For most known nuclear domains (ND), specific functions have been identified. In this report we used murine mAbs and human autoantibodies to investigate precisely circumscribed structures 0.2-0.3 micron in diameter which appear as "nuclear dots" distributed throughout the nucleoplasm. Nuclear dots are metabolically stable and resistant to nuclease digestion and salt extraction. The localization of nuclear dots is separate from kinetochores, centromeres, sites of mRNA processing and tRNA synthesis, nuclear bodies, and chromosomes. The nuclear dots, therefore, represent a novel ND. Nuclear dots break down as cells enter metaphase and reassemble at telophase. In interphase cells, nuclear dots are frequently "paired," and some are visible as "doublets" when stained with one particular antiserum. The number of dot doublets increased when quiescent cells were stimulated with serum although the total number of dots did not change substantially. One of the antigens was identified as a protein with a molecular mass of approximately 55 kD showing three charge isomers in the pI range of 7.4 to 7.7. Autoantibodies affinity purified from this nuclear dot protein (NDP-55) show nuclear dots exclusively. Nuclear dot-negative rat liver parenchymal cells became positive after chemical hepatectomy, suggesting involvement of the NDP-55 in the proliferative state of cells.

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