelling platforms for synthetic biology and discuss emerging improvements of cell-free technologies. We also select and highlight recent and emerging applications of cell-free synthetic biology.

A continuous-exchange cell-free protein synthesis system based on extracts from cultured insect cells

In this study, we present a novel technique for the synthesis of complex prokaryotic and eukaryotic proteins by using a continuous-exchange cell-free (CECF) protein synthesis system based on extracts from cultured insect cells. Our approach consists of two basic elements: First, protein synthesis is performed in insect cell lysates which harbor endogenous microsomal vesicles, enabling a translocation of de novo synthesized target proteins into the lumen of the insect vesicles or, in the case of membrane proteins, their embedding into a natural membrane scaffold. Second, cell-free reactions are performed in a two chamber dialysis device for 48 h. The combination of the eukaryotic cell-free translation system based on insect cell extracts and the CECF translation system results in significantly prolonged reaction life times and increased protein yields compared to conventional batch reactions. In this context, we demonstrate the synthesis of various representative model proteins, among them cytosolic proteins, pharmacological relevant membrane proteins and glycosylated proteins in an endotoxin-free environment. Furthermore, the cell-free system used in this study is well-suited for the synthesis of biologically active tissue-type-plasminogen activator, a complex eukaryotic protein harboring multiple disulfide bonds.

Antibody mediated fluorescence enhancement of nucleoside analogue 1,3-diaza-2-oxophenoxazine (tC°)

We report on the generation and analytical application of the monoclonal antibody G93-ED2 raised against the tricyclic fluorescent nucleoside analogue 1,3-diaza-2-oxophenoxazine (tC°). G93-ED2 is specifically binding this deoxycytidine analogue and was found to raise its fluorescence intensity by a factor of 5. This unique feature makes it a valuable tool in fluorescence dependent immunoassays. G93-ED2 was successfully applied in a homogeneous fluorescence quenching immunoassay (DNA-Q) for the sequence specific determination of DNA.