Gene. OverExpressTM C41 (DE3) and C43 (DE3) were purchased from Lucigen. DNA encoding the humanopioid receptor was provided by Qiagen (Germany). Ni-NTA was purchased from Qiagen (Germany). Superdex 200 (16/60) and analytical grade Superdex 200 HR 10/30 size exclusion chromatography were from GE Healthcare. All other chemicals were from either Sigma-Aldrich or Fluka. Fos-12 was purchased from Anatrace (Maumee, OH) and other detergents were purchased from GLYCON (Germany). Buffer A: 20 mM Tris Cl, 150 mM NaCl, 1676428 10 Glycerol, pH 8. Solubilisation buffer: 20 mM Tris?HCl, 300 mM NaCl, 10 Glycerol, pH 8, 1 Fos-12, 5 mM imidazole. Wash buffer: 20 mM Tris Cl, 300 mM NaCl, 10 Glycerol, pH 8, 0.1 Fos-12, 25 mM imidazole. Elution buffer: 20 mM Tris Cl, 300 mM NaCl, 10 Glycerol, pH 8, 0.1 Fos-12, 300 mM imidazole. Gel filtration buffer: 20 mM Tris?HCl, 150 mM NaCl, 10 Glycerol, pH 8, 0.1 Fos-12. Buffer B: 5 mM NaHPO4, 10 glycerol, 0.07 Fos-12, pH 7.5 (with or without 1 mM TCEP, as required).Expression of Recombinant OPRMFigure 7. Secondary structural analysis of purified OPRM protein. The Circular dichroism spectrum of OPRM at 25uC. Mean residue ellipticity [h] in degrees6cm26dmol21. doi:10.1371/journal.pone.0056500.gThe synthetic human mu opioid receptor gene (GENEART) was constructed into the Qiagen plasmid pQE-2 thereby encoding full-length OPRM with either an N-terminal or C-terminal decahistidine tag. Any codons that are get Triptorelin rarely used in E. coli were avoided.OPRM from E. coliHigh Pressure Homogenizer EmulsiFlex-C3 (Avestin, Solvent Yellow 14 Canada) or Constant Cell Disruption Systems (Constant Systems, UK) in buffer A plus 5 mM MgCl2, 2 mM ?ME, 1 mM EDTA, DNAse, lysozyme (1 mg/ml), supplemented with EDTA-free protease inhibitors (one tablet/50?00 ml, Roche). The 
cell lysate was centrifuged at 1000 g to remove unbroken cell and cell debris, followed by another centrifugation at 10000 g for 40 min to collect white inclusion bodies. The supernatant was further centrifuged at 100,000 g for 1 h to harvest a membrane fraction. Pellets were flash frozen and stored at 280uC until further use.Detergent Screening: Small Scale Solubilisation of OPRM1 g of the resulting membrane pellet was solubilised in 10?20 ml of solubilisation buffer (buffer A containing detergents or chaotropic agents). The following detergents were used as the solubilisers: 1 LDAO, 1 Fos-12, 1 DDM, 1 Cy6, 0.8 laurysarcosine, 1 SDS, 6 M urea. The solubilisation was allowed to proceed with gentle agitation at 4uC for 2 h. The solubilised supernatant was separated by centrifugation at 20,000 g (4uC, 0.5 h). The respective membrane fractions before and after solubilisation and the residue pellet were analyzed by western blot.Isolation of OPRMFigure 8. Interaction of OPRM with Endomorphin-1 by Surface Plasmon Resonance (SPR). SPR shows the apparent association increases in RU response with the addition of EM-1 at 25uC. The binding constant (KD) of EM-1 to OPRM was obtained from (Rmax-R)*C/R, where C is concentration of EM-1, total concentration of OPRM is proportional to maximum binding capacity Rmax, Concentration of complex is measured directly as Response Unit in R. A KD of 60.9618.1 nM for EM-1 was determined by fitting the data with a 1:1 interaction model. Error bars represent values of two duplicates. doi:10.1371/journal.pone.0056500.gExpression with autoinduction was carried out at 37uC [39]. Plasmids were transformed into the different E. coli expression strains: BL21-CodonPlus-RIL.Gene. OverExpressTM C41 (DE3) and C43 (DE3) were purchased from Lucigen. DNA encoding the humanopioid receptor was provided by Qiagen (Germany). Ni-NTA was purchased from Qiagen (Germany). Superdex 200 (16/60) and analytical grade Superdex 200 HR 10/30 size exclusion chromatography were from GE Healthcare. All other chemicals were from either Sigma-Aldrich or Fluka. Fos-12 was purchased from Anatrace (Maumee, OH) and other detergents 
