FIG. 1.

Ubiquitin is present in the nucleolus, and the pre-rRNP complexes are ubiquitinated. (a) Immunofluorescence detection of ubiquitin in the nucleolus. Antibodies against ubiquitin stain the cytoplasm and nucleus as expected but also consistently colocalize with nucleoli, identified here in HeLa cells by fibrillarin expression. (b) 3617 cells, a derivative cell line from 3134, stably expressing GFP-tagged glucocorticoid receptor (GFP-GR) were also used (27). GFP-GR stains the nucleoplasm but is absent from nucleoli. These nucleoli are clearly stained with an anti-ubiquitin antibody. (c) In addition, MBA-15 cells also show a clear nuclear localization of ubiquitin as seen from the overlay with the DAPI DNA staining. Some of the nucleoli are indicated with arrows. Scale bar, 2 μm.