Sequential binding of import ligands to distinct nucleopore regions during their nuclear import.

Protein import into nuclei is mediated by the nuclear pore complex (NPC) and by cellular factors. To structurally characterize this process, nuclear import of gold-labeled nucleoplasmin was followed by electron microscopy to identify NPC components interacting with the import ligand complex in vivo. Before translocation into the nucleus, nucleoplasmin sequentially bound to two distinct regions: first to the distal part of the cytoplasmic filaments and then at the cytoplasmic entry to the central gated channel. Evidence that the delivery of the import ligand from the first to the second binding region occurred by bending of the cytoplasmic filaments is presented here.