Cell-free expression is emerging as a prime method for the rapid production of preparative quantities of high-quality membrane protein samples. The technology facilitates easy access to large numbers of proteins that have been extremely difficult to obtain. Most frequently used are cell-free systems based on extracts of Escherichia coli cells, and the reaction procedures are reliable and efficient. This protocol describes the preparation of all essential reaction components such as the E. coli cell extract, T7 RNA polymerase, DNA templates as well as the individual stock solutions. The setups of expression reactions in analytical and preparative scales, including a variety of reaction designs, are illustrated. We provide detailed reaction schemes that allow the preparation of milligram amounts of functionally folded membrane proteins of prokaryotic and eukaryotic origin in less than 24 h.

Allosteric modulators have been identified for several G protein-coupled receptors, most notably muscarinic receptors. To study their mechanism of action, we made use of a recently developed technique to generate fluorescence resonance energy transfer (FRET)-based sensors to monitor G protein-coupled receptor activation. Cyan fluorescent protein was fused to the C terminus of the M 2 muscarinic receptor, and a specific binding sequence for the small fluorescent compound fluorescein arsenical hairpin binder, FlAsH, was inserted into the third intracellular loop; the latter site was labeled in intact cells by incubation with FlAsH. We then measured FRET between the donor cyan fluorescent protein and the acceptor FlAsH in intact cells and monitored its changes in real time. Agonists such as acetylcholine and carbachol induced rapid changes in FRET, indicative of agonist-induced conformational changes. Removal of the agonists or addition of an antagonist caused a reversal of this signal with rate constants between 400 and 1100 ms. The allosteric ligands gallamine and dimethyl-W84 caused no changes in FRET when given alone, but increased FRET when given in the presence of an agonist, compatible with an inactivation of the receptors. The kinetics of these effects were very rapid, with rate constants of 80 -100 ms and ≈200 ms for saturating concentrations of gallamine and dimethyl-W84, respectively. Because these speeds are significantly faster than the responses to antagonists, these data indicate that gallamine and dimethyl-W84 are allosteric ligands and actively induce a conformation of the M 2 receptor with a reduced affinity for its agonists.

Morphine and several other opioids are important drugs for the treatment of acute and chronic pain. Opioid-induced analgesia is predominantly mediated by the μ-opioid receptor (MOR). When administered to humans, complex metabolic pathways lead to generation of many metabolites, nine of which may be considered major metabolites. While the properties of the two main compounds, morphine-6-glucuronide and morphine-3-glucuronide, are well described, the activity of other morphine metabolites is largely unknown. Here we performed an extensive pharmacological characterization by comparing efficacies and potencies of morphine and its nine major metabolites for the two main signaling pathways engaged by the human MOR, which occur via G(i)-protein activation and β-arrestins, respectively. We used radioligand binding studies and FRET-based methods to monitor MOR-mediated G(i)-protein activation and β-arrestin recruitment in single intact 293T cells. This approach identified two major groups of morphine metabolites, which we classified into "strong" and "weak" receptor ligands. Strong partial agonists morphine, morphine-6-glucuronide, normorphine, morphine-6-sulfate, 6-acetylmorphine and 3-acetylmorphine showed efficacies in the nanomolar range, while the weak metabolites morphine-N-oxide, morphine-3-sulfate, morphine-3-glucuronide and pseudomorphine activated MOR pathways only in the micromolar range. Interestingly, three metabolites, normorphine, 6-acetylmorphine and morphine-6-glucuronide, had lower potencies for Gi-protein activation but higher potencies and efficacies for β-arrestin recruitment than morphine itself, suggesting that they are biased towards β-arrestin pathways.