High-yield cell-free synthesis of human EGFR by IRES-mediated protein translation in a continuous exchange cell-free reaction format

Beyond In Vivo, Pharmaceutical Molecule Production in Cell-Free Systems and the Use of Noncanonical Amino Acids Therein

Throughout history, we have looked to nature to discover and copy pharmaceutical solutions to prevent and heal diseases. Due to the advances in metabolic engineering and the production of pharmaceutical proteins in different host cells, we have moved from mimicking nature to the delicate engineering of cells and proteins. We can now produce novel drug molecules, which are fusions of small chemical drugs and proteins. Currently we are at the brink of yet another step to venture beyond nature's border with the use of unnatural amino acids and manufacturing without the use of living cells using cell-free systems. In this review, we summarize the progress and limitations of the last decades in the development of pharmaceutical protein development, production in cells, and cell-free systems. We also discuss possible future directions of the field.

Cell-Free Gene Expression: Methods and Applications

Cell-free gene expression (CFE) systems empower synthetic biologists to build biological molecules and processes outside of living intact cells. The foundational principle is that precise, complex biomolecular transformations can be conducted in purified enzyme or crude cell lysate systems. This concept circumvents mechanisms that have evolved to facilitate species survival, bypasses limitations on molecular transport across the cell wall, and provides a significant departure from traditional, cell-based processes that rely on microscopic cellular "reactors." In addition, cell-free systems are inherently distributable through freeze-drying, which allows simple distribution before rehydration at the point-of-use. Furthermore, as cell-free systems are nonliving, they provide built-in safeguards for biocontainment without the constraints attendant on genetically modified organisms. These features have led to a significant increase in the development and use of CFE systems over the past two decades. Here, we discuss recent advances in CFE systems and highlight how they are transforming efforts to build cells, control genetic networks, and manufacture biobased products.

Synthesis of biologically active Shiga toxins in cell-free systems

Shiga toxins (Stx) produced by pathogenic bacteria can cause mild to severe diseases in humans. Thus, the analysis of such toxins is of utmost importance. As an AB5 toxin, Stx consist of a catalytic A-subunit acting as a ribosome-inactivating protein (RIP) and a B-pentamer binding domain. In this study we synthesized the subunits and holotoxins from Stx and Stx2a using different cell-free systems, namely an E. coli- and CHO-based cell-free protein synthesis (CFPS) system. The functional activity of the protein toxins was analyzed in two ways. First, activity of the A-subunits was assessed using an in vitro protein inhibition assay. StxA produced in an E. coli cell-free system showed significant RIP activity at concentrations of 0.02 nM, whereas toxins synthesized in a CHO cell-free system revealed significant activity at concentrations of 0.2 nM. Cell-free synthesized StxA2a was compared to StxA2a expressed in E. coli cells. Cell-based StxA2a had to be added at concentrations of 20 to 200 nM to yield a significant RIP activity. Furthermore, holotoxin analysis on cultured HeLa cells using an O-propargyl-puromycin assay showed significant protein translation reduction at concentrations of 10 nM and 5 nM for cell-free synthesized toxins derived from E. coli and CHO systems, respectively. Overall, these results show that Stx can be synthesized using different cell-free systems while remaining functionally active. In addition, we were able to use CFPS to assess the activity of different Stx variants which can further be used for RIPs in general.

Solubilization of Oligomeric Cell-Free Synthesized Proteins Using SMA Copolymers

Although membrane proteins are abundant in nature, their investigation is limited due to bottlenecks in heterologous overexpression and consequently restricted accessibility for downstream applications. In this chapter, we address these challenges by presenting a fast and straightforward synthesis platform based on eukaryotic cell-free protein synthesis (CFPS) and an efficient solubilization strategy using styrene-maleic acid (SMA) copolymers. We demonstrate CFPS of TWIK-1, a dimeric ion channel, based on Sf21 (Spodoptera frugiperda) insect lysate showing homooligomerization and N-glycosylation enabled by endoplasmic reticulum-derived microsomes. Furthermore, we employ SMA copolymers for protein solubilization, which preserves the native-like microsomal environment. This approach not only retains the solubilized protein's suitability for downstream applications but also maintains the oligomerization and glycosylation of TWIK-1 post-solubilization. We validate the solubilization procedure using autoradiography, particle size analysis, and biomolecular fluorescence assay and confirm the very efficient, structurally intact solubilization of cell-free synthesized TWIK-1.

