Ng the receptor was located in the membrane. Solubilisation using mild detergents turned out to be only moderately successful. Extraction of OPRM with SDS, laurylsarcosine alone, or 6 M urea with 0.8 (w/v) laurylsarcosine proved to be most efficient (Title Loaded From File Figure 3A and B). The detergent Fos-12 was outstanding in solubilisation of the receptor. No residual receptor was found in the pellet after solubilisation.Isolation of OPRMPurification of OPRM was carried out with several purification 1655472 strategies such as affinity chromatography, ionic exchange chromatography and size exclusion chromatography. Ionic exchange chromatography was found to be of limited value in purification of the membrane protein especially when solubilised with an ionic or zwitterionic detergent. OPRM extracted from membrane was purified through metal chelate affinity chromatography (NI-NTA) two times, followed by size exclusion (Superdex 200) chromatography. In the first purification step the majority of OPRM can be captured by NiNTA (Figure 4A). A second Ni-NTA chromatography of the diluted sample improves the purity to ca. 85 . Residual impurities and aggregated material were removed by (SEC) size exclusion chromatography (Figure 4B). It was also used to assess the state of aggregation of OPRM (Figure 5): Peak 1 (Superdex 200 HR 10/30, GE Healthcare in 0.1 (w/v) Fos-12) shows aggregated protein. It was regarded to be caused by the instability of the protein in detergent, respectively the presence of misfolded and unfolded protein. Thus a final yield of 0.17 mg/liter of culture was obtained by Ni-NTA and size exclusion chromatography (Figure 4B). The elution profile of the receptor shows a peak with an apparent molecular weight of the Fos-12/receptor Title Loaded From File complex of ca. 158 kDa (underlined in Figure 5). The expected molecular weight of the Fos-12/receptor complex is ca. 65 kDa (Mw of OPRM 46 kD, and Mw of Fos-12 micelle (in H2O) ,19 kD). It appears that the apparent molecular weight for this Fos-12/receptor complex does not agree with the expected molecular weight of the monomeric detergent-receptor complex. The difference between the predicted and the observed Mw might be due to non-ideal behavior of the detergent/receptor complex in the size exclusion column or dimerisation.Figure 1. Expression of the N-terminally his-tagged OPRM protein. Western blot on His-tag. A, Expression by autoinduction at 37uC in different E.coli strains (RP, RIL, C41, and C43). Lane 1 uninduced, lane 2 nclusion body fraction (induced 4 h), lane 3?Membrane fraction (induced 4 h), 1317923 lane 4 nclusion body fraction (induced 20 h), lane 5 embrane fraction (induced 20 h). B, Optimised expression of OPRM using C43 cells, TB medium with 0.4 mM IPTG at 18uC. Western blot showed inclusion body (IB) and membrane fractions (M) of OPRM. doi:10.1371/journal.pone.0056500.gConfirmation of Full Length of OPRMOPRM, western blot positive for the N-terminal his-tag, was found at a position of around 38 kDa on 12 SDS-PAGE (Figure 4), though the expected Mw is 46 kDa. Several integral membrane proteins including several GPCRs were found to migrate anomalously smaller than expected on SDS AGE due toOPRM from E. coliFigure 2. Growth conditions of OPRM in different E.coli strains. Expression of OPRM was induced by IPTG. Cell culture density (OD600) and weight of cell pellet (g) after different induction times with two different media (TB and DYT) was measured. Cell pellet (g) was obtained from 1 liter of culture medium. doi:1.Ng the receptor was located in the membrane. Solubilisation using mild detergents turned out to be only moderately successful. Extraction of OPRM with SDS, laurylsarcosine alone, or 6 M urea with 0.8 (w/v) laurylsarcosine proved to be most efficient (Figure 3A and B). The detergent Fos-12 was outstanding in solubilisation of the receptor. No residual receptor was found in the pellet after solubilisation.Isolation of OPRMPurification of OPRM was carried out with several purification 1655472 strategies such as affinity chromatography, ionic exchange chromatography and size exclusion chromatography. Ionic exchange chromatography was found to be of limited value in purification of the membrane protein especially when solubilised with an ionic or zwitterionic detergent. OPRM extracted from membrane was purified through metal chelate affinity chromatography (NI-NTA) two times, followed by size exclusion (Superdex 200) chromatography. In the first purification step the majority of OPRM can be captured by NiNTA (Figure 4A). A second Ni-NTA chromatography of the diluted sample improves the purity to ca. 85 . Residual impurities and aggregated material were removed by (SEC) size exclusion chromatography (Figure 4B). It was also used to assess the state of aggregation of OPRM (Figure 5): Peak 1 (Superdex 200 HR 10/30, GE Healthcare in 0.1 (w/v) Fos-12) shows aggregated protein. It was regarded to be caused by the instability of the protein in detergent, respectively the presence of misfolded and unfolded protein. Thus a final yield of 0.17 mg/liter of culture was obtained by Ni-NTA and size exclusion chromatography (Figure 4B). The elution profile of the receptor shows a peak with an apparent molecular weight of the Fos-12/receptor complex of ca. 158 kDa (underlined in Figure 5). The expected molecular weight of the Fos-12/receptor complex is ca. 65 kDa (Mw of OPRM 46 kD, and Mw of Fos-12 micelle (in H2O) ,19 kD). It appears that the apparent molecular weight for this Fos-12/receptor complex does not agree with the expected molecular weight of the monomeric detergent-receptor complex. The difference between the predicted and the observed Mw might be due to non-ideal behavior of the detergent/receptor complex in the size exclusion column or dimerisation.Figure 1. Expression of the N-terminally his-tagged OPRM protein. Western blot on His-tag. A, Expression by autoinduction at 37uC in different E.coli strains (RP, RIL, C41, and C43). Lane 1 uninduced, lane 2 nclusion body fraction (induced 4 h), lane 3?Membrane fraction (induced 4 h), 1317923 lane 4 nclusion body fraction (induced 20 h), lane 5 embrane fraction (induced 20 h). B, Optimised expression of OPRM using C43 cells, TB medium with 0.4 mM IPTG at 18uC. Western blot showed inclusion body (IB) and membrane fractions (M) of OPRM. doi:10.1371/journal.pone.0056500.gConfirmation of Full Length of OPRMOPRM, western blot positive for the N-terminal his-tag, was found at a position of around 38 kDa on 12 SDS-PAGE (Figure 4), though the expected Mw is 46 kDa. Several integral membrane proteins including several GPCRs were found to migrate anomalously smaller than expected on SDS AGE due toOPRM from E. coliFigure 2. Growth conditions of OPRM in different E.coli strains. Expression of OPRM was induced by IPTG. Cell culture density (OD600) and weight of cell pellet (g) after different induction times with two different media (TB and DYT) was measured. Cell pellet (g) was obtained from 1 liter of culture medium. doi:1.
