001, Wilcoxon/Kruskal Wallis test). Volumes were grouped into 50 μm3 bins and plotted. Inset: The average cell volume (±SD) of Crenigacestat clinical trial the three strains with shading as in main figure. C) PA-expressing yeast had increased sensitivity to hydroxyrurea, a potent Ralimetinib clinical trial inhibitor of RNR activity, when compared to the control strain and the vATPase-defective strain YPL234C, as determined through MIC measurements. Error bars represent standard deviation of 5 biological replicates. The noted changes in cell volume are consistent with the hypothesis that PAp interferes with yeast Rnr1p function. Additional support for this idea came from our

observation that PA-expressing yeast had an increased sensitivity to hydroxyurea over the control strain when grown in YPD (Figure 4C) but not on YPRaf/Gal (Additional file 1: Figure S4). Since hydroxyurea is a potent and specific inhibitor of RNR catalytic activity [26], this increased sensitivity to hydroxyurea provided further indications that low-level expression of PAp interferes with Rnr1p functions in yeast. PA-expressing strains contain a non-reducible PAp-Rnr1p protein complex Immunoblotting

methods were used to determine whether PAp binds to the yeast Rnr1p. Previously, it was reported that the oxidation state of yeast Rnr1p can be determined by SDS-PAGE [27]. In yeast, the RNR holoenzyme uses free radical chemistry to generate dNDPs from the respective NDPs. During 2′ hydroxyl group removal from the ribose moiety of the NDP, a disulphide bridge ATM Kinase Inhibitor is formed between two cysteine residues in the catalytic site of Rnr1p. Once the newly formed dNDP is released from the catalytic site, Tau-protein kinase the flexible C-terminus of the adjacent R1 subunit enters into the catalytic site and the disulphide bridge in the catalytic site is transferred to two cysteine residues located on the flexible C-terminus.

The C-terminus arm then swings out of the catalytic site and this disulphide bridge is finally reduced by glutaredoxin or thioredoxin to reactivate the RNR holoenzyme [8, 9]. When examined using SDS-PAGE, non-reducing conditions cause Rnr1p to resolve as two bands: the top band (lower mobility) represents the oxidized form (i.e., having a disulphide bridge between cysteine residues at the catalytic site) and the lower, high-mobility band represents the reduced form. When proteins are extracted under reducing conditions, only the lower band of reduced Rnr1p is evident [27]. We found that under non-reducing conditions (no DTT or β-mercaptoethanol) Rnr1p from the control strain grown on YPD was resolved on immunoblots into reduced Rnr1p and oxidized Rnr1p (Figure 5A). In contrast, protein extracts of PA-expressing yeast showed the reduced form of Rnr1p (100 kDa), but little or none of the oxidized form. Interestingly, an intense band of ~155 kDa, the expected size of a complex consisting of PAp (55 kDa) and Rnr1p, was also observed from the PA-expressing yeast strain.