Research progress on the application of cell-free synthesis systems for enzymatic processes

Cell-free synthesis systems can complete the transcription and translation process in vitro to produce complex proteins that are difficult to be expressed in traditional cell-based systems. Such systems also can be used for the assembly of efficient localized multienzyme cascades to synthesize products that are toxic to cells. Cell-free synthesis systems provide a simpler and faster engineering solution than living cells, allowing unprecedented design freedom. This paper reviews the latest progress on the application of cell-free synthesis systems in the field of enzymatic catalysis, including cell-free protein synthesis and cell-free metabolic engineering. In cell-free protein synthesis: complex proteins, toxic proteins, membrane proteins, and artificial proteins containing non-natural amino acids can be easily synthesized by directly controlling the reaction conditions in the cell-free system. In cell-free metabolic engineering, the synthesis of desired products can be made more specific and efficient by designing metabolic pathways and screening biocatalysts based on purified enzymes or crude extracts. Through the combination of cell-free synthesis systems and emerging technologies, such as: synthetic biology, microfluidic control, cofactor regeneration, and artificial scaffolds, we will be able to build increasingly complex biomolecule systems. In the next few years, these technologies are expected to mature and reach industrialization, providing innovative platforms for a wide range of biotechnological applications.

Rapid One-Step Capturing of Native, Cell-Free Synthesized and Membrane-Embedded GLP-1R

G protein-coupled receptors (GPCRs) are of outstanding pharmacological interest as they are abundant in cell membranes where they perform diverse functions that are closely related to the vitality of cells. The analysis of GPCRs in natural membranes is laborious, as established methods are almost exclusively cell culture-based and only a few methods for immobilization in a natural membrane outside the cell are known. Within this study, we present a one-step, fast and robust immobilization strategy of the GPCR glucagon-like peptide 1 receptor (GLP-1R). GLP-1R was synthesized in eukaryotic lysates harboring endogenous endoplasmic reticulum-derived microsomes enabling the embedment of GLP-1R in a natural membrane. Interestingly, we found that these microsomes spontaneously adsorbed to magnetic Neutravidin beads thus providing immobilized membrane protein preparations which required no additional manipulation of the target receptor or its supporting membrane. The accessibility of the extracellular domain of membrane-embedded and bead-immobilized GLP-1R was demonstrated by bead-based enzyme-linked immunosorbent assay (ELISA) using GLP-1R-specific monoclonal antibodies. In addition, ligand binding of immobilized GLP-1R was verified in a radioligand binding assay. In summary, we present an easy and straightforward synthesis and immobilization methodology of an active GPCR which can be beneficial for studying membrane proteins in general.

Cell-Free Synthesis and Electrophysiological Analysis of Multipass Voltage-Gated Ion Channels Tethered in Microsomal Membranes

Cell-free protein synthesis (CFPS) has emerged as a powerful tool for the rapid synthesis and analysis of various structurally and functionally distinct proteins. These include 'difficult-to-express' membrane proteins such as large multipass ion channel receptors. Owing to their membrane localization, eukaryotic CFPS supplemented with endoplasmic reticulum (ER)-derived microsomal vesicles has proven to be an efficient system for the synthesis of functional membrane proteins. Here we demonstrate the applicability of the eukaryotic cell-free systems based on lysates from the mammalian Chinese Hamster Ovary (CHO) and insect Spodoptera frugiperda (Sf21) cells. We demonstrate the efficiency of the systems in the de novo cell-free synthesis of the human cardiac ion channels: ether-a-go-go potassium channel (hERG) KV11.1 and the voltage-gated sodium channel hNaV1.5.

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.

High-Throughput Cell-Free Screening of Eukaryotic Membrane Proteins in Lipidic Mimetics

Membrane proteins (MPs) carry out important functions in the metabolism of cells, such as the detection of extracellular activities and the transport of small molecules across the plasma and organelle membranes. Expression of MPs for biochemical, biophysical, and structural analysis is in most cases achieved by overexpression of the desired target in an appropriate host, such as a bacterium. However, overexpression of MPs is usually toxic to the host cells and can lead to aggregation of target protein and to resistance to detergent extraction. An alternative to cell-based MP expression is cell-free (CF), or in vitro, expression. CF expression of MPs has several advantages over cell-based methods, including lack of toxicity issues, no requirement for detergent extraction, and direct incorporation of target proteins in various lipidic mimetics. This article describes a high-throughput method for the expression and purification of eukaryotic membrane proteins used in the author's lab. Basic Protocol 1 describes the selection and cloning of target genes into appropriate vectors for CF expression. Basic Protocol 2 describes the assembly of CF reactions for high-throughput screening. Basic Protocol 3 outlines methods for purification and detection of target proteins. Support Protocols 1-6 describe various accessory procedures: amplification of target, treatment of vectors to prepare them for ligation-independent cloning, and the preparation of S30 extract, T7 RNA polymerase, liposomes, and nanodiscs. © 2022 Wiley Periodicals LLC. Basic Protocol 1: Target selection, construct design, and cloning into pET-based expression vectors Support Protocol 1: Amplification of target DNA Support Protocol 2: Preparation of ligation-independent cloning (LIC)-compatible vectors Basic Protocol 2: Assembly of small-scale cell-free reactions for high-throughput screening Support Protocol 3: Preparation of Escherichia coli S30 extract Support Protocol 4: Preparation of T7 RNA polymerase Support Protocol 5: Preparation of liposomes Support Protocol 6: Preparation of nanodiscs Basic Protocol 3: Purification and detection of cell-free reaction products.

