Different kinases phosphorylate nucleophosmin/B23 at different sites during G2 and M phases of the cell cycle

The recombinant GST-nucleophosmin/B23 and the truncated mutants were tested for phosphorylation in cell-free extracts of G2 and M phases or by purified kinases. Our results indicated that a threonine residue at amino acids (a.a.) 185–240 was phosphorylated by cdc2 kinase during the entry of mitosis while the serine phosphorylation site at the middle acidic portion of the molecule (a.a. 83–152) was phosphorylated by casein kinase II during G2 phase. Our results also showed that there was possibly another serine phosphorylation at site other than the middle portion of nucleophosmin/B23 (a.a. 83–152) during the entry of cells into mitosis. The demonstration of the characteristic changes in phosphorylation of nucleophosmin/B23 during the cell cycle implicates important role of nucleophosmin/B23 in the control of the fate of nucleoli and cell growth.