THE PHOSPHOPROTEIN PP135 IS AN ESSENTIAL CONSTITUENT OF THE FIBRILLARCOMPONENTS OF NUCLEOLI AND OF COILED BODIES

We examined the distribution of the silver-stainable phosphoprotein, p p135, within Ehrlich tumor and HEp-2 cells by a postembedding Lowicryl immunogold labeling procedure. Identical labeling patterns were obtai ned in both cell types. During interphase, gold particles were found n ot 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 inhibi ted by actinomycin D, the same labeling was observed in segregated nuc leoli; both fibrillar components were labeled. Aside from the nucleola r labeling, label was also consistently present in coiled bodies. Duri ng metaphase, label was visualized in silver-stainable material of the nucleolus organizing regions. It thus appears that, unlike the two ma jor silver-stained proteins, nucleolin/C23 and B23, pp135 remains loca ted in all major silver-stainable structures during the whole cell cyc le. This finding strongly suggests that pp135 could be the component r esponsible for in situ silver staining. On the other hand, the mainten ance of pp135 in the fibrillar centers throughout the cell cycle, like RNA polymerase I, upstream binding factor, and DNA topoisomerase I, s uggests that pp135 could be a component involved in transcription of t he rRNA genes.