NAP-2: histone chaperone function and phosphorylation state through the cell cycle.

We have recently cloned the human nucleosome assembly protein 2 (NAP-2). Here, we demonstrate that casein kinase 2 (CKII) from HeLa cell nuclear extracts interacts with immobilized NAP-II, and phosphorylates both NAP-2 and nucleosome assembly protein 1 (NAP-1) in vitro. Furthermore, NAP-1 and NAP-2 phosphorylation in crude HeLa cell extracts ...
is abolished by heparin, a specific inhibitor of CKII. Addition of core histones can stimulate phosphorylation of NAP-1 and NAP-2 by CKII. NAP-2 is also a phosphoprotein in vivo. The protein is phosphorylated at the G0/G1 boundary but it is not phosphorylated in S-phase. Here, we show that NAP-2 is a histone chaperone throughout the cell cycle and that its cell-cycle distribution might be governed by its phosphorylation status. Phosphorylated NAP-2 remains in the cytoplasm in a complex with histones during the G0/G1 transition, whereas its dephosphorylation triggers its transport into the nucleus, at the G1/S-boundary, with the histone cargo, suggesting that binding to histones does not depend on phosphorylation status. Finally, indirect immunofluorescence shows that NAP-2 is present during metaphase of HeLa and COS cells, and its localization is distinct from metaphase chromosomes.
Mesh Terms:
Animals, Biological Transport, Casein Kinase II, Cell Cycle, Cell Cycle Proteins, Cell Extracts, Cell Line, Cell Nucleus, Chromatography, Affinity, Chromosomes, Cytoplasm, Enzyme Activation, G1 Phase, Hela Cells, Heparin, Histones, Humans, Metaphase, Molecular Chaperones, Nuclear Proteins, Nucleosome Assembly Protein 1, Phosphoproteins, Phosphorylation, Protein Binding, Protein-Serine-Threonine Kinases, Proteins, S Phase
J. Mol. Biol.
Date: Apr. 28, 2000
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