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Nucleophosmin/B23, is a major nucleolar phosphoprotein which is
more abundant in tumor cells as compared to normal resting
cells. When cells are treatment with an anti-tumor drug, such as
actinomycin D, that inhibits cell growth or during the
stationary phase of growth, nucleophosmin/B23 translocates from
nucleolus to nucleoplasm. Nucleophosmin/B23 has been proposed to
be involved in the preribosomal particle assembly and nucleolin/
C23 has been proposed to be involved in rRNA processing. In this
report, cross-linking reagent, dithiobis succinimidyl propionate
(DSP), was used to detect nucleophosmin/B23-nucleolin/C23
complex in HeLa cells. By using immunoprecipitation,
immunofluorescence and confocal microscopy, I have found that
nucleophosmin/B23 interacts with nucleolin/C23 in interphase as
well as in cytokinesis, but not in prometaphase nor in metaphase
of cell cycle. In actinomycin D-induced inhibition of cell
growth, the interaction of nucleophosmin/B23 and nucleolin/C23
is found not only in cells in which both proteins are localized
in nucleolus but also in cells in which they have translocated
from nucleolus to nucleoplasm. In cell free kinase and
immunoprecipitation assays, the nucleophosmin/B23-nucleolin/C23
interaction is not related to the cell cycle dependent
phosphorylation states of nucleophosmin/B23. These results
indicate: (1) The associated complex of nucleophosmin/B23 and
nucleolin/C23 during cytokinesis and interphase may be a part of
the assembly line responsible for the synthesis and processing
of pre-ribosomal particles and rRNA. (2) Factors other than
phosphorylations of nucleophosmin/B23 to interact with
nucleolin/C23, resulting in the cease of nucleolar activities
and mitotically structural dissolution of the nucleolus.
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