were purchased from GLYCON (Germany). Buffer A: 20 mM Tris Cl, 150 mM NaCl, 1676428 10 Glycerol, pH 8. Solubilisation buffer: 20 mM Tris?HCl, 300 mM NaCl, 10 Glycerol, pH 8, 1 Fos-12, 5 mM imidazole. Wash buffer: 20 mM Tris Cl, 300 mM NaCl, 10 Glycerol, pH 8, 0.1 Fos-12, 25 mM imidazole. Elution buffer: 20 mM Tris Cl, 300 mM NaCl, 10 Glycerol, pH 8, 0.1 Fos-12, 300 mM imidazole. Gel filtration buffer: 20 mM Tris?HCl, 150 mM NaCl, 10 Glycerol, pH 8, 0.1 Fos-12. Buffer B: 5 mM NaHPO4, 10 glycerol, 0.07 Fos-12, pH 7.5 (with or without 1 mM TCEP, as required).Expression of Recombinant OPRMFigure 7. Secondary structural analysis of purified OPRM protein. The Circular dichroism spectrum of OPRM at 25uC. Mean residue ellipticity [h] in degrees6cm26dmol21. doi:10.1371/journal.pone.0056500.gThe synthetic human mu opioid receptor gene (GENEART) was constructed into the Qiagen plasmid pQE-2 thereby encoding full-length OPRM with either an N-terminal or C-terminal decahistidine tag. Any codons that are rarely used in E. coli were avoided.OPRM from E. coliHigh Pressure Homogenizer EmulsiFlex-C3 (Avestin, Canada) or Constant Cell Disruption Systems (Constant Systems, UK) in buffer A plus 5 mM MgCl2, 2 mM ?ME, 1 mM EDTA, DNAse, lysozyme (1 mg/ml), supplemented with EDTA-free protease inhibitors (one tablet/50?00 ml, Roche). The cell lysate was centrifuged at 1000 g to remove unbroken cell and cell debris, followed by another centrifugation at 10000 g for 40 min to collect white inclusion bodies. The supernatant was further centrifuged at 100,000 g for 1 h to harvest a membrane fraction. Pellets were flash frozen and stored at 280uC until further use.Detergent Screening: Small Scale Solubilisation of OPRM1 g of the resulting membrane pellet was solubilised in 10?20 ml of solubilisation buffer (buffer A containing detergents or chaotropic agents). The following detergents were used as the solubilisers: 1 LDAO, 1 Fos-12, 1 DDM, 1 Cy6, 0.8 laurysarcosine, 1 SDS, 6 M urea. The solubilisation was allowed to proceed with gentle agitation at 4uC for 2 h. The solubilised supernatant was separated by centrifugation at 20,000 g (4uC, 0.5 h). The respective membrane fractions before and after solubilisation and the residue pellet were analyzed by western blot.Isolation of OPRMFigure 8. Interaction of OPRM with Endomorphin-1 by Surface Plasmon Resonance (SPR). SPR shows the apparent association increases in RU response with the addition of EM-1 at 25uC. The binding constant (KD) of EM-1 to OPRM was obtained from (Rmax-R)*C/R, where C is concentration of EM-1, total concentration of OPRM is proportional to maximum binding capacity Rmax, Concentration of complex is measured directly as Response Unit in R. A KD of 60.9618.1 nM for EM-1 was determined by fitting the data with a 1:1 interaction model. Error bars represent values of two duplicates. doi:10.1371/journal.pone.0056500.gExpression with autoinduction was carried out at 37uC [39]. Plasmids were transformed into the different E. coli expression strains: BL21-CodonPlus-RIL.