Preparative scale cell-free production and quality optimization of MraY homologues in different expression modes

MraY translocase catalyzes the first committed membrane-bound step of bacterial peptidoglycan synthesis leading to the formation of lipid I. The essential membrane protein therefore has a high potential as target for drug screening approaches to develop antibiotics against gram-positive as well as gram-negative bacteria. However, the production of large integral membrane proteins in conventional cellular expression systems is still very challenging. Cell-free expression technologies have been optimized in recent times for the production of membrane proteins in the presence of detergents (D-CF), lipids (L-CF), or as precipitates (P-CF). We report the development of preparative scale production protocols for the MraY homologues of Escherichia coli and Bacillus subtilis in all three cell-free expression modes followed by their subsequent quality evaluation. Although both proteins can be cell-free produced at comparable high levels, their requirements for optimal expression conditions differ markedly. B. subtilus MraY was stably folded in all three expression modes and showed highest translocase activities after P-CF production followed by defined treatment with detergents. In contrast, the E. coli MraY appears to be unstable after post- or cotranslational solubilization in detergent micelles. Expression kinetics and reducing conditions were identified as optimization parameters for the quality improvement of E. coli MraY. Most remarkably, in contrast to B. subtilis MraY the E. coli MraY has to be stabilized by lipids and only the production in the L-CF mode in the presence of preformed liposomes resulted in stable and translocase active protein samples.     

Preparative scale cell-free expression systems: new tools for the large scale preparation of integral membrane proteins for functional and structural studies

Cell-free expression techniques have emerged as promising tools for the production of membrane proteins for structural and functional analysis. Elimination of toxic effects and a variety of options to stabilize the synthesized proteins enable the synthesis of otherwise difficult to obtain proteins. Modifications in the reaction design result in preparative scale production rates of cell-free reactions and yield in milligram amounts of membrane proteins per one millilitre of reaction volume. A diverse selection of detergents can be supplied into the reaction system without inhibitory effects to the translation machinery. This offers the unique opportunity to produce a membrane protein directly into micelles of a detergent of choice. We present detailed protocols for the cell-free production of membrane proteins in different modes and we summarize the current knowledge of this technique. A special emphasize will be on the production of soluble and functionally folded membrane proteins in presence of suitable detergents. In addition, we will highlight the advantages of cell-free expression for the structural analysis of membrane proteins especially by liquid state nuclear magnetic resonance spectroscopy and we will discuss new strategies for structural approaches.     

Expression of G protein coupled receptors in a cell-free translational system using detergents and thioredoxin-fusion vectors

In Escherichia coli and other cell-based expression systems, there are critical difficulties in synthesizing membrane proteins, such as the low protein expression levels and the formation of insoluble aggregates. However, structure determinations by X-ray crystallography require the purification of milligram quantities of membrane proteins. In this study, we tried to solve these problems by using cell-free protein expression with an E. coli S30 extract, with G protein coupled receptors (GPCRs) as the target integral membrane proteins. In this system, the thioredoxin-fusion vector induced high protein expression levels as compared with the non-fusion and hexa-histidine-tagged proteins. Two detergents, Brij35 and digitonin, effectively solubilized the produced GPCRs, with little or no effect on the protein yields. The synthesized proteins were detected by Coomassie brilliant blue staining within 1h of reaction initiation, and were easily reconstituted within phospholipid vesicles. Surprisingly, the unpurified, reconstituted thioredoxin-fused receptor proteins had functional activity, in that a specific affinity binding value of an antagonist was obtained for the receptor. This cell-free translation system (about 1mg/ml of reaction volume for 6-8 h) has biophysical and biochemical advantages for the synthesis of integral membrane proteins.     

Preparative expression of secreted proteins in bacteria: status report and future prospects

The expression of heterologous secreted proteins in Escherichia coli is widely employed for laboratory and preparative purposes. Thanks to advances in expression technologies over the past 25 years, many mammalian proteins can now be produced routinely in secreted form with yields in the gram/litre scale. Nonetheless, ensuring efficient secretion across the inner membrane, and preventing proteolytic degradation, incorrect disulfide-bond formation and aggregation into periplasmic inclusion bodies, frequently presents significant challenges. Recent advances in the understanding of the periplasmic folding quality control system are leading to new strategies to facilitate the expression of heterologous secreted proteins. In parallel, protein design and directed evolution approaches are beginning to be exploited for engineering of the cellular protein folding machinery to achieve further improvements in protein expression.     

