Naim et al Text_revision_211106

Nuclear pore complexes (NPCs) provide the sole gateway for the exchange of material between nucleus and cytoplasm of interphase eukaryotic cells. They support two modes of transport: passive diffusion of ions, metabolites and intermediate-sized macromolecules, and facilitated, receptor-mediated translocation of proteins, RNA, and RNPs complexes. It is generally assumed that both modes of transport occur through a single diffusion channel located within the central pore of the NPC. To test this hypothesis, we studied the mutual effects between transporting molecules utilizing either the same or different modes of translocation. We find that the two modes of transport do not interfere with each other, but molecules utilizing a particular mode of transport do hinder motion of others utilizing the same pathway. We therefore conclude that the two modes of transport are largely segregated. Eukaryotic cell nuclei are separated from the cytoplasm by a double lipid bilayer system known as the nuclear envelope (NE). Exchange of material between the two compartments proceeds through nuclear pore complexes (NPCs), large protein assemblies that span the NE and provide the sole medium for exchange. The vertebrate NPC has a molecular mass of ~125 MDa (1) and is made up of ~30 different proteins, called nucleoporins, most of which are present in multiples of eight (2,3). The core of the NPC consists of a symmetrical framework, measuring approximately 120 x 90 nm, which is made of two coaxial rings sandwiching a wheel-like array of eight spoke-shaped domains. The spoke-ring assembly encircles the central pore channel, which resembles an hourglass, 45-50 nm wide at its waist (4-7). In addition to the central framework, NPCs contain peripheral structures, which are anchored to the ring moieties of the spoke-ring assembly and serve as docking sites for nuclear transport receptors and effectors. These structures include eight short (~50 nm) filaments that protrude towards the cytoplasm and a massive, fish traplike structure, termed the nuclear basket, which extends into the nucleus (4-11). Yeast NPCs have an overall similar architecture, but are smaller (3,12-14). Transport across the NPC has been reviewed in detail (15-22), and can be devided into 2 modes: Small molecules, such as ions, metabolites and intermediate-sized macromolecules can pass, unassisted, by diffusion which becomes increasingly restricted as the particle approaches a size limit of ~10 nm in diameter (23,24). In contrast, proteins, RNAs, and their complexes are ushered selectively by dedicated soluble transport receptors, which recognize specific import (NLS) or export (NES) signal peptides displayed by the cargo. In most cases, assembly and disassembly of transport complexes in the appropriate cellular compartment are coordinated by the small GTPase Ran, which performs these activities by binding to transport receptors in its GTP-bound form. Facilitated translocation is often coupled to an input of metabolic energy that permits transport against concentration gradients and can accommodate objects with a diameter of up to ~40 nm (25). It has been estimated that in proliferating HeLa cells a single NPC supports the facilitated translocation of ≥10-20 MDa of macromolecules http://www.jbc.org/cgi/doi/10.1074/jbc.M608329200 The latest version is at JBC Papers in Press. Published on December 12, 2006 as Manuscript M608329200