Structure of Importin-4 bound to the H3-H4·ASF1 histone·histone chaperone complex

Abstract

Importin-4 is the primary nuclear import receptor of core histones H3 and H4. Importin-4 binds the H3-H4 dimer and histone-chaperone ASF1 prior to nuclear import, but available structures of Importin-4{middle dot}histone tail complexes do not explain how Importin-4 recognizes the biologically relevant heterotrimeric H3-H4{middle dot}ASF1 cargo. Our 3.5 [A] Importin-4{middle dot}H3-H4{middle dot}ASF1 cryo-electron microscopy structure revealed interactions with H3-H4{middle dot}ASF1 different those suggested by previous Importin-H3 tail peptide structures. The N-terminal half of Importin-4 clamps the globular histone domain and the H3 N helix while its C-terminal half binds the H3 N-terminal tail weakly, with negligible tail contribution to binding energy; ASF1 binds H3-H4 without contacting Importin-4. Together, ASF1 and Importin-4 shield nucleosomal interfaces of H3-H4 to chaperone and import it into the nucleus, where Importin-4 undergoes large conformational changes as RanGTP binds to release H3-H4{middle dot}ASF1. This work explains the mechanisms of nuclear import of full-length H3-H4.