High-throughput,genome-scale protein production method based on the wheat germ cell-free expression system

Current cell-free protein expression systems are capable of synthesizing proteins with high speed and accuracy; however, the yields are low due to their instability over time. Escherichia coli based systems are not always sufficient for expression of eukaryotic proteins. This report reviews a high-throughput protein production method based on the cell-free system prepared from eukaryote, wheat embryos. We first demonstrate a method for preparation of this extract that exhibited a high degree of stability and activity. To maximize translation yield and throughput, we address and resolve the following issues: (1) optimization of the ORF flanking regions; (2) PCR-based generation of DNA for mRNA production; (3) expression vectors for large-scale protein production; and (4) a translation reaction that does not require a membrane. The combination of these elemental processes with robotic automation resulted in high-throughput protein synthesis.     

Wheat germ systems for cell-free protein expression

Cell-free protein expression plays an important role in biochemical research. However, only recent developments led to new methods to rapidly synthesize preparative amounts of protein that make cell-free protein expression an attractive alternative to cell-based methods. In particular the wheat germ system provides the highest translation efficiency among eukaryotic cell-free protein expression approaches and has a very high success rate for the expression of soluble proteins of good quality. As an open in vitro method, the wheat germ system is a preferable choice for many applications in protein research including options for protein labeling and the expression of difficult-to-express proteins like membrane proteins and multiple protein complexes. Here I describe wheat germ cell-free protein expression systems and give examples how they have been used in genome-wide expression studies, preparation of labeled proteins for structural genomics and protein mass spectroscopy, automated protein synthesis, and screening of enzymatic activities. Future directions for the use of cell-free expression methods are discussed.     

Large-scale proteomic analysis of membrane proteins

Proteomic analysis of membrane proteins is a promising approach for the identification of novel drug targets and/or disease biomarkers. Despite notable technological developments, obstacles related to extraction and solublization of membrane proteins are encountered. A critical discussion of the different preparative methods of membrane proteins is offered in relation to downstream proteomic applications, mainly gel-based analyses and mass spectrometry. Frequently, unknown proteins are identified by high-throughput profiling of membrane proteins. In search for novel membrane proteins, analysis of protein sequences using computational tools is performed to predict the presence of transmembrane domains. This review also presents these bioinformatic tools with the human proteome as a case study. Along with technological innovations, advancements in the areas of sample preparation and computational prediction of membrane proteins will lead to exciting discoveries.     

Large-scale proteomic analysis of membrane proteins

Proteomic analysis of membrane proteins is a promising approach for the identification of novel drug targets and/or disease biomarkers. Despite notable technological developments, obstacles related to extraction and solublization of membrane proteins are encountered. A critical discussion of the different preparative methods of membrane proteins is offered in relation to downstream proteomic applications, mainly gel-based analyses and mass spectrometry. Frequently, unknown proteins are identified by high-throughput profiling of membrane proteins. In search for novel membrane proteins, analysis of protein sequences using computational tools is performed to predict the presence of transmembrane domains. This review also presents these bioinformatic tools with the human proteome as a case study. Along with technological innovations, advancements in the areas of sample preparation and computational prediction of membrane proteins will lead to exciting discoveries.     

Mistic and TarCF as fusion protein partners for functional expression of the cannabinoid receptor 2 in Escherichia coli

G protein coupled receptors (GPCRs) are key players in signal recognition and cellular communication making them important therapeutic targets. Large-scale production of these membrane proteins in their native form is crucial for understanding their mechanism of action and target-based drug design. Here we report the overexpression system for a GPCR, the cannabinoid receptor subtype 2 (CB2), in Escherichia coli C43(DE3) facilitated by two fusion partners: Mistic, an integral membrane protein expression enhancer at the N-terminal, and TarCF, a C-terminal fragment of the bacterial chemosensory transducer Tar at the C-terminal of the CB2 open reading frame region. Multiple histidine tags were added on both ends of the fusion protein to facilitate purification. Using individual and combined fusion partners, we found that CB2 fusion protein expression was maximized only when both partners were used. Variable growth and induction conditions were conducted to determine and optimize protein expression. More importantly, this fusion protein Mistic-CB2-TarCF can localize into the E. coli membrane and exhibit functional binding activities with known CB2 ligands including CP55,940, WIN55,212-2 and SR144,528. These results indicate that this novel expression and purification system provides us with a promising strategy for the preparation of biologically active GPCRs, as well as general application for the preparation of membrane-bound proteins using the two new fusion partners described.     

