Discrete foci containing RNase A are found in nucleoli of HeLa cells after aging in culture

Abstract

We have studied by means of ultrastructural immunocytochemistry the localization of RNase A in nuclei of HeLa cells in control conditions and following cell ageing in culture. We have found that roundish, electron dense foci, which contain a significant amount of RNase A, can be detected within nucleoli of aged cells. These bodies also contain RNA and lack ribosomal S3 proteins, and may represent either simple storage sites or areas where RNA degradation takes place.

Figure 1

a) Aged HeLa cell after EDTA staining. The labelling for RNAase A is present exclusively over a dense area near the periphery of the nucleolus. Bar: 0.25 µm. b) Aged HeLa cell after terbium staining specific for RNA. The peripheral roundish area (arrow) contains RNA fibers. In the inset, the fibrils are clearly visible. Bar: 0.25 µm. c) After immunofluorescence labeling for RNase A, the nucleolus is clearly not homogeneously labelled. The arrow points to a bright spot. Bar: 10 µm. d) Terbium staining and labelling for RNAse demonstrate the colocalization of these components in the dense structure. Bar: 0.25 µm. e) The dense body is labelled only at the periphery by the anti-S3 antibody. The ribosomal protein is present in the DFC e GC. Bar: 0.25 µm. f) Terbium staining and labelling for RNAse A. A dense body (arrow) in the close vicinity of a fibrillar centre (FC). Bar: 0.25 µm.