Cell-free expression and in meso crystallisation of an integral membrane kinase for structure determination

Membrane proteins are key elements in cell physiology and drug targeting, but getting a high-resolution structure by crystallographic means is still enormously challenging. Novel strategies are in big demand to facilitate the structure determination process that will ultimately hasten the day when sequence information alone can provide a three-dimensional model. Cell-free or in vitro expression enables rapid access to large quantities of high-quality membrane proteins suitable for an array of applications. Despite its impressive efficiency, to date only two membrane proteins produced by the in vitro approach have yielded crystal structures. Here, we have analysed synergies of cell-free expression and crystallisation in lipid mesophases for generating an X-ray structure of the integral membrane enzyme diacylglycerol kinase to 2.28-Å resolution. The quality of cellular and cell-free-expressed kinase samples has been evaluated systematically by comparing (1) spectroscopic properties, (2) purity and oligomer formation, (3) lipid content and (4) functionality. DgkA is the first membrane enzyme crystallised based on cell-free expression. The study provides a basic standard for the crystallisation of cell-free-expressed membrane proteins and the methods detailed here should prove generally useful and contribute to accelerating the pace at which membrane protein structures are solved.

High-level cell-free production of membrane proteins with nanodiscs

This chapter addresses two major bottlenecks in cell-free membrane protein production. Firstly, we describe the optimization of expression templates for obtaining membrane proteins in preparative scales. We present details for a newly established tag variation screen providing high success rates in improving expression efficiencies while having only minimal impacts on the target protein structure. Secondly, we present protocols for the efficient co-translational insertion of membrane proteins into defined lipid bilayers. We describe the production of nanodiscs and their implementation into cell-free expression reactions for the co-translational reconstitution of membrane proteins. In addition we give guidelines for the loading of nanodiscs with different lipids in order to systematically analyze effects of lipids on the translocation, functional folding, and stability of cell-free expressed membrane proteins.

A systematic approach to increase the efficiency of membrane protein production in cell-free expression systems

High amounts of membrane protein samples are needed for structural or functional analysis and a first bottleneck is often to obtain sufficient production efficiencies. The reduced complexity of protein production in cell-free expression systems results in a frequent correlation of efficiency problems with the essential transcription/translation process. We present a systematic tag variation strategy for the rapid improvement of cell-free expression efficiencies of membrane proteins based on the optimization of translation initiation. A small number of rationally designed short expression tags is attached via overlap PCR to the 5-prime end of the target protein coding sequence. The generated pool of DNA templates is analyzed in a cell-free expression screen and the most efficient template is selected for further preparative scale protein production. The expression tags can be minimized to only a few codons and no further impact on the coding sequence is required. The complete process takes only few days and the synthesized PCR fragments can be used directly as templates for preparative scale cell-free reactions. The strategy is exemplified with the production of a set of G-protein coupled receptors and yield improvements of up to 32-fold were obtained. All proteins were finally synthesized in amounts sufficient for further quality optimization and initial crystallization screens.

Systems for the cell-free synthesis of proteins

We describe a system for the cell-free expression of proteins based on extracts from Escherichia coli. Two reaction configurations, batch and continuous exchange, are discussed and analytical scale as well as preparative scale setups are documented. Guidelines for the systematic development and optimization of cell-free expression protocols are given in detail. We further provide specific protocols and parameters for the cell-free production of membrane proteins. High-throughput screening applications of CF expression systems are exemplified as new tools for genomics and proteomics studies.