And P, C41 (DE3) and C43 (DE3). 500 ml of an overnight preculture was used to innoculate 500 ml autoinduction media in a 2 l flask. Cultures were grown at 37uC for 5 h with shaking at 200 rpm until glucose was used up (tested by glucose test strips). Cultures were continually grown at 37uC for 5 h or overnight with shaking at 200 rpm. Expression with IPTG at 18uC was carried out in the same E. coli strains. After transforming into the different strains (BL21CodonPlus -RIL and P, C41(DE3) and C43 (DE3)) 2? fresh colonies from plates complemented with 2 glucose, were inoculated into DYT or TB containing 50 mg/ml kanamycin and 2 D-glucose at 37uC. Routinely 7 hours later, a fraction of the preculture with high cell density (typically at OD600 = 5?) was diluted into 500 ml of the respective medium containing 50 mg/ml kanamycin to obtain a cell density at OD600 of 0.1?.2 and grown with vigorous shaking until the OD600 reached 0.6?0.8 (usually 1 h) at 37uC. IPTG was added to a final concentration of 0.2, 0.4, 0.8 and 1 mM, and growth was continued for a further 3? h at 37uC or 8?2 h at 18uC. Bacteria were then harvested by 
centrifugation at 6000 rpm (JLA-8.1000, Beckman) for 20 min. Pellets were flash frozen in liquid nitrogen and stored at 280uC until used for purification.The membrane pellets were solubilised at 1 g/10?0 ml of membrane/solubilisation buffer for 1? h at 4uC. The supernatant was separated by ultracentrifugation at 100,000 g (1 h, 4uC) and subsequently incubated with Ni-NTA resin at a ratio of 0.2?0.4 mg target protein/ml Ni-NTA resin in batch mode. The resin was then poured into a column and excess solution collected (flowthrough). After washing with 10 bed volumes of wash buffer, proteins bound to the resin were eluted with 5 bed volumes with a final concentration of 300 mM imidazole in wash buffer. All the elution fractions were pooled and diluted to a final concentration of 25 mM imidazole, and then passed again onto Ni-NTA to conduct wash and elution steps as described above. As a final purification step the OPRM sample was subjected to a superdex 200 (16/60) gel SIS3 site filtration column (GE Healthcare) in gel filtration buffer. SDS-PAGE and western blotting were used to identify fractions containing OPRM.the samples were flash-frozen in liquid nitrogen and then stored at 220uC until further use.Mass SpectroscopyTo confirm the identity of the isolated protein, peptide mass fingerprints were determined by mass spectroscopy. The coomassie stained band was excised and tert-Butylhydroquinone web destained. The protein was reduced with 10 mM DTT in 25 mM NH4HCO3 solution at 56uC for 1 hour and then incubated in 55 mM iodoacetamide in 25 mM NH4HCO3 for 45 min at room temperature in darkness. Gel pieces were then incubated with 20 ml trypsin (Sigma) solution (20 mg/ml) to be digested overnight at 37uC. The resulting fragments were then analyzed by a Bruker Daltonic Ultraflex III TOF/TOF mass spectrometer.Circular Dichroism MeasurementsAll CD spectra were acquired at 25uC on a Jasco J-810 spectropolarimeter using 0.1 cm path length cylindrical quartz cuvettes. The purified protein was desalted by a PD10 (GE Healthcare) column and subsequently concentrated by ultrafiltration (Amicon, Millipore). Measurements were performed in Buffer B to reduce background signal. Protein concentrations were determined by UV-spectrometry using extinction coefficients [40]. The CD spectra were obtained from 190 to 280 nm using a bandwidth of 1 nm, a step width of 0.5 nm, a scann.And P, C41 (DE3) and C43 (DE3). 500 ml of an overnight preculture was used to innoculate 500 ml autoinduction media in a 2 l flask. Cultures were grown at 37uC for 5 h with shaking at 200 rpm until glucose was used up (tested by glucose test strips). Cultures were continually grown at 37uC for 5 h or overnight with shaking at 200 rpm. Expression with IPTG at 18uC was carried out in the same E. coli strains. After transforming into the different strains (BL21CodonPlus -RIL and P, C41(DE3) and 
C43 (DE3)) 2? fresh colonies from plates complemented with 2 glucose, were inoculated into DYT or TB containing 50 mg/ml kanamycin and 2 D-glucose at 37uC. Routinely 7 hours later, a fraction of the preculture with high cell density (typically at OD600 = 5?) was diluted into 500 ml of the respective medium containing 50 mg/ml kanamycin to obtain a cell density at OD600 of 0.1?.2 and grown with vigorous shaking until the OD600 reached 0.6?0.8 (usually 1 h) at 37uC. IPTG was added to a final concentration of 0.2, 0.4, 0.8 and 1 mM, and growth was continued for a further 3? h at 37uC or 8?2 h at 18uC. Bacteria were then harvested by centrifugation at 6000 rpm (JLA-8.1000, Beckman) for 20 min. Pellets were flash frozen in liquid nitrogen and stored at 280uC until used for purification.The membrane pellets were solubilised at 1 g/10?0 ml of membrane/solubilisation buffer for 1? h at 4uC. The supernatant was separated by ultracentrifugation at 100,000 g (1 h, 4uC) and subsequently incubated with Ni-NTA resin at a ratio of 0.2?0.4 mg target protein/ml Ni-NTA resin in batch mode. The resin was then poured into a column and excess solution collected (flowthrough). After washing with 10 bed volumes of wash buffer, proteins bound to the resin were eluted with 5 bed volumes with a final concentration of 300 mM imidazole in wash buffer. All the elution fractions were pooled and diluted to a final concentration of 25 mM imidazole, and then passed again onto Ni-NTA to conduct wash and elution steps as described above. As a final purification step the OPRM sample was subjected to a superdex 200 (16/60) gel filtration column (GE Healthcare) in gel filtration buffer. SDS-PAGE and western blotting were used to identify fractions containing OPRM.the samples were flash-frozen in liquid nitrogen and then stored at 220uC until further use.Mass SpectroscopyTo confirm the identity of the isolated protein, peptide mass fingerprints were determined by mass spectroscopy. The coomassie stained band was excised and destained. The protein was reduced with 10 mM DTT in 25 mM NH4HCO3 solution at 56uC for 1 hour and then incubated in 55 mM iodoacetamide in 25 mM NH4HCO3 for 45 min at room temperature in darkness. Gel pieces were then incubated with 20 ml trypsin (Sigma) solution (20 mg/ml) to be digested overnight at 37uC. The resulting fragments were then analyzed by a Bruker Daltonic Ultraflex III TOF/TOF mass spectrometer.Circular Dichroism MeasurementsAll CD spectra were acquired at 25uC on a Jasco J-810 spectropolarimeter using 0.1 cm path length cylindrical quartz cuvettes. The purified protein was desalted by a PD10 (GE Healthcare) column and subsequently concentrated by ultrafiltration (Amicon, Millipore). Measurements were performed in Buffer B to reduce background signal. Protein concentrations were determined by UV-spectrometry using extinction coefficients [40]. The CD spectra were obtained from 190 to 280 nm using a bandwidth of 1 nm, a step width of 0.5 nm, a scann.
