Structural organization of the nuclear pore permeability barrier

The efficiency of gene therapy in non-dividing cells is particularly poor due to restricted nuclear delivery rates of exogenously applied macromolecules across the nuclear pore complexes (NPCs). Therefore, improved intranuclear delivery of transgenes requires an ability to modulate the barrier function of the NPC. Despite a large body of experimental evidence accumulated to date, the contribution of individual NPC proteins (nucleoporins) to the formation of the NPC permeability barrier as well as their structural organization within the NPC remains under debate. In the present study, we revisit the view on the spatial arrangement of the Phe-Gly rich domains (FG-domains) of a subset of nucleoporins known as FG-nucleoporins. They are generally believed to be the key constituents of the NPC permeability barrier. Comparison of the binding pattern of a transport receptor importin β fragment, that binds specifically to FG-domains, with the binding pattern of wheat germ agglutinin that binds elsewhere in the NPC, reveals that FG-domains tend to cluster in the very center of the NPC. Furthermore, a controlled sequential release of the barrier-forming nucleoporins results in a gradual breakdown of the NPC permeability barrier. The breakdown is initiated by a dissociation of Nup62 from the NPC. This is accompanied by an increased passive diffusion of small molecules across the NPC. Subsequent dissociation of Nup98 and possibly other nucleoporins results in a collapse of the barrier for larger molecules. We therefore conclude that FG-nucleoporins do not contribute equally to the maintenance of the NPC permeability barrier exclusion limit. This implies that a controlled release of nucleoporins that contribute most to the formation and maintenance of the NPC barrier can facilitate access of therapeutic macromolecules into the nucleus.