Multiple pathways contribute to nuclear import of core histones

Nuclear import of the four core histones H2A, H2B, H3 and H4 is one of the main nuclear import activities during S-phase of the cell cycle. However, the molecular machinery facilitating nuclear import of core histones has not been elucidated. Here, we investigated the pathways by which histone import can occur. First, we show that core histone import can be competed by the BIB (beta -like import receptor binding) domain of ribosomal protein L23a suggesting that histone import is an importin mediated process. Secondly, affinity chromatography on immobilized core histones revealed that several members of the importin beta family of transport receptors are able to interact with core histones. Finally, we demonstrate that at least four known and one novel importin, importin 9, can mediate nuclear import of core histones into the nuclei of permeabilized cells. Our results suggest that multiple pathways of import exist to provide efficient nuclear uptake of these abundant, essential proteins.