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Mechanism of RanGTP priming H2A-H2B release from Kap114 in an atypical RanGTP•Kap114•H2A-H2B complex.

Jenny Jiou1, Joy M Shaffer2, Natalia E Bernades1

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Summary

Nuclear import receptor Importin-9 and its homolog Kap114 bind histone H2A-H2B and RanGTP simultaneously. Cryo-EM structures reveal how this complex facilitates H2A-H2B release for nucleosome assembly.

Keywords:
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Area of Science:

  • Molecular Biology
  • Cell Biology
  • Structural Biology

Background:

  • Importin-9 facilitates nuclear transport of histone H2A-H2B.
  • RanGTP typically dissociates cargo from importins, but stably binds the Importin-9•H2A-H2B complex.
  • The mechanism of simultaneous binding and cargo release remained unclear.

Purpose of the Study:

  • To elucidate the structural basis for simultaneous binding of H2A-H2B and RanGTP to Importin-9 homolog Kap114.
  • To understand how the ternary complex positions H2A-H2B for release and nucleosome assembly.

Main Methods:

  • Cryo-electron microscopy (cryo-EM) was used to determine structures.
  • Structures of Kap114•RanGTP, Kap114•H2A-H2B, and RanGTP•Kap114•H2A-H2B complexes were solved.
  • Structural analysis of conserved Kap114 interactions with cargo and GTPase.

Main Results:

  • Cryo-EM structures revealed how Kap114 binds H2A-H2B and RanGTP simultaneously.
  • RanGTP binds Kap114's N-terminal repeats, while H2A-H2B binds via its acidic patch to C-terminal repeats.
  • In the ternary complex, RanGTP induces a conformational change in Kap114, releasing the H2A-H2B docking domain.

Conclusions:

  • Kap114's conserved structure enables simultaneous binding of histone H2A-H2B and RanGTP.
  • RanGTP binding to Kap114 repositions H2A-H2B for efficient transfer to the nucleosome.
  • This mechanism is crucial for nuclear import and nucleosome assembly.