DECREASED ACCUMULATION AND DEPHOSPHORYLATION OF THE MITOSIS-SPECIFIC FORM OF NUCLEOPHOSMIN B23 IN STAUROSPORINE-INDUCED CHROMOSOME DECONDENSATION/

Nucleophosmin/B23 is highly phosphorylated by cdc2 kinase during mitos is, and this phosphorylation most probably has a role in initiating an d controlling the entry of cells into mitosis [Peter, Nakagawa, Doree, Labbe and Nigg (1990) Cell 60, 791-801]. In the present study, the pr otein kinase inhibitor staurosporine has been used to examine possible changes in nucleophosmin/B23 at mitosis in HeLa cells. Addition of st aurosporine to HeLa cells already arrested at mitosis by nocodazole ca uses: (i) decreased accumulation of the mitosis-specific form of nucle ophosmin/B23, (ii) dephosphorylation of nucleophosmin/B23, (iii) redis tribution of nucleophosmin/B23 to the cytosol, and (iv) concomitant de condensation of chromosomes. These results suggest that the mitosis-sp ecific phosphorylated form of nucleophosmin/B23 may play a role in mai ntaining mitotic chromosomes in their condensed state.