Wheat-germ cell-free production of prion proteins for solid-state NMR structural studies

Half a Century of Progress: The Evolution of Wheat Germ-Based In Vitro Translation into a Versatile Protein Production Method

The first demonstration of wheat germ extract (WGE)-based in vitro translation synthesising a protein from exogenously introduced messenger ribonucleic acid (mRNA) was published approximately fifty years ago. Since then, there have been numerous crucial improvements to the WGE-based in vitro translation, resulting in a significant increase in yield and the development of high-throughput protein-producing platforms. These developments have transformed the original setup into a versatile eukaryotic protein production method with broad applications. The present review explores the theoretical background of the implemented modifications and brings a panel of examples for WGE applications in high-throughput protein studies and synthesis of challenging-to-produce proteins such as protein complexes, extracellular proteins, and membrane proteins. It also highlights the unique advantages of in vitro translation as an open system for synthesising radioactively labelled proteins, as illustrated by numerous publications using WGE to meet the protein demands of these studies. This review aims to orientate readers in finding the most appropriate WGE arrangement for their specific needs and demonstrate that a deeper understanding of the system modifications will help them make further adjustments to the reaction conditions for synthesising difficult-to-express proteins.

Cell-free expression, purification, and membrane reconstitution for NMR studies of the nonstructural protein 4B from hepatitis C virus

We describe the expression of the hepatitis C virus nonstructural protein 4B (NS4B), which is an integral membrane protein, in a wheat germ cell-free system, the subsequent purification and characterization of NS4B and its insertion into proteoliposomes in amounts sufficient for multidimensional solid-state NMR spectroscopy. First spectra of the isotopically [(2)H,(13)C,(15)N]-labeled protein are shown to yield narrow (13)C resonance lines and a proper, predominantly α-helical fold. Clean residue-selective leucine, isoleucine and threonine-labeling is demonstrated. These results evidence the suitability of the wheat germ-produced integral membrane protein NS4B for solid-state NMR. Still, the proton linewidth under fast magic angle spinning is broader than expected for a perfect sample and possible causes are discussed.

Functional expression, purification, characterization, and membrane reconstitution of non-structural protein 2 from hepatitis C virus

Non-structural protein 2 (NS2) of the hepatitis C virus (HCV) is an integral membrane protein that contains a cysteine protease and that plays a central organizing role in assembly of infectious progeny virions. While the crystal structure of the protease domain has been solved, the NS2 full-length form remains biochemically and structurally uncharacterized because recombinant NS2 could not be prepared in sufficient quantities from cell-based systems. We show here that functional NS2 in the context of the NS2-NS3pro precursor protein, ensuring NS2-NS3 cleavage, can be efficiently expressed by using a wheat germ cell-free expression system. In this same system, we subsequently successfully produce and purify milligram amounts of a detergent-solubilized form of full-length NS2 exhibiting the expected secondary structure content. Furthermore, immuno-electron microscopy analyses of reconstituted proteoliposomes demonstrate NS2 association with model membranes.

Wheat germ cell-free expression: Two detergents with a low critical micelle concentration allow for production of soluble HCV membrane proteins

Membrane proteins are notoriously difficult to express in a soluble form. Here, we use wheat germ cell-free expression in the presence of various detergents to produce the non-structural membrane proteins 2, 4B and 5A of the hepatitis C virus (HCV). We show that lauryl maltose neopentyl glycol (MNG-3) and dodecyl octaethylene glycol ether (C12E8) detergents can yield essentially soluble membrane proteins at detergent concentrations that do not inhibit the cell-free reaction. This finding can be explained by the low critical micelle concentration (CMC) of these detergents, which keeps the monomer concentrations low while at the same time providing the necessary excess of detergent concentration above CMC required for full target protein solubilization. We estimate that a tenfold excess of detergent micelles with respect to the protein concentration is sufficient for solubilization, a number that we propose as a guideline for detergent screening assays.

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.

How to investigate interactions between membrane proteins and ligands by solid-state NMR

Solid-state NMR is an established method for biophysical studies of membrane proteins within the lipid bilayers and an emerging technique for structural biology in general. In particular magic angle sample spinning has been found to be very useful for the investigation of large membrane proteins and their interaction with small molecules within the lipid bilayer. Using a number of examples, we illustrate and discuss in this chapter, which information can be gained and which experimental parameters need to be considered when planning such experiments. We focus especially on the interaction of diffusive ligands with membrane proteins.

Establishment and optimization of a wheat germ cell-free protein synthesis system and its application in venom kallikrein

Wheat germ cell-free protein synthesis systems have the potential to synthesize functional proteins safely and with high accuracy, but the poor energy supply and the instability of mRNA templates reduce the productivity of this system, which restricts its applications. In this report, phosphocreatine and pyruvate were added to the system to supply ATP as a secondary energy source. After comparing the protein yield, we found that phosphocreatine is more suitable for use in the wheat germ cell-free protein synthesis system. To stabilize the mRNA template, the plasmid vector, SP6 RNA polymerase, and Cu(2+) were optimized, and a wheat germ cell-free protein synthesis system with high yield and speed was established. When plasmid vector (30 ng/μl), SP6 RNA polymerase (15 U), phosphocreatine (25 mM), and Cu(2+) (5 mM) were added to the system and incubated at 26°C for 16 h, the yield of venom kallikrein increased from 0.13 to 0.74 mg/ml. The specific activity of the recombinant protein was 1.3 U/mg, which is only slightly lower than the crude venom kallikrein (1.74 U/mg) due to the lack of the sugar chain. In this study, the yield of venom kallikrein was improved by optimizing the system, and a good foundation has been laid for industrial applications and for further studies.