A De Novo Optimized Cell-Free System for the Expression of Soluble and Active Human Tumor Necrosis Factor-Alpha

Cell-free (in vitro) expression is a robust alternative platform to the cell-based (in vivo) system for recombinant protein production. Tumor necrosis factor-alpha (TNF-α) is an effective pro-inflammatory cytokine with pleiotropic effects. The aim of the current study was de novo optimized expression of soluble and active human TNF-α by an in vitro method in an E. coli-based cell-free protein synthesis (CFPS) system and its biological activity evaluation. The codon-optimized synthetic human TNF-α gene was constructed by a two-step PCR, cloned into pET101/D-TOPO vector and then expressed by the E. coli CFPS system. Cell-free expression of the soluble protein was optimized using a response surface methodology (RSM). The anticancer activity of purified human TNF-α was assessed against three human cancer cell lines: Caco-2, HepG-2 and MCF-7. Data from RSM revealed that the lowest value (7.2 µg/mL) of cell-free production of recombinant human TNF-α (rhTNF-α) was obtained at a certain incubation time (6 h) and incubation temperature (20 °C), while the highest value (350 µg/mL) was recorded at 4 h and 35 °C. This rhTNF-α showed a significant anticancer potency. Our findings suggest a cell-free expression system as an alternative platform for producing soluble and functionally active recombinant TNF-α for further research and clinical trials.

Recombinant Protein Production and Purification of Insoluble Proteins

Proteins are synthesized in heterologous systems because of the impossibility to obtain satisfactory yields from natural sources. The efficient production of soluble and functional recombinant proteins is among the main goals in the biotechnological field. In this context, it is important to point out that under stress conditions, protein folding machinery is saturated and this promotes protein misfolding and, consequently, protein aggregation. Thus, the selection of the optimal expression organism and its growth conditions to minimize the formation of insoluble protein aggregates should be done according to the protein characteristics and downstream requirements. Escherichia coli is the most popular recombinant protein expression system despite the great development achieved so far by eukaryotic expression systems. Besides, other prokaryotic expression systems, such as lactic acid bacteria and psychrophilic bacteria, are gaining interest in this field. However, it is worth mentioning that prokaryotic expression system poses, in many cases, severe restrictions for a successful heterologous protein production. Thus, eukaryotic systems such as mammalian cells, insect cells, yeast, filamentous fungus, and microalgae are an interesting alternative for the production of these difficult-to-express proteins.

Targeted esterase-induced dye (TED) loading supports direct calcium imaging in eukaryotic cell-free systems

Calcium imaging is an important functional tool for analysing ion channels, transporters and pumps for drug screening in living cells. Depicted eukaryotic cell-free systems utilize microsomes, derived from the endoplasmic reticulum to incorporate the synthesized membrane proteins-like ion channels. Carboxylesterase is required to cleave the acetoxymethyl ester moiety of the chemical calcium indicators in order to ensure its immobility across the endoplasmic reticulum membrane. Absence or an inadequate amount of carboxylesterase in the endoplasmic reticulum of different eukaryotic cells poses a hindrance to perform calcium imaging in microsomes. In this work, we try to overcome this drawback and adapt the cell-based calcium imaging principle to a cell-free protein synthesis platform. Carboxylesterase synthesized in a Spodoptera frugiperda Sf21 lysate translation system is established as a viable calcium imaging tool in microsomes. Cell-free synthesized carboxylesterase inside microsomes is validated with esterase and dye loading assays. Native proteins from the endoplasmic reticulum, such as ryanodine channels and calcium ATPase, are analysed. Cell-free synthesized transient receptor potential channels are used as model proteins to demonstrate the realization of this concept.

Carboxylesterase, the key enzyme to handle ester-based dyes, is synthesized in microsomes using eukaryotic cell-free protein synthesis platform and established as a viable calcium imaging tool to analyze native and cell-free synthesized ion channels.

