Nuclear import of histone H2A and H2B is mediated by a network of karyopherins

The first step in the assembly of new chromatin is the cell cycle-regulated synthesis and nuclear import of core histones. The core histones include H 2A and H2B, which are assembled into nucleosomes as heterodimers. We show h ere that the import of histone H2A and H2B is mediated by several members o f the karyopherin (Kap; importin) family, An abundant complex of H2A, H2B, and Kap114p was detected in cytosol. In addition, two other Kaps, Kap121p a nd Kap123p, and the histone chaperone Nap1p were isolated with H2A and H2B. Nap1p is not necessary for the formation of the Kap114p-H2A/H2B complex or for import of H2A and H2B, We demonstrate that both histones contain a nuc lear localization sequence (NLS) in the aminoterminal tail. Fusions of the NLSs to green fluorescent protein were specifically mislocalized to the cyt oplasm in kap mutant strains. In addition, we detected a specific mislocali zation in a kap95 temperature-sensitive strain, suggesting that this Kap is also involved in the import of H2A and H2B in vivo. Importantly, we show t hat Kap114p, Kap121p, and Kap95 interact directly with both histone NLSs an d that RanGTP inhibits this association. These data suggest that the import of H2A and H2B is mediated by a network of Kaps, in which Kap114p may play the major role.