Importin-9 wraps around the H2A-H2B core to act as nuclear importer and histone chaperone
- University of Texas Southwestern Medical Center, United States
- University of Texas at Dallas, United States
- Korea Advanced Institute of Science and Technology (KAIST), Korea (South), Republic of
- University of California, San Francisco, United States
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 suggest 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.
Data availability
Diffraction data have been deposited in PDB under the accession code 6N1Z
Article and author information
Author details
Funding
National Institutes of Health
- Yuh Min Chook
Welch Foundation
- Yuh Min Chook
Leukemia and Lymphoma Society
- Yuh Min Chook
National Institutes of Health
- Abhilash Padavannil
- Tolga Cagatay
- Jenny Jiou
National Institutes of Health
- Chad A Brautigam
- Diana R Tomchick
- Andrej Sali
The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.
Reviewing Editor
- Andrea Musacchio, Max Planck Institute of Molecular Physiology, Germany
Publication history
- Received: November 14, 2018
- Accepted: March 9, 2019
- Accepted Manuscript published: March 11, 2019 (version 1)
- Version of Record published: April 8, 2019 (version 2)
Copyright
© 2019, Padavannil et al.
This article is distributed under the terms of the Creative Commons Attribution License permitting unrestricted use and redistribution provided that the original author and source are credited.
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Further reading
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- Structural Biology and Molecular Biophysics
Lipid droplets (LDs) are organelles formed in the endoplasmic reticulum (ER) to store triacylglycerol (TG) and sterol esters. The ER protein seipin is key for LD biogenesis. Seipin forms a cage-like structure, with each seipin monomer containing a conserved hydrophobic helix (HH) and two transmembrane (TM) segments. How the different parts of seipin function in TG nucleation and LD budding is poorly understood. Here, we utilized molecular dynamics simulations of human seipin, along with cell-based experiments, to study seipin's functions in protein-lipid interactions, lipid diffusion, and LD maturation. An all-atom (AA) simulation indicates that seipin TM segment residues and hydrophobic helices residues located in the phospholipid (PL) tail region of the bilayer attract TG. Simulating larger, growing LDs with coarse-grained (CG) models, we find that the seipin TM segments form a constricted neck structure to facilitate conversion of a flat oil lens into a budding LD. Using cell experiments and simulations, we also show that conserved, positively charged residues at the end of seipin's TM segments affect LD maturation. We propose a model in which seipin TM segments critically function in TG nucleation and LD growth.
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