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Natively unfolded phenylalanine-glycine (FG)-repeat domains are alleged to form the physical constituents of the selective barrier-gate in nuclear pore complexes during nucleocytoplasmic transport. Presently, the biophysical mechanism behind the selective gate remains speculative because of a lack of information regarding the nanomechanical properties of the FG domains. In this work, we have applied the atomic force microscope to measure the mechanical response of individual and clusters of FG molecules. Single-molecule force spectroscopy reveals that FG molecules are unfolded and highly flexible. To provide insight into the selective gating mechanism, an experimental platform has been constructed to study the collective behavior of surface-tethered FG molecules at the nanoscale. Measurements indicate that the collective behavior of such FG molecules gives rise to an exponentially decaying long-range steric repulsive force. This finding indicates that the molecules are thermally mobile in an extended polymer brush-like conformation. This assertion is confirmed by observing that the brush-like conformation undergoes a reversible collapse transition in less polar solvent conditions. These findings reveal how FG-repeat domains may simultaneously function as an entropic barrier and a selective trap in the near-field interaction zone of nuclear pore complexes; i.e., selective gate. force spectroscopy | nanomechanics | natively unfolded proteins | nuclear pore complex | selective gating

The nuclear pore complex (NPC) is the only gateway between the cell nucleus and the cytoplasm, and it mediates all movement between these two compartments. The past few years have witnessed much progress on its structure and molecular architecture, its interaction with soluble transport receptors, and how nucleocytoplasmic transport is regulated by the small GTPase Ran. However, the exact molecular mechanisms underlying nucleocytoplasmic transport and in particular cargo translocation through the NPC are understood only poorly. Recent findings on the interaction between the NPC and transport factors might provide insights into how efficient NPC translocation is accomplished. Moreover, in addition to mediating nucleocytoplasmic transport, the NPC seems to be involved, either directly or indirectly, in many other cellular processes such as chromosome segregation, gene expression, the secretory pathway and apoptosis. Here we review recent findings on these unexpected roles of the NPC.