The Structure and Composition of the Yeast NPC

Abstract

The double-membraned nuclear envelope (NE) behaves as a selective barrier that segregates the genome from all cytosolic processes. A highly regulated exchange system between these two compartments is essential for proper cell growth, progression through the cell cycle, accurate responses to developmental and extracellular signals and to maintain the functional integrity of the nucleus. The sole mediators of controlled nucleocytoplasmic transport are the nuclear pore complexes (NPCs), large proteinaceous machineries embedded within specialized circular pores that traverse the NE. In actively growing cells it is estimated that every minute hundreds of proteins and ribonucleoprotein particles (RNPs) traverse each NPC in both directions. The basic mechanisms of nuclear transport appear to be highly conserved across distantly related species (reviewed in Nigg 1997; Mattaj and Englmeier 1998; Gorlich and Kutay 1999; Wente 2000). Although metabolites, water, ions and small macromolecules can freely diffuse through aqueous channels of 10 nm in the NPC, large macromolecular particles with a diameter of up to 30 nm are selectively transported across the NPC via a highly regulated energy-dependent process. Active transport requires specific soluble transport factors that recognize individual substrates both inside and outside the nucleus and mediate their interaction with the stationary phase of the NPC translocation machinery. Specifically, the translocation of transport substrates is known to require the docking of the transport complex to the NPC, the active translocation of the docked complexes across the NPC and the release of the substrate into the target compartment. Various models have been proposed to explain how this docked complex is actively translocated across the 50–60 nm long NPC transporter and then subsequently released into the nucleoplasm, and the matter is still highly controversial (see below). All models agree in attributing a crucial importance to the protein Ran in maintaining vectorial cargo transport and regulating the binding and release steps that take place during translocation. As a member of the Ras superfamily of small GTPases, Ran exists within the cell in a GDP-bound and in a GTP-bound form. The balance between these two forms is regulated by a variety of Ran cofactors that are asymmetrically distributed within the cell. As a consequence cytoplasmic Ran is thought to exist prevalently in the GDP-bound form, while Ran-GTP is thought to predominate in the nucleus. This differential distribution of Ran-GTP versus Ran-GDP would establish directional transport by ensuring that transport complexes are formed in one compartment and disassembled in the other (reviewed in Cole and Hammell 1998; Mattaj and Englmeier 1998; Pemberton et al. 1998; Wozniak et al. 1998; Gorlich and Kutay 1999). Understanding this regulated transport demands an understanding of the detailed three-dimensional map of the NPC and of the interactions and relationships between the soluble and stationary phases of nuclear transport.