Intranucleolar Localization of Human Proliferating Cell Nucleolar Antigen pl20*

The human proliferation-associated nucleolar antigen p120 was local ized to substructures within HeLa cell nucleoli by immunofluorescence and immunoelectron microscopy of cells whose nucleoli were segregated by drug treatment or extracted with nucleases. By indirect immunofluo rescence, protein pi 20 was localized diffusely throughout all interphase nucleoli. However, high resolution immunoelectron microscopy demon strated that protein pl20 staining delineated a network of 20-30-nm diameter beaded fibrils distributed throughout the nucleolus. This distri bution was unique compared to that of the nucleolar proteins p145, RNA polymerase I, or B23 which were examined simultaneously. Drug-induced segregation of nucleoli by actinomycin D or dichlorobenzimidazole riboside, followed by immunoelectron microscopy, indicated that protein pi 20 was concentrated at the periphery of the granular region in segregated nucleoli. In situ nuclease digestion of cells with DNase I and/or RNase A did not release pi20 from the nucleolus. Instead, pi20 immunoreactivity was retained within phase-dense residual nucleoli. These results provide evidence that protein pi 20 is associated with, and in fact deline ates, a network of fibrils which is retained in the nucleolar residue fraction of proliferating cells.