The phosphoprotein pp135 is an essential constituent of the fibrillar components of nucleoli and of coiled bodies.

We examined the distribution of the silver-stainable phosphoprotein, pp135, within Ehrlich tumor and HEp-2 cells by a postembedding Lowicryl immunogold labeling procedure. Identical labeling patterns were obtained in both cell types. During interphase, gold particles were found not only over the dense fibrillar component but were also evident over the fibrillar centers of nucleoli. By contrast, the granular component did not display any significant label. When rRNA synthesis was inhibited by actinomycin D, the same labeling was observed in segregated nucleoli; both fibrillar components were labeled. Aside from the nucleolar labeling, label was also consistently present in coiled bodies. During metaphase, label was visualized in silver-stainable material of the nucleolus organizing regions. It thus appears that, unlike the two major silver-stained proteins, nucleolin/C23 and B23, pp135 remains located in all major silver-stainable structures during the whole cell cycle. This finding strongly suggests that pp135 could be the component responsible for in situ silver staining. On the other hand, the maintenance of pp135 in the fibrillar centers throughout the cell cycle, like RNA polymerase I, upstream binding factor, and DNA topoisomerase I, suggests that pp135 could be a component involved in transcription of the rRNA genes.