Localization of nucleolar phosphoproteins B23 and C23 during mitosis.

Nucleolar phosphoproteins B23 and C23 were simultaneously localized in unsynchronized male rat-kangaroo PtK2 cells during mitosis using a mouse monoclonal antibody against protein B23 and a rabbit antibody against protein C23. The distribution of proteins B23 and C23 during mitosis was compared with the distribution of the silver staining protein. During interphase, proteins B23 and C23 were both localized to the nucleolus. As the nucleolus disappeared in prophase, the distribution of protein B23 became nucleoplasmic, whereas most of protein C23 remained associated with the disappearing nucleolus. Throughout metaphase and anaphase protein B23 was found associated with the chromosomes, whereas protein C23 seemed to disappear. When the nucleolus reformed during telophase, protein C23 appeared first in 'prenucleolar bodies' and then in the nucleolus, whereas protein B23 did not appear in the nucleolus until late telophase or early G1 phase. Silver staining during mitosis closely paralleled the distribution of protein C23, supporting previous conclusions that protein C23 is a silver staining nucleolus organizer region (NOR) protein [19, 20].