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.
Highlights
Eukaryotic chromatin is organized into nucleosomes, which are structural and functional units that are composed of 147 base pairs of DNA wrapped around two H3-H4 dimers and two H2A-H2B dimers (Luger et al, 1997)
The 5-residue 28KKRRK32 segment of the H2B tail contacts Imp9 even though weak electron density and high atomic displacement parameters of the H2B tail suggests that these interactions are dynamic
Our structural observations that Imp9 binds mostly to the globular domain of the H2AH2B are consistent with the lack of effect in Imp9 binding when either or both histone tails are deleted (Table 1), and with the previously reported nuclear localization of a mutant of H2A-H2B that lacks both its N-terminal tails (Thiriet and Hayes, 2001)
Summary
Eukaryotic chromatin is organized into nucleosomes, which are structural and functional units that are composed of 147 base pairs of DNA wrapped around two H3-H4 dimers and two H2A-H2B dimers (Luger et al, 1997). Nucleosomes are assembled in the nucleus during S-phase as new H2A, H2B, H3 and H4 proteins are synthesized in the cytoplasm (Adams and Kamakaka, 1999; Annunziato, 2012; Verreault, 2000). Translated histones are folded and assembled into H2A-H2B and H3-H4 dimers, which are imported into the nucleus for deposition onto replicating chromatin. Despite their small sizes, histones do not diffuse into the nucleus but are transported by nuclear import receptors of the Karyopherin-b family termed importins
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