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.

Optimising protein synthesis in cell‐free systems, a review

Over the last decades, cell-free systems have been extensively used for in vitro protein expression. A vast range of protocols and cellular sources varying from prokaryotes and eukaryotes are now available for cell-free technology. However, exploiting the maximum capacity of cell free systems is not achieved by using traditional protocols. Here, what are the strategies and choices one can apply to optimise cell-free protein synthesis have been reviewed. These strategies provide robust and informative improvements regarding transcription, translation and protein folding which can later be used for the establishment of individual best cell-free reactions per lysate batch.

Cell-free protein synthesis: advances on production process for biopharmaceuticals and immunobiological products

Biopharmaceutical products are of great importance in the treatment or prevention of many diseases and represent a growing share of the global pharmaceutical market. The usual technology for protein synthesis (cell-based expression) faces certain obstacles, especially with 'difficult-to-express' proteins. Cell-free protein synthesis (CFPS) can overcome the main bottlenecks of cell-based expression. This review aims to present recent advances in the production process of biologic products by CFPS. First, key aspects of CFPS systems are summarized. A description of several biologic products that have been successfully produced using the CFPS system is provided. Finally, the CFPS system's ability to scale up and scale down, its main limitations and its application for biologics production are discussed.

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 systems for accelerating glycoprotein expression and biomanufacturing

Protein glycosylation, the enzymatic modification of amino acid sidechains with sugar moieties, plays critical roles in cellular function, human health, and biotechnology. However, studying and producing defined glycoproteins remains challenging. Cell-free glycoprotein synthesis systems, in which protein synthesis and glycosylation are performed in crude cell extracts, offer new approaches to address these challenges. Here, we review versatile, state-of-the-art systems for biomanufacturing glycoproteins in prokaryotic and eukaryotic cell-free systems with natural and synthetic N-linked glycosylation pathways. We discuss existing challenges and future opportunities in the use of cell-free systems for the design, manufacture, and study of glycoprotein biomedicines.

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.

Exploring the Potential of Cell-Free Protein Synthesis for Extending the Abilities of Biological Systems

Cell-free protein synthesis (CFPS) system is a simple, rapid, and sensitive tool that is devoid of membrane-bound barriers, yet contains all the mandatory substrates, biomolecules, and machineries required for the synthesis of the desired proteins. It has the potential to overcome loopholes in the current in vivo production systems and is a promising tool in both basic and applied scientific research. It facilitates a simplified organization of desired experiments with a variety of reaction conditions, making CFPS a powerful tool in biological research. It has been used for the expansion of genetic code, assembly of viruses, and in metabolic engineering for production of toxic and complex proteins. Subsequently, CFPS systems have emerged as potent technology for high-throughput production of membrane proteins, enzymes, and therapeutics. The present review highlights the recent advances and uses of CFPS systems in biomedical, therapeutic, and biotechnological applications. Additionally, we highlight possible solutions to the potential biosafety issues that may be encountered while using CFPS technology.

Functional Reconstitution of Membrane Proteins Derived From Eukaryotic Cell-Free Systems

Cell-free protein synthesis (CFPS) based on eukaryotic Sf21 lysate is gaining interest among researchers due to its ability to handle the synthesis of complex human membrane proteins (MPs). Additionally Sf21 cell-free systems contain endogenous microsomal vesicles originally derived from the endoplasmic reticulum (ER). After CFPS, MPs will be translocated into the microsomal vesicles membranes present in the lysates. Thus microsomal membranes offer a natural environment for de novo synthesized MPs. Despite the advantage of synthesizing complex MPs with post translational modifications directly into the microsomal membranes without any additional solubilization supplements, batch based Sf21 cell-free synthesis suffers from low yields. The bottleneck for MPs in particular after the synthesis and incorporation into the microsomal membranes is to analyze their functionality. Apart from low yields of the synthesized MPs with batch based cell-free synthesis, the challenges arise in the form of cytoskeleton elements and peripheral endogenous proteins surrounding the microsomes which may impede the functional analysis of the synthesized proteins. So careful sample processing after the synthesis is particularly important for developing the appropriate functional assays. Here we demonstrate how MPs (native and batch synthesized) from ER derived microsomes can be processed for functional analysis by electrophysiology and radioactive uptake assay methods. Treatment of the microsomal membranes either with a sucrose washing step in the case of human serotonin transporter (hSERT) and sarco/endoplasmic reticulum Ca2+/ATPase (SERCA) pump or with mild detergents followed by the preparation of proteoliposomes in the case of the human voltage dependent anionic channel (hVDAC1) helps to analyze the functional properties of MPs.

