Oral Presentation 49th Lorne Conference on Protein Structure and Function 2024

Shield and Shuttle: Nuclear Import Mechanism of Core Histones H2A-H2B (#7)

Jenny Jiou 1 2 , Joy M Shaffer 3 , Natalia E Bernardes 2 , Ho Yee J Fung 2 , Sheena D'Arcy Ph.D. 3 , Yuh Min Chook 1 2
  1. Department of Biophysics, UT Southwestern, Dallas, Texas, United States
  2. Department of Pharmacology, UT Southwestern, Dallas, Texas, United States
  3. Department of Chemistry and Biochemistry, UT Dallas, Richardson, Texas, United States

In the nucleus, core histones H2A, H2B, H3, and H4 organize DNA into nucleosomes as 147 base pairs of DNA wrap around the H3-H4 tetramer and two H2A-H2B dimers. However, before core histones can undergo nucleosome assembly, they are first individually synthesized in the cytoplasm and folded into H3-H4 and H2A-H2B dimers by cytoplasmic chaperones. Then, nuclear import receptors (Importins) of the Karyopherin-β family actively import these histones into the nucleus for assembly into nucleosomes. Previously we showed that Importin-9, the major nuclear import receptor of histone H2A-H2B, wraps around the H2A-H2B core to chaperone the histone and transport it from the cytoplasm to the nucleus. However, unlike most nuclear import systems where RanGTP dissociates cargoes from their importins, RanGTP does not dissociate H2A-H2B from Importin-9 but binds stably to Imp9•H2A-H2B. Formation of the RanGTP•Importin-9•H2A-H2B complex facilitates H2A-H2B release to the assembling nucleosome (Padavannil et al, 2019). Here we show Imp9 S. cerevisiae homolog, Kap114, undergoes conserved biochemical and biophysical traits of histone chaperone that needs RanGTP to prime the release of H2A-H2B onto assembling nucleosomes. The cryo-EM structures of Imp9•RanGTP and of Kap114, including Kap114•RanGTP, Kap114•H2A-H2B, and RanGTP•Kap114•H2A-H2B elaborates how structurally conserved is the nuclear import machinery is for H2A-H2B. These structures explain how RanGTP binding to Kap114•H2A-H2B changes the conformation of the Kap114/Imp9 N-terminal region such that it no longer contacts the surface of H2A-H2B proximal to the H2A docking domain that drives nucleosome assembly. 

  1. Jiou, J., Shaffer, J. M., Bernades, N. E., Fung, H. Y. J., Kikumoto Dias, J., D’Arcy, S., & Chook, Y. M. (2023). Mechanism of RanGTP priming H2A-H2B release from Kap114 in an atypical RanGTP• Kap114• H2A-H2B complex. Proceedings of the National Academy of Sciences, 120(29), e2301199120.
  2. Shaffer, J. M., Jiou, J., Tripathi, K., Olaluwoye, O. S., Fung, H. Y. J., Chook, Y. M., & D’Arcy, S. (2023). Molecular basis of RanGTP-activated release of Histones H2A-H2B from Importin-9. Structure, 31(8), 903-911.
  3. Padavannil, A., Sarkar, P., Kim, S. J., Cagatay, T., Jiou, J., Brautigam, C. A., ... & Chook, Y. M. (2019). Importin-9 wraps around the H2A-H2B core to act as nuclear importer and histone chaperone. Elife, 8, e43630.