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 G₁ and G₁/S phases. The incidence of the changes of localizations of nucleophosmin / B23 decreases as cells enter into S and G₂ 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 G₁ and G₁/S phases. As cells progressed into S, G₂ 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 G₁ and G₁/S phases as demonstrated by the dynamic, reversible changes of localization and oligomerization states of nucleophosmin / B23.