Structural insight into co-translational membrane protein folding

Membrane protein folding studies lag behind those of water-soluble proteins due to immense difficulties of experimental study, resulting from the need to provide a hydrophobic lipid-bilayer environment when investigated in vitro. A sound understanding of folding mechanisms is important for membrane proteins as they contribute to a third of the proteome and are frequently associated with disease when mutated and/or misfolded. Membrane proteins largely consist of α-helical, hydrophobic transmembrane domains, which insert into the membrane, often using the SecYEG/Sec61 translocase system. This mini-review highlights recent advances in techniques that can further our understanding of co-translational folding and notably, the structure and insertion of nascent chains as they emerge from translating ribosomes. This article is part of a Special Issue entitled: Molecular biophysics of membranes and membrane proteins.

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.

Lipids modulate the insertion and folding of the nascent chains of alpha helical membrane proteins

Membrane proteins must be inserted into a membrane and folded into their correct structure to function correctly. This insertion occurs during translation and synthesis by the ribosome for most α-helical membrane proteins. Precisely how this co-translational insertion and folding occurs, and the role played by the surrounding lipids, is still not understood. Most of the work on the influence of the lipid environment on folding and insertion has focussed on denatured, fully translated proteins, and thus does not replicate folding during unidirectional elongation of nascent chains that occurs in the cell. This review aims to highlight recent advances in elucidating lipid composition and bilayer properties optimal for insertion and folding of nascent chains in the membrane and in the assembly of oligomeric proteins.

Cell-free protein synthesis as a novel tool for directed glycoengineering of active erythropoietin

As one of the most complex post-translational modification, glycosylation is widely involved in cell adhesion, cell proliferation and immune response. Nevertheless glycoproteins with an identical polypeptide backbone mostly differ in their glycosylation patterns. Due to this heterogeneity, the mapping of different glycosylation patterns to their associated function is nearly impossible. In the last years, glycoengineering tools including cell line engineering, chemoenzymatic remodeling and site-specific glycosylation have attracted increasing interest. The therapeutic hormone erythropoietin (EPO) has been investigated in particular by various groups to establish a production process resulting in a defined glycosylation pattern. However commercially available recombinant human EPO shows batch-to-batch variations in its glycoforms. Therefore we present an alternative method for the synthesis of active glycosylated EPO with an engineered O-glycosylation site by combining eukaryotic cell-free protein synthesis and site-directed incorporation of non-canonical amino acids with subsequent chemoselective modifications.

Nanodelivery of a functional membrane receptor to manipulate cellular phenotype

Modification of membrane receptor makeup is one of the most efficient ways to control input-output signals but is usually achieved by expressing DNA or RNA-encoded proteins or by using other genome-editing methods, which can be technically challenging and produce unwanted side effects. Here we develop and validate a nanodelivery approach to transfer in vitro synthesized, functional membrane receptors into the plasma membrane of living cells. Using β₂-adrenergic receptor (β₂AR), a prototypical G-protein coupled receptor, as an example, we demonstrated efficient incorporation of a full-length β₂AR into a variety of mammalian cells, which imparts pharmacologic control over cellular signaling and affects cellular phenotype in an ex-vivo wound-healing model. Our approach for nanodelivery of functional membrane receptors expands the current toolkit for DNA and RNA-free manipulation of cellular function. We expect this approach to be readily applicable to the synthesis and nanodelivery of other types of GPCRs and membrane receptors, opening new doors for therapeutic development at the intersection between synthetic biology and nanomedicine.

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.

Comparison of cell-based versus cell-free mammalian systems for the production of a recombinant human bone morphogenic growth factor

The human bone morphogenetic protein-2 (hBMP2) is a glycoprotein, which induces de novo bone formation. Here, recombinant production in stably transfected Chinese Hamster Ovary (CHO) cells is compared to transient expression in Human Embryo Kidney (HEK) cells and cell-free synthesis in CHO cell lysates containing microsomal structures as sites of post-translational processing. In case of the stably transfected cells, growth rates and viabilities were similar to those of the parent cells, while entry into the death phase of the culture was delayed. The maximum achievable rhBMP2 concentration in these cultures was 153 pg/mL. Up to 280 ng/mL could be produced in the transient expression system. In both cases the rhBMP-2 was found to interact with the producer cells, which presumably contributed to the low yields. In the cell-free system, hBMP2 yields could be increased to almost 40 μg/mL, reached within three hours. The cell-free system thus approached productivities for the active (renatured) protein previously only recorded for bacterial hosts, while assuring comprehensive post-translational processing.

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.