Applications of Cell-Free Synthesized Membrane Protein Precipitates

Cell-free protein expression systems are new core platforms for membrane protein synthesis. Expression in the presence of supplied artificial hydrophobic environments such as nanomembranes or micelles allows the co-translational solubilization and folding of membrane proteins. In the absence of hydrophobic compounds, the synthesized membrane proteins quantitatively precipitate, while frequently still retaining a significant part of folded structural elements. This so-called precipitate-forming cell-free (P-CF) expression mode is a very effective and reliable approach for numerous applications. Even from complex membrane proteins such as G-protein coupled receptors or large transporters, significant amounts of such precipitates can be synthesized within few hours. The precipitates can be solubilized in detergents or reconstituted into membranes for subsequent structural or functional analysis. Harsh denaturation and refolding procedures as known from the treatment of bacterial inclusion bodies are usually not required.This strategy is particularly interesting for applications requiring large amounts of membrane protein or fast access to a sample. It is further an excellent tool for the production of membrane protein antigens suitable for antibody generation. The purification of the precipitates in downstream processing is streamlined as only few proteins from the cell-free lysate may co-precipitate with the synthesized membrane protein. For most applications, a one-step affinity chromatography by taking advantage of small purification tags attached to the membrane protein target is sufficient. We give an overview on current applications of P-CF precipitates and describe the underlying techniques in detail. We furthermore provide protocols for the successful crystallization and NMR analysis of P-CF synthesized membrane proteins exemplified with the diacylglycerol kinase (DAGK). In addition, we describe the functional characterization of a P-CF synthesized large eukaryotic transporter.