Observation of histone nuclear import in living cells: implications in the processing of newly synthesised H3.1 & H4

Abstract

Highlights-Small-molecule-gated tether-and-release system for rapid pulse-chase of nuclear proteins-Tracking nuclear import of histone H3.1 and H4 and their incorporation at sites of active replication-Tethered H3.1 and H4 are monomeric and do not associate with ASF1, NASP, RbAp46 or HAT1 in the cytosol-Importin-β proteins as cytosolic binders of monomeric histonesSummaryWe present here a cytosolic tether-and-release system to study the import and dynamics of newly synthesised nuclear proteins. Release is gated by rapamycin-induced recruitment and activation of a viral protease, with cleavage of a peptide linker releasing the tethered cargo. We use this system to investigate nucleo-cytoplasmic divisions in the histone H3.1 & H4 deposition pathway, revealing that, contrary to previous analyses, H3.1 and H4 are predominantly monomeric in the cytosol, and only associate with the core histone chaperoning machinery after translocation to the nucleus. Whilst we do not detect interaction with known H3-H4 chaperones in the cytosol we do detect interaction with a number of importin-β proteins, that may serve a dual import and chaperoning function, preventing aggregation of histones until they are handed-off to the core histone chaperoning machinery in the nucleus.