Differential thiol trapping of CadC in vivo The thiol/disulfide state of the periplasmic cysteines of CadC was monitored in vivo by differential thiol trapping according to [16]. The procedure was modified as follows: E. coli BL21(DE3)pLysS carrying one of the plasmids pET-CadC-C172A, pET-CadC-C172A,C208A or pET-CadC-C172A,C208A,C272A

was grown in phosphate buffered minimal medium with a pH of 7.6 or 5.8 to an OD600 of 0.5. Subsequently, overproduction of the CadC derivatives was induced by addition of 0.5 mM IPTG. After an additional hour of growth at 37°C, the OD600 was adjusted to 1, and 5 mM iodoacetamide (dissolved in 0.1 M Tris) was added to 1 ml cell suspension. At pH 7.6, incubation was performed for 15 min (37°C),

at pH 5.8 the incubation time was prolonged to 150 min to compensate the lower alkylation rate of iodoacetamide at low pH. selleck kinase inhibitor This first alkylation procedure irreversibly modified all free thiol groups directly in the living cells. Subsequently, cells were harvested into 100 μl ice-cold 100% (w/v) trichloric acid (TCA) and stored on ice for at least 30 min. The TCA treated cells were centrifuged (16.000 g, 4°C, 15 min), and the resulting pellet was washed with 200 μl of ice-cold 10% (w/v) TCA followed Idasanutlin order by a wash with 100 μl of ice-cold 5% (w/v) TCA. The supernatant was removed completely, and the pellet was resuspended in 100 μl of denaturing buffer [6 M urea, 200 mM Tris-HCl (pH 8.5), 10 mM EDTA, 0.5% (w/v) SDS] supplemented with 10 mM DTT to reduce disulfide bonds. After one hour of incubation in the dark (37°C, gentle agitation at 1300 rpm), 10 μl ice-cold 100% (w/v) TCA was added, and the sample was stored on ice for at least

Cell press 30 min. After centrifugation, the resulting pellet was again washed with 200 μl of ice-cold 10% (w/v) TCA followed by a wash with 100 μl of ice-cold 5% (w/v) TCA. Finally, the pellet was resuspended in 50 μl of denaturing buffer containing 10 mM PEG-maleimide (Iris Biotech GmbH, Marktredwitz/Germany) to alkylate all newly reduced cysteines. The reaction (37°C, gentle agitation at 1300 rpm, in the dark) was stopped after one hour by addition of 5 μl ice-cold 100% (w/v) TCA. After precipitation on ice (30 min) and centrifugation, the pellet was washed first with 100 μl of 10% and then with 50 μl of 5% ice-cold (w/v) TCA. After removing the TCA, the pellet was washed twice with 500 μl acetone and resuspended in 50 μl denaturing buffer. Samples were mixed with non-reducing SDS-sample buffer and loaded onto 12.5% SDS-polyacrylamide gels [42]. CadC was detected by Western blot analysis [11]. Analysis of intermolecular disulfide bonds For the detection of intermolecular disulfide bonds, wild-type CadC and all available CadC derivatives with Cys replacements (CadC_C172A; CadC_C208A; CadC_C272A; CadC_C172A,C208A; CadC_C172A,C272A; CadC_C208A,C272A; CadC_C172A,C208A,C272A) were overproduced in E.