Ains, C41 (DE3) and C43 (DE3), in over-expressing toxic and membrane proteins has been previously demonstrated. The strain C43 (DE3) was derived from C41 (DE3) by selecting for resistance to a different toxic protein [31]. Compared with the other E.coli strains RP and RIL, the C41 and C43 strains were observed to yield more membrane mass per cell mass. This finding may explain one of the reasons whyFigure 3. Solubilisation of OPRM with detergents. A, solubilisation with urea or detergents. B, solubilisation with urea and laurylsarcosine. T -total membrane fraction, S -solubilised membrane fraction, P –3687-18-1 site pellet after solubilisation. doi:10.1371/journal.pone.0056500.gOPRM from E. coliFigure 4. Purification of OPRM from C43 in Fos-12. A, Purification of OPRM solubilised with Fos-12 by Ni-NTA. T -total membrane fraction, S solubilised membrane fraction, FT -flowthrough. W ?wash fractions (25 mM imidazole), E ?elution fractions (300 mM imidazole). The arrow denotes the monomeric OPRM. B, SEC -purified OPRM after size exclusion chromatography. doi:10.1371/journal.pone.0056500.gOverExpressTM C41 (DE3) and C43 (DE3), have been found to be superior for over-expressing toxic and membrane proteins. The over-expression of OPRM was largely tolerated by C43 with the conditions of 0.4 mM IPTG at 18uC for 8?2 h. Preferential membrane-insertion of OPRM instead of formation of inclusion bodies may be due to the larger mass of membrane in these strains. The membrane insertion of the protein represented first evidence that a correctly folded and stable OPRM was obtained. Interestingly, in contrast to the N-terminally tagged OPRM the expression of C-terminal decahis-tag OPRM in C43 gave only a poor expression level of the protein. This result appears to contradict the conclusion that a hexahistidine tag fused at the amino terminus of opioid receptor decreased expression levels markedly in baculovirus-infected insect cells [32] due to the “positive inside rule” described for E.coli membrane proteins and GPCRs [33]. The so-called positive inside rule states thatFigure 5. Size exclusion chromatography of OPRM in Fos-12. Purification of OPRM was performed in analytical grade Superdex 200 HR 10/30 size exclusion chromatography. Peak 1 identifies the aggregation of OPRM. The underlined Peak 2 shows the monomeric form of OPRM. doi:10.1371/journal.pone.0056500.gcytoplasmic segments contain more positive charges than extracytoplasmic segments. This is also true for OPRM. It was also previously claimed that due to poor expression of OPRM with a C-terminal his-tag, E.coli may be not a suitable expression system for OPRM [14]. In the light of our results this conclusion appears to be overly generalising. In our case, the expression 15900046 level of the N-terminally his-tagged receptor could be obtained in yields of 0.3?.5 mg/liter of culture, which is the highest yield obtained for GPCRs from E.coli membrane ever reported. The obtained yield of purified OPRM is 0.17 mg/liter of culture, which corresponds to 30?0 of expressed OPRM. Several mild Rubusoside detergents were used for solubilisation of the receptor, only to find solubilisation efficiency was too low and none of them was able to extract sufficient amounts of receptor except Fos-12, probably due to poor membrane breakage and solubilisation for the target protein. Further investigation of the optimal detergent e.g. Fos-14 may allow increasing the yield:expression ratio even further. The detergent Fos-14 has been reported previou.Ains, C41 (DE3) and C43 (DE3), in over-expressing toxic and membrane proteins has been previously demonstrated. The strain C43 (DE3) was derived from C41 (DE3) by selecting for resistance to a different toxic protein [31]. Compared with the other E.coli strains RP and RIL, the C41 and C43 strains were observed to yield more membrane mass per cell mass. This finding may explain one of the reasons whyFigure 3. Solubilisation of OPRM with detergents. A, solubilisation with urea or detergents. B, solubilisation with urea and laurylsarcosine. T -total membrane fraction, S -solubilised membrane fraction, P -pellet after solubilisation. doi:10.1371/journal.pone.0056500.gOPRM from E. coliFigure 4. Purification of OPRM from C43 in Fos-12. A, Purification of OPRM solubilised with Fos-12 by Ni-NTA. T -total membrane fraction, S solubilised membrane fraction, FT -flowthrough. W ?wash fractions (25 mM imidazole), E ?elution fractions (300 mM imidazole). The arrow denotes the monomeric OPRM. B, SEC -purified OPRM after size exclusion chromatography. doi:10.1371/journal.pone.0056500.gOverExpressTM C41 (DE3) and C43 (DE3), have been found to be superior for over-expressing toxic and membrane proteins. The over-expression of OPRM was largely tolerated by C43 with the conditions of 0.4 mM IPTG at 18uC for 8?2 h. Preferential membrane-insertion of OPRM instead of formation of inclusion bodies may be due to the larger mass of membrane in these strains. The membrane insertion of the protein represented first evidence that a correctly folded and stable OPRM was obtained. Interestingly, in contrast to the N-terminally tagged OPRM the expression of C-terminal decahis-tag OPRM in C43 gave only a poor expression level of the protein. This result appears to contradict the conclusion that a hexahistidine tag fused at the amino terminus of opioid receptor decreased expression levels markedly in baculovirus-infected insect cells [32] due to the “positive inside rule” described for E.coli membrane proteins and GPCRs [33]. The so-called positive inside rule states thatFigure 5. Size exclusion chromatography of OPRM in Fos-12. Purification of OPRM was performed in analytical grade Superdex 200 HR 10/30 size exclusion chromatography. Peak 1 identifies the aggregation of OPRM. The underlined Peak 2 shows the monomeric form of OPRM. doi:10.1371/journal.pone.0056500.gcytoplasmic segments contain more positive charges than extracytoplasmic segments. This is also true for OPRM. It was also previously claimed that due to poor expression of OPRM with a C-terminal his-tag, E.coli may be not a suitable expression system for OPRM [14]. In the light of our results this conclusion appears to be overly generalising. In our case, the expression 15900046 level of the N-terminally his-tagged receptor could be obtained in yields of 0.3?.5 mg/liter of culture, which is the highest yield obtained for GPCRs from E.coli membrane ever reported. The obtained yield of purified OPRM is 0.17 mg/liter of culture, which corresponds to 30?0 of expressed OPRM. Several mild detergents were used for solubilisation of the receptor, only to find solubilisation efficiency was too low and none of them was able to extract sufficient amounts of receptor except Fos-12, probably due to poor membrane breakage and solubilisation for the target protein. Further investigation of the optimal detergent e.g. Fos-14 may allow increasing the yield:expression ratio even further. The detergent Fos-14 has been reported previou.