Solubilization, isolation, and immunochemical characterization of the major outer membrane protein from Rhodopseudomonas sphaeroides

Solubilization of the major outer membrane protein of Rhodopseudomonas sphaeroides, and subsequent isolation, has been achieved by both non-detergent- and detergent-based methods. The protein was differentially solubilized from other outer membrane proteins in 5 M guanidine thiocyanate which was exchanged by dialysis for 7 M urea. The urea-soluble protein was purified to homogeneity by a combination of DEAE-Sephadex chromatography and preparative electrophoretic techniques. Similar to the peptidoglycan-associated proteins of other Gram-negative bacteria, the protein was also purified by differential temperature extraction of the outer membrane in the presence of sodium dodecyl sulfate (SDS) followed by preparative SDS-polyacrylamide gel electrophoresis. Immunochemical analysis of the proteins isolated by the two techniques established the immunochemical identity and homogeneity of each preparation. Immunoblots of SDS-polyacrylamide gels revealed that antibody directed against the major outer membrane protein reacted with the three high molecular weight aggregates present in the outer membrane which we have previously shown to be composed of the major outer membrane protein and three nonidentical small molecular weight proteins.     

A systematic assessment of mature MBP in membrane protein production: overexpression, membrane targeting and purification

Obtaining enough membrane protein in native or native-like status is still a challenge in membrane protein structure biology. Maltose binding protein (MBP) has been widely used as a fusion partner in improving membrane protein production. In the present work, a systematic assessment on the application of mature MBP (mMBP) for membrane protein overexpression and purification was performed on 42 membrane proteins, most of which showed no or poor expression level in membrane fraction fused with an N-terminal Histag. It was found that most of the small membrane proteins were overexpressed in the native membrane of Escherichia coli when using mMBP. In addition, the proteolysis of the fusions were performed on the membrane without solubilization with detergents, leading to the development of an efficient protocol to directly purify the target membrane proteins from the membrane fraction through a one-step affinity chromatography. Our results indicated that mMBP is an excellent fusion partner for overexpression, membrane targeting and purification of small membrane proteins. The present expression and purification method may be a good solution for the large scale preparation of small membrane proteins in structural and functional studies.     

Green Fluorescent Protein-based Expression Screening of Membrane Proteins in Escherichia coli

The production of recombinant membrane proteins for structural and functional studies remains technically challenging due to low levels of expression and the inherent instability of many membrane proteins once solubilized in detergents. A protocol is described that combines ligation independent cloning of membrane proteins as GFP fusions with expression in Escherichia coli detected by GFP fluorescence. This enables the construction and expression screening of multiple membrane protein/variants to identify candidates suitable for further investment of time and effort. The GFP reporter is used in a primary screen of expression by visualizing GFP fluorescence following SDS polyacrylamide gel electrophoresis (SDS-PAGE). Membrane proteins that show both a high expression level with minimum degradation as indicated by the absence of free GFP, are selected for a secondary screen. These constructs are scaled and a total membrane fraction prepared and solubilized in four different detergents. Following ultracentrifugation to remove detergent-insoluble material, lysates are analyzed by fluorescence detection size exclusion chromatography (FSEC). Monitoring the size exclusion profile by GFP fluorescence provides information about the mono-dispersity and integrity of the membrane proteins in different detergents. Protein: detergent combinations that elute with a symmetrical peak with little or no free GFP and minimum aggregation are candidates for subsequent purification. Using the above methodology, the heterologous expression in E. coli of SED (shape, elongation, division, and sporulation) proteins from 47 different species of bacteria was analyzed. These proteins typically have ten transmembrane domains and are essential for cell division. The results show that the production of the SEDs orthologues in E. coli was highly variable with respect to the expression levels and integrity of the GFP fusion proteins. The experiment identified a subset for further investigation.     

