CELL-CYCLE PHASE-DEPENDENT CHANGES OF LOCALIZATION AND OLIGOMERIZATION STATES OF NUCLEOPHOSMIN/B23

Nucleophosmin ! B23, an abundant nucleolar phosphoprotein, accumulates in the nucleoplasm of cells during the stationary phase of growth or after exposure to selected cytotoxic drugs [Chan, P.K. (1992) Exp. Cell Res. 203, 174-181]. Monomeric and hexameric forms of nucleophosmin ! B23 are present in cells [Yung, B.Y.M. and Chan, P.K. (1987) Biochim. Biophys. Acta. 925,74-82]. Using indirect immunofluorescence, here we show that there are changes in nucleophosmin / B23's cellular localizations throughout the cell cycle. The alternation of the nuclear and nucleolar localizations of nucleophosmin ! B23 is most frequently observed in cells of G(1) and G(1)/S phases. The incidence of the changes of localizations of nucleophosmin ! B23 decreases as cells enter into S and G(2) phases. In parallel, using Western blotting, the reversible change of oligomerization states between the hexameric and monomeric forms of nucleophosmin ! B23 is also found to occur most frequently in cells of G(1) and G(1)/S phases. As cells progressed into S, G(2) and M phases, the frequency of the reversible change of hexameric and monomeric forms of nucleophosmin ! B23 decreases. These findings suggest that nucleophosmin / B23 being possibly involved in rRNA processing and transport, is highly active at G(1) and G(1)/S phases as demonstrated by the dynamic, reversible changes of localization and oligomerization states of nucleophosmin !B23. (C) 1995 Academic Press. Inc.