Identification of Ag-NOR proteins, markers of proliferation related to ribosomal gene activity

Abstract

During mitosis, ribosomal genes are associated with a set of silver-stained nucleolar proteins designated Ag-nucleolar organizer region (NOR) proteins. The amount of Ag-NOR protein, estimated during interphase, may be used as marker of cell proliferation with a prognostic value for several human cancers. Our objective was to identify the Ag-NOR proteins in human transformed cell lines at specific phases of the cell cycle and in a hamster cell line that serves as model for active ribosomal transcription. During interphase, the major Ag-NOR proteins in both human and hamster cells were nucleolin and protein B23 and also proteins of 42, 40, and 29 kDa, which accounted for a small amount of the silver stain. The pIs of these proteins were between 4.5 and 5.6. During mitosis, the Ag-NOR proteins were either solubilized in the cytoplasm, distributed around the chromosomes, or associated with the ribosomal genes, i.e., in the NORs. The major Ag-NOR proteins associated with the ribosomal genes were the largest RNA polymerase I subunit, the 135-kDa NOR protein, the UBF transcription factor, and a 50-kDa protein. Less than 5% of the total nucleolin remained associated with ribosomal genes during mitosis. Using the purified RNA polymerase I complex from yeast, we demonstrate that the 190-, 43-, and 34.5-kDa subunits are Ag-NOR proteins in this species. This study demonstrates that the major Ag-NOR proteins in nucleoli during interphase are not the same as those associated with the ribosomal genes during mitosis. We conclude that the prognostic test for human cancer cell proliferation is largely based on the amount of the nucleolar proteins, nucleolin, and protein B23, which are not directly involved in ribosomal gene transcription. In contrast, the evaluation of active NORs in karyotypes during mitosis is based on the presence of some proteins of the ribosomal gene transcription machinery.