Fig. 6. Distribution of proteasomes in COS-7 cells in the presence and absence of proteasome inhibitors. (A) Proteasomes are recruited to the nucleolus in proteasome-inhibitor-treated cells. Cells were cultured in the presence or absence of 100 µM ALLN, and immunostained for proteasomes using a rabbit pAb (1:10,000) and a mouse mAb (1:500). Proteasomes were detected in the nucleus and the cytoplasm, but were not detected in the nucleoli of untreated cells (upper panels). In cells treated with ALLN for 5 hours, a subpopulation of proteasomes was detected in the nucleoli (lower panels). (B) Localisation of the proteasomes (red), and fibrillarin (green) in proteasome-inhibitor-treated cells is shown at higher magnification (left panels). Confocal images show the accumulation of proteasomes adjacent to the fibrillarin marker for the dense fibrillar region of the nucleolus in the nucleolus (right panels). Rabbit pAb was used for proteasomes immuno-staining.