Structural mechanism of co-import of H2A-H2B and its chaperone Nap1 by nuclear import receptor Kap114.

Free core histones are highly toxic and must be sequestered by other macromolecules in the cell. The mechanism of H3-H4 import by karyopherin Importin-4 in the presence of its chaperone ASF1 is understood, but the mechanism of how histone chaperone Nap1 influences H2A-H2B import is not resolved. We present biophysical interaction analysis and cryo-EM structures that reveal how Kap114, Nap1 and H2A-H2B assemble into an import complex. These results lead us to a structural mechanism of how Nap1 delivers H2A-H2B to Kap114 in the cytosol, how Nap1 and H2A-H2B are co-imported into the nucleus, and how RanGTP may influence Kap114/Nap1/H2A-H2B interactions to assemble nucleosomes in the nucleus.