人早胚发育相关蛋白ZNF268的抗体制备

目的:获得纯化抗原用于制备ZNF268的多克隆抗体。方法:PCR扩增目的基因片Spacer区序列,亚克隆入融合蛋白表达载体pET28a+,构建了重组质粒pET28a+/SPA。然后将该重组质粒转化E.coliDE3(Rosseta),IPTG诱导表达,SDS-PAGE分离,获纯化融合蛋白6His-SPA。用6His-SPA免疫家兔,颈动脉取血,分离血清,从血清中获得ZNF268特异的多克隆抗体。结论:通过构建融合蛋白重组表达质粒pET28a+/SPA,用获得的初步纯化融合蛋白6His-SPA,制备了特异的ZNF268的多克隆抗体6His-SPA Ab。     

Cell-free expression as an emerging technique for the large scale production of integral membrane protein

Membrane proteins are highly underrepresented in structural data banks due to tremendous difficulties that occur upon approaching their structural analysis. Inefficient sample preparation from conventional cellular expression systems is in many cases the first major bottleneck. Preparative scale cell-free expression has now become an emerging alternative tool for the high level production of integral membrane proteins. Many toxic effects attributed to the overproduction of recombinant proteins are eliminated by cell-free expression as viable host cells are no longer required. A unique characteristic is the open nature of cell-free systems that offers a variety of options to manipulate the reaction conditions in order to protect or to stabilize the synthesized recombinant proteins. Detergents or lipids can easily be supplemented and membrane proteins can therefore be synthesized directly into a defined hydrophobic environment of choice that permits solubility and allows the functional folding of the proteins. Alternatively, cell-free produced precipitates of membrane proteins can efficiently be solubilized in mild detergents after expression. Highly valuable for structural approaches is the fast and efficient cell-free production of uniformly or specifically labeled proteins. A considerable number of membrane proteins from diverse families like prokaryotic small multidrug transporters or eukaryotic G-protein coupled receptors have been produced in cell-free systems in high amounts and in functionally active forms. We will give an overview about the current state of the art of this new approach with special emphasis on technical aspects as well as on the functional and structural characterization of cell-free produced membrane proteins.     

An efficient strategy for high-throughput expression screening of recombinant integral membrane proteins

The recombinant expression of integral membrane proteins is considered a major challenge, and together with the crystallization step, the major hurdle toward routine structure determination of membrane proteins. Basic methodologies for high-throughput (HTP) expression optimization of soluble proteins have recently emerged, providing statistically significant success rates for producing such proteins. Experimental procedures for handling integral membrane proteins are generally more challenging, and there have been no previous comprehensive reports of HTP technology for membrane protein production. Here, we present a generic and integrated parallel HTP strategy for cloning and expression screening of membrane proteins in their detergent solubilized form. Based on this strategy, we provide overall success rates for membrane protein production in Escherichia coli, as well as initial benchmarking statistics of parameters such as expression vectors, strains, and solubilizing detergents. The technologies were applied to 49 E. coli integral membrane proteins with human homologs and revealed that 71% of these proteins could be produced at sufficient levels to allow milligram amounts of protein to be relatively easily purified, which is a significantly higher success rate than anticipated. We attribute the high success rate to the quality and robustness of the methodology used, and to introducing multiple parameters such as different vectors, strains, and detergents. The presented strategy demonstrates the usefulness of HTP technologies for membrane protein production, and the feasibility of large-scale programs for elucidation of structure and function of bacterial integral membrane proteins.     

Optimizing synthesis and expression of transmembrane peptides and proteins

Structural studies of full-length membrane proteins have been hindered by their hydrophobicity and low expression in a variety of systems. However, a simplifying aspect of membrane protein folding is that individual transmembrane segments or membrane protein fragments have been observed to represent independent folding domains, and as such, can facilitate the study of packing interactions between TM helices, and the collection of structural information regarding membrane proteins. This review focuses on two categories of techniques--total peptide synthesis and bacterial expression--that can each be optimized for preparation of transmembrane protein segments. First, synthesis of hydrophobic transmembrane peptides that are N- and/or C-tagged with solubilizing residues such as lysine can improve manipulation of the transmembrane core in a variety of biophysical experiments. In this context, we describe general protocol considerations during the synthesis, cleavage, and purification stages of these peptides to identify appropriate parameters that combine to improve yields of hydrophobic peptides. Second, bacterial expression of membrane protein fragments is a useful tool for producing large quantities of hydrophobic protein segments. Targeting protein expression within Escherichia coli can facilitate purification, while attaching the hydrophobic construct to a hydrophilic fusion protein can amplify expression. We show that adapting protein constructs to comply with expression host specifications, in concert with thorough exploration of expression conditions such as the type of media used for expression, temperature, and cell strain, can significantly improve protein yields.