Nucleoporin foci are stress‐sensitive condensates dispensable for C. elegans nuclear pore assembly

Abstract

Nucleoporins (Nups) assemble nuclear pores that form the permeability barrier between nucleoplasm and cytoplasm. Nucleoporins also localize in cytoplasmic foci proposed to function as pore preassembly intermediates. Here, we characterize the composition and incidence of cytoplasmic Nup foci in an intact animal, C. elegans. We find that, in young non-stressed animals, Nup foci only appear in developing sperm, oocytes and embryos, tissues that express high levels of nucleoporins. The foci are condensates of highly cohesive FG repeat-containing nucleoporins (FG-Nups), which are maintained near their solubility limit in the cytoplasm by posttranslational modifications and chaperone activity. Only a minor fraction of FG-Nup molecules concentrate in Nup foci, which dissolve during M phase and are dispensable for nuclear pore assembly. Nucleoporin condensation is enhanced by stress and advancing age, and overexpression of a single FG-Nup in postmitotic neurons is sufficient to induce ectopic condensation and organismal paralysis. We speculate that Nup foci are non-essential and potentially toxic condensates whose assembly is actively suppressed in healthy cells.

Coordinating nucleoporin condensation and nuclear pore complex assembly

“…While NTRs and chaperone complexes are emerging as important players, the molecular basis for the exquisite temporal synchrony of inside-out NPC biogenesis and the related membrane expansion leading to herniations is still enigmatic. Furthermore, with cytosolic NUP condensation being observed independent of NPC biogenesis [108], it is tempting to speculate that such condensates might fulfill additional roles that are unrelated to nuclear transport, which remain to be determined. Lastly, the fusogenic machinery acting within the nuclear envelope of higher eukaryotes remains to be identified.…”

“…NPC biogenesis starts in some cases with an abundant amount of condensation-prone Nups, as seen for Nup358 in D. melanogaster [43], NSP1 in yeast [106,107]. Condensates of these Nups can be precursors for NPC biogenesis, but are also observed independent of NPC biogenesis in D. melanogaster [108]. In yeast, Nup condensates comprise newly synthesized Nups that are inherited by the daughter cell [106].…”

Nuclear-import receptors as gatekeepers of pathological phase transitions in ALS/FTD

“…At high concentrations, Nup98 and other FG-Nups can spontaneously phase separate into liquid droplets and hydrogels with NPC-like permeability properties [ 19 ] and can transition into amyloid fibrils over time [ 45 , 103 , 104 ]. Aside from their canonical role in forming the permeability barrier for selective NCT, FG-Nups are highly mobile and can be found outside the NPC, where they may contribute to the formation of phase-separated membraneless organelles, such as SGs and processing bodies (P-bodies) [ 60 , 105 ].…”

Puzzling out nuclear pore complex assembly

“…Strikingly, these condensates enrich pairs of Nups that assemble co‐translationally (i.e., yNsp1‐yNup57 and yNup116‐yNup82 [30,102]), and are targeted to the cell bud via the actomyosin cytoskeleton [57], as shown for yeast translation factories [119], suggesting that they could couple Nup synthesis, assembly and transport. Similarly, cytoplasmic condensates encompassing several distinct FG and scaffold Nups were also observed in human cells deficient for fragile X‐related (FXR) proteins, which are RBPs involved in translational processes [126], or in worm embryos [128].…”

“…2A), and/or aberrant condensates formed upon accumulation of cytosolic precursors, remains however an opened question (discussed in Ref. [128]).…”