Nt expression of GNA1-sGFP. Controls without any additives were taken as 100 . Black, 160?80 ; Dots, 120?60 ; Lines, 80?20 ; Gray, 0?0 . doi:10.1371/journal.pone.0056637.gcompatibility of choline was lower if compared with the two other polyions and below approximately 30 mM final concentration.Improving the Soluble CF Expression of Human GNA1 and of CurA Halogenase by Addition of StabilizersAs a first proof of principle, we approached to improve the CF expression of two targets known to partly precipitate as aggregates.Figure 6. Effect of protein stabilizers on the soluble expression of CurA halogenase. The CurA halogenase domain was expressed in the batch configuration with different additives. Protein production was quantified by immunoblotting. The results were normalized with the control as 100 corresponding to a protein concentration of 80 ng/ml. A: Immunoblot with anti-penta-His antibody. M, marker proteins in 
kDa; P, positive control for quantification (PositopeTM, invitrogene). B: Quantification of band intensity. 1, control; 2, 6 D-trehalose; 3, 10 mM L-arginine; 4, 10 mM choline. doi:10.1371/journal.pone.0056637.gChemical Chaperones for Improving Protein QualityThe human glucosamine 6-phosphate N-acetyltransferase (GNA1) is required for the de novo synthesis of N-acetyl-D-glucosamine-6phosphate representing an essential precursor in UDP-GlcNAc biosynthesis [31]. The protein was purchase NT-157 synthesized with a C-terminal fusion to sGFP. The 40.5 kDa halogenase domain of the polyketide synthetase CurA from Lynbya majuscula was synthesized with a N-terminal poly(His)6-tag [16]. Efficient CF expression protocols for both enzymes have been established with yields exceeding 1 mg/ml. However, solubility is limited and approximately 30?0 of the expressed proteins precipitate during the reaction. Considering the screening results of the analyzed types of additives, only representative compounds shown to be tolerated by the CF system were analyzed for potential stabilizing effects on the two proteins. The addition of sucrose, D-sorbitol, ectoine or betaine in the tolerated concentration ranges had no effects on the soluble expression of GNA1-sGFP as monitored by sGFP fluorescence (data not shown). However, either 10 mM choline or 10 mM L-arginine increased the GNA1-sGFP fluorescence by approximately 20 (Fig. 5A). The addition of choline and Larginine could either stabilize the general expression machinery resulting into higher yields, and/or they could stabilize the synthesized protein resulting in increased solubility. In order to investigate the reason for increased GNA1-sGFP fluorescence, the total protein production in the CF reaction was quantified by 35SMet incorporation measurements. In addition, CF sGFP expression was included as a second reference reaction and the specific enzymatic activity of GNA1-sGFP was AN-3199 furthermore determined. The total sGFP 1326631 expression as determined by 35S-Met incorporation was increased with either 10 mM L-arginine or 10 mM choline to 10 and 20 , respectively (Fig. 5A). However, in contrast with GNA1-sGFP only a slight increase with 10 mM choline was detectable while even a minor reduction of the total yield was measured with 10 mM L-arginine. Moreover, the increase in GNA1-sGFP fluorescence correlated with higher specific activity of the GNA1 enzyme upon addition of 10 mM choline into the CF reaction. Choline therefore appears to have multiple stabilizing effects in the CF expression reaction. The increased tot.Nt expression of GNA1-sGFP. Controls without any additives were taken as 100 . Black, 160?80 ; Dots, 120?60 ; Lines, 80?20 ; Gray, 0?0 . doi:10.1371/journal.pone.0056637.gcompatibility of choline was lower if compared with the two other polyions and below approximately 30 mM final concentration.Improving the Soluble CF Expression of Human GNA1 and of CurA Halogenase by Addition of StabilizersAs a first proof of principle, we approached to improve the CF expression of two targets known to partly precipitate as aggregates.Figure 6. Effect of 
protein stabilizers on the soluble expression of CurA halogenase. The CurA halogenase domain was expressed in the batch configuration with different additives. Protein production was quantified by immunoblotting. The results were normalized with the control as 100 corresponding to a protein concentration of 80 ng/ml. A: Immunoblot with anti-penta-His antibody. M, marker proteins in kDa; P, positive control for quantification (PositopeTM, invitrogene). B: Quantification of band intensity. 1, control; 2, 6 D-trehalose; 3, 10 mM L-arginine; 4, 10 mM choline. doi:10.1371/journal.pone.0056637.gChemical Chaperones for Improving Protein QualityThe human glucosamine 6-phosphate N-acetyltransferase (GNA1) is required for the de novo synthesis of N-acetyl-D-glucosamine-6phosphate representing an essential precursor in UDP-GlcNAc biosynthesis [31]. The protein was synthesized with a C-terminal fusion to sGFP. The 40.5 kDa halogenase domain of the polyketide synthetase CurA from Lynbya majuscula was synthesized with a N-terminal poly(His)6-tag [16]. Efficient CF expression protocols for both enzymes have been established with yields exceeding 1 mg/ml. However, solubility is limited and approximately 30?0 of the expressed proteins precipitate during the reaction. Considering the screening results of the analyzed types of additives, only representative compounds shown to be tolerated by the CF system were analyzed for potential stabilizing effects on the two proteins. The addition of sucrose, D-sorbitol, ectoine or betaine in the tolerated concentration ranges had no effects on the soluble expression of GNA1-sGFP as monitored by sGFP fluorescence (data not shown). However, either 10 mM choline or 10 mM L-arginine increased the GNA1-sGFP fluorescence by approximately 20 (Fig. 5A). The addition of choline and Larginine could either stabilize the general expression machinery resulting into higher yields, and/or they could stabilize the synthesized protein resulting in increased solubility. In order to investigate the reason for increased GNA1-sGFP fluorescence, the total protein production in the CF reaction was quantified by 35SMet incorporation measurements. In addition, CF sGFP expression was included as a second reference reaction and the specific enzymatic activity of GNA1-sGFP was furthermore determined. The total sGFP 1326631 expression as determined by 35S-Met incorporation was increased with either 10 mM L-arginine or 10 mM choline to 10 and 20 , respectively (Fig. 5A). However, in contrast with GNA1-sGFP only a slight increase with 10 mM choline was detectable while even a minor reduction of the total yield was measured with 10 mM L-arginine. Moreover, the increase in GNA1-sGFP fluorescence correlated with higher specific activity of the GNA1 enzyme upon addition of 10 mM choline into the CF reaction. Choline therefore appears to have multiple stabilizing effects in the CF expression reaction. The increased tot.