On the contrary, roGFP1 expressed in the ER of P. pastoris wild-type cells was always fully oxidized, which is comparable to the results received for the ER of an S. cerevisiae wild-type strain with roGFP2 (Merksamer et al., 2008) and for the ER of mammalian cells with roGFP1 (Schwarzer et al., 2007). Based on the hypothesis that the midpoint potential of the compartment

has an influence on the functionality of the biosensor, we analyzed the P. pastoris wild-type strain with the constructs roGFP1_iE and roGFP1_iL, which theoretically have the optimal midpoint potential for the ER. Both of these constructs indicate a more oxidizing ER environment compared with the cytosol, but are not completely oxidized, as it was claimed when using roGFP1 and roGFP2 (Meyer et al., 2007; Schwarzer et al., 2007; Merksamer AZD6244 in vitro et al., 2008). The results obtained here appear to be more plausible than the redox ratios obtained with roGFP1. Comparing the redox ratios and the SDs determined with both variants, there is not much difference between roGFP1_iE and roGFP1_iL,

but according to the range of fluorescence between the oxidized and the reduced form of the protein, roGFP1_iE was chosen for further experiments. Determination of the thiol/disulfide equilibrium in different cell compartments has been of interest for years. Hwang et al. (1992) report that in CRL-1606 cells (murine B-lymphocytes), the reduction potential (E) for the redox pair GSSG/2GSH in the ER is −180 mV, while GSK126 molecular weight the cytosol has a value of −232 mV. Using the Nernst equation and the standard redox potential of the applied roGFP, the cellular reduction potential can be calculated based on the fluorescence data [Eqns (1)–(3)]. According to this calculation, the cytosol of P. pastoris has a reduction potential of −295 mV, which is in accordance with the results obtained

for S. cerevisiae (−289 mV; Ostergaard et al., 2004). As the ER is much more oxidizing, the reduction potential of this compartment should differ clearly from that of the cytosol. After targeting roGFP1 into the ER of the epithelial cell line CF15, the calculation Thiamine-diphosphate kinase of the reduction potential of this organelle seemed to be quite difficult, because the roGFP1 ratios were nearly saturated, indicating that the ER was more oxidized than −250 mV (Schwarzer et al., 2007). These data show similarities to the results of Merksamer et al. (2008) calculated for the S. cerevisiae ER when expressing roGFP2 in this compartment, but no reduction potentials were reported. This might be due to the fact that for fully oxidized redox sensors, the calculation yields no results. In the present study, the reduction potential of the ER was determined using the fluorescence data from the experiments with roGFP1_iE.