Further evidence that phosphoprotein C23 (110 kD/pI 5.1) is the nucleolar silver staining protein.

Previously we demonstrated a similar distribution between nucleolar organizing region-(NOR)-specific silver staining and localization of nucleolar phosphoprotein C23 (MW 110 kD/pI 5.1) [1, 2]. We now report that under fixation conditions which allow for antibody binding and subsequent silver staining, monoclonal antibody against protein C23 blocks NOR silver staining as well as silver staining in interphase nucleoli. Monoclonal antibody against nucleolar phosphoprotein B23 (MW 37 kD/pI 5.1) did not block silver staining in either NORs or interphase nucleoli. These, along with earlier observations, provide evidence that nucleolar phosphoprotein C23 is the major silver staining protein of the nucleolus and that it is directly or indirectly associated with rDNA.