Accumulation of polyubiquitylated proteins coincides with depletion of uH2A and chromatin remodeling. (A) Fluorescence images of living GFP-Ub cells before and after 2 h incubation with 25 μM MG132. Bar, 10 μm. (B) Quantification of GFP-Ub levels in the cytoplasm (open circles) and nucleus (closed squares) during proteasome inhibition with 25 μM MG132. Black line is the ratio of the nucleus to the cytoplasm as plotted on the left y axis. The relative fluorescence of the nucleus and cytoplasm is plotted on the right y axis. (C) Diffusion rates of GFP-Ub in nucleus and cytoplasm without treatment (black bars) and after 2 h of MG132 treatment (gray bars). P values are indicated (unpaired t tests). Error bars represent the mean and SD of 15 independent cells in one representative experiment. (D) Western blot analysis of nuclear (N) and cytosolic (C) fractions of GFP-Ub cells. Cells were left untreated or treated for 2 h with DMSO, MG132, or a heat shock, and nuclear (N) and cytosolic (C) fractions were isolated. Nuclear fractions contained, on average, two times less protein than the cytosolic fractions. For analysis, 15- and 30-μg proteins were loaded for the nuclear and cytosolic fractions, respectively. Membranes were probed with an anti-GFP antibody. Molecular mass markers are indicated. (E) FRAP curves for GFP-Ub in the nucleus of untreated cells (black line) and after 2 h incubation with MG132 (gray line). (F) Western blot analysis of uH2A in GFP-Ub cells left untreated or exposed for 2 h to DMSO, MG132, or a heat shock. The membranes were probed with an anti-uH2A antibody. Molecular mass markers are indicated. (G) Lysates of Mel JuSo cells expressing GFP-Ub that were treated for various time periods with MG132 were probed with an anti-GFP antibody (top) and uH2A antibody (bottom). (H) Quantification of two independent experiments as shown in G. The values of experiment 1 (circles), experiment 2 (diamonds), and the mean of the two experiments (bars) shown. The values were standardized to the intensities of the corresponding band in untreated cells. Deubiquitylation of GFP-Ub–histone and uH2A followed similar kinetics.