Phosphoprotein pp135 is an essential component of the nucleolus organizer region (NOR)

The association of phosphoproteins pp135 and pp105 with distinct substructures of the nucleolus was studied by cytochemical and immunological methods at the light microscopic and electron microscopic level. Both phosphoproteins exhibited a very high affinity for silver and Giemsa staining compared to other nucleolar proteins. Immunolocalization of pp135 and pp105 during mitosis by light microscopy revealed a tight association of pp135 with the silver staining nucleolus organizer region (NOR), whereas pp105 (cross-reacting with C23) appeared to be only partially associated with the NOR, exclusively at telophase. At the immunoelectron microscopic level the distribution of pp135 and pp105 was investigated in interphase nucleoli. Phosphoprotein pp135 was located in the fibrillar shell and pp105 in the fibrillar shell and the granular zone. The fibrillar centers were essentially free of both phosphoproteins..