The central channel of the nuclear pore complex (NPC) is occupied by non-structured polypeptides with a high content of Phe-Gly (FG) motifs. This protein-rich environment functions as an entropic barrier that prevents the passage of molecules, as well as the binding sites for karyopherins, to regulate macromolecular traffic between the nucleoplasm and the cytoplasm. In this study, we expressed individual Nups fused with a crowding-sensitive probe (GimRET) to determine the spatial distribution of protein-rich domains within the central channel in vivo, and characterize the properties of the entropic barrier. Analyses of the probe signal revealed that the central channel contains two protein-rich domains at both the nucleoplasmic and cytoplasmic peripheries, and a less-crowded central cavity. Karyopherins and other soluble proteins are not the constituents of the protein-rich domains. The time-lapse observation of the post-mitotic reassembly process also revealed how individual protein-rich domains are constructed by a sequential assembly of nucleoporins.

The reductive coupling of 4-bromo-3,3,4,4-tetrafluorobut-1-ene with 2.4 equivalents of various carbonyl compounds proceeded very smoothly in the presence of 2.4 equivalents of MeLi/LiBr-free at -78 °C for two hours, giving the corresponding adducts in high to excellent yields. On the other hand, triethyl(1,1,2,2-tetrafluorobut-3-enyl)silane, which could be prepared by treatment of 4-bromo-3,3,4,4-tetrafluorobut-1-ene and chlorotriethylsilane with magnesium at 0 °C for three hours, reacted with some aldehydes in the presence of 1 mol% of tetrabutylammonium fluoride, the desired adducts being obtained in good to high yields.

The karyopherin family of nuclear transport receptors is composed of a long array of amphiphilic α-helices and undergoes flexible conformational changes to pass through the hydrophobic crowding barrier of the nuclear pore. Here, we focused on the characteristic enrichment of prolines in the middle of the outer α-helices of importin-β. When these prolines were substituted with alanine, nuclear transport activity was reduced drastically and, and caused a severe defect in mitotic progression. These mutations did not alter the overall folding of the helical repeat or affect its interaction with cargo or the regulatory factor Ran. However, and analyses revealed that the mutant lost structural flexibility and could not undergo rapid conformational changes when transferring from a hydrophilic to hydrophobic environment or vice versa. These findings reveal the essential roles of prolines in ensuring the structural flexibility and functional integrity of karyopherins.

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Abbreviations: FG-motif, phenylalanine-glycine motif; IDR, intrinsically disordered region; NPC, nuclear pore complex. AbstractIn this study, we examined how channel-forming subunits of the nuclear pore complex (NPC) are assembled into a selective channel within a highly structured scaffold ring during postmitotic assembly. We focused on non-structured domains of the scaffold Nups and performed in vitro self-assembled particle assays with those derived from channel-forming FG-Nups. We found that non-structured domains of ELYS and Nup35N interacted with channel-forming FG-Nups to form a self-assembled particle. Sequential addition of FG-Nups into the scaffold particle revealed that ELYS, which initiates postmitotic NPC reassembly, interacts with early assembling FG-Nups (Nups98 and 153) but not middle stage-assembling FG-Nups (Nups58 and 62). Nup35, which assembles between the early and middle stages, facilitated the assembly of Nup62 into the early assembling Nups both in vitro and in vivo. These results demonstrate that ELYS and Nup35 have a role of facilitator in the ordered assembly of channel-forming FG-Nups during mitosis. K E Y W O R D Sintrinsically disordered region, molecular crowding, nuclear pore complex, self-assembly