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Importin-9 wraps around the H2A-H2B core to act as nuclear importer and histone chaperone.

  • Author(s): Padavannil, Abhilash;
  • Sarkar, Prithwijit;
  • Kim, Seung Joong;
  • Cagatay, Tolga;
  • Jiou, Jenny;
  • Brautigam, Chad A;
  • Tomchick, Diana R;
  • Sali, Andrej;
  • D'Arcy, Sheena;
  • Chook, Yuh Min
  • et al.
Abstract

We report the crystal structure of nuclear import receptor Importin-9 bound to its cargo, the histones H2A-H2B. Importin-9 wraps around the core, globular region of H2A-H2B to form an extensive interface. The nature of this interface coupled with quantitative analysis of deletion mutants of H2A-H2B suggests that the NLS-like sequences in the H2A-H2B tails play a minor role in import. Importin-9•H2A-H2B is reminiscent of interactions between histones and histone chaperones in that it precludes H2A-H2B interactions with DNA and H3-H4 as seen in the nucleosome. Like many histone chaperones, which prevent inappropriate non-nucleosomal interactions, Importin-9 also sequesters H2A-H2B from DNA. Importin-9 appears to act as a storage chaperone for H2A-H2B while escorting it to the nucleus. Surprisingly, RanGTP does not dissociate Importin-9•H2A-H2B but assembles into a RanGTP•Importin-9•H2A-H2B complex. The presence of Ran in the complex, however, modulates Imp9-H2A-H2B interactions to facilitate its dissociation by DNA and assembly into a nucleosome.

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