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First published online June 24, 2005; 10.1104/pp.105.060509 Plant Physiology 138:1446-1456 (2005) © 2005 American Society of Plant Biologists Interacting Proteins and Differences in Nuclear Transport Reveal Specific Functions for the NAP1 Family Proteins in Plants1State Key Laboratory of Genetic Engineering, Department of Biochemistry, School of Life Sciences, Fudan University, Shanghai 200433, People's Republic of China (A.D., Z.L., Y.Z., F.Y., Z.L., K.C.); and Institut de Biologie Moléculaire des Plantes du Centre National de la Recherche Scientifique, Université Louis Pasteur de Strasbourg, 67084 Strasbourg cedex, France (Y.Z., F.Y., W.-H.S.)
Nucleosome assembly protein 1 (NAP1) is conserved from yeast to human and facilitates the in vitro assembly of nucleosomes as a histone chaperone. Inconsistent with their proposed function in the nucleus, however, many NAP1 proteins had been reported to localize in the cytoplasm. We investigated the subcellular localization of tobacco (Nicotiana tabacum) and rice (Oryza sativa) NAP1 family proteins first by identification of interacting partners and by direct examination of the localization of green fluorescent protein-tagged proteins. Through treatment of tobacco cells with leptomycin B and mutagenesis of nuclear export signal, we demonstrated that Nicta;NAP1;1 and Orysa;NAP1;1 shuttle between the cytoplasm and the nucleus. Together with the demonstration that tobacco NAP1 proteins bind histone H2A and H2B, our results support the current model and provide additional evidence that function of NAP1 as histone chaperones appears to be conserved in plants. In addition, we show that tobacco NAP1 proteins interact with tubulin and the mitotic cyclin Nicta;CYCB1;1, suggesting a role for NAP1 in microtubule dynamics. Interestingly, in spite of their high homology with the above NAP1 proteins, the other three tobacco proteins and Orysa;NAP1;2 did not show nucleocytoplasmic shuttling and were localized only in the cytoplasm. Moreover, Orysa;NAP1;3 that lacks a typical nuclear localization signal sequence was localized in both the cytoplasm and the nucleus. Finally, we show that only Orysa;NAP1;3 could be phosphorylated by casein kinase 2
1 This work was supported by the Centre National de la Recherche Scientifique (PICS no. 2391 to W.-H.S.), by the National Natural Science Foundation of China (grant no. NSF30100094), by the Scientific and Technological Council Foundation of Shanghai (grant no. 04JC14017), and by the Shanghai Rising Star Program (to A.D.). Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.060509. * Corresponding author; e-mail awdong001{at}yahoo.com.cn; fax 86–21–65650149. Received January 31, 2005; returned for revision March 14, 2005; accepted March 22, 2005. This article has been cited by other articles:
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in vitro. However, this phosphorylation was not responsible for nuclear import of Orysa;NAP1;3 as being demonstrated through mutagenesis studies. Together, our results provide an important step toward elucidating the molecular mechanism of function of the NAP1 family proteins in plants. 
