We have recently cloned the human nucleosome assembly protein 2 (NAP-2). He
re, we demonstrate that casein kinase 2 (CKII) from HeLa cell nuclear extra
cts interacts with immobilized NAP-II, and phosphorylates both NAP-2 and nu
cleosome assembly protein 1 (NAP-1) in vitro. Furthermore, NAP-1 and NAP-2
phosphorylation in crude HeLa cell extracts is abolished by heparin, a spec
ific inhibitor of CKII. Addition of core histones can stimulate phosphoryla
tion of NAP-1 and NAP-2 by CKII. NAP-2 is also a phosphoprotein in vivo. Th
e protein is phosphorylated at the G0/G1 boundary but it is not phosphoryla
ted 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 it
s phosphorylation status. Phosphorylated NAP-2 remains in the cytoplasm in
a complex with histones during the G0/G1 transition, whereas its dephosphor
ylation 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. (C) 2000 Academic Press.