B: Ubc9 D100A is severely impaired for disulfide bond formation with Uba2. Stable U2OS cells were transfected with siRNA against Ubc9 for 72h, treated with 250 µM H2O2 for different time points, from 2 min to 2 hours, lysed in non-reducing buffer and analyzed by immunoblotting against Ubc9 to detect the disulfide (upper panel) or free Uba2 as loading control (see also Appendix Fig. S5). Lower panel: relative intensities of Ubc9 bands were measured using imageJ. Highest intensities in both cell population were set to 1. 