6 – 8 Electron microscopy has revealed the general morphology of the NPC at nanometer-level resolution. 1 – 5 These Nups form a selective gate that allows passive diffusion of small molecules (<20–40 kDa) and transport-receptor-facilitated translocation of larger molecules (up to 25–50 MDa). The NPC is a large assembly (∼60–100 MDa) that is embedded in the nuclear envelope (NE) and composed of approximately 30 different protein components, known as nucleoporins (Nups), each present in an integer multiple of eight copies. In eukaryotic cells, the exchange of genetic materials generated by transcription in the nucleus and proteins synthesized in the cytoplasm is mediated by nuclear pore complexes (NPCs). Here we speculate that the selective permeability barrier in the NPC could be formed by clustered FG-Nups. The 3D distribution of interaction sites may indicate some native properties of the FG-Nups barrier. Moreover, we found that these interaction sites are spatially clustered into distinct groups in the periphery around a central axial channel with a diameter of approximately 10–20 nm in the NPC. Recently, we have shown that three-dimensional (3D) density maps of transient interactions between the FG-Nups barrier and a cargo-free or a cargo-bound transport receptor in native NPCs can be obtained by an advanced single-molecule fluorescence microscopy approach. Understanding the structure and function of the FG-Nups barrier under real-time trafficking conditions is still a formidable challenge due to the dynamic nature of a channeled membranous environment. ‘Natively unfolded’ nucleoporins (Nups) with domains rich in phenylalanine-glycine (FG) repeats form the selective permeability barrier and provide binding sites for mobile transport receptors in the NPC.
The nuclear pore complex (NPC) acts as a selective gate that mediates the bidirectional transport of macromolecules between the cytoplasm and the nucleus of eukaryotic cells.
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