Protein Import through the Nuclear Pore Complex Is a Multistep Process

The transport of macromolecules across the nuclear envelope is mediated by the nuclear pore complex (NPC). Using cryo-electron microscopy and image processing we have mapped the interaction of three specific gold probes with the NPC and obtained projection maps of two possible intermediates in nuclear import. The probes used in these experiments were (a) mAb-414, which cross-reacts with Xenopus nucleoporins containing O-linked N-acetyl glucosamines; (b) wheat germ agglutinin, a transport inhibitor; and (c) nucleoplasmin, a transport substrate. Strong binding sites of the three probes are circularly arrayed on NPCs between radii of 100 and 125 Å and may be coextensive. These results suggest that nucleoplasmin-gold (NP-gold) can form at least three distinct complexes with a central transport assembly of the NPC, which may represent intermediates of a multistep protein import pathway. Initially, NP-gold appears to bind at multiple sites located around the periphery of the closed NPC transporter and also directly over the center where it can dock. In a subsequent step NP-gold is translocated through the nuclear pore.