Cytoplasmic nucleoporin foci are stress-sensitive, non-essential condensates in C. elegans

Nucleoporins (Nups) assemble nuclear pores that form the permeability barrier that separates nucleoplasm from cytoplasm. Nups have also been observed in cytoplasmic foci proposed to function as pore pre-assembly 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 Nup levels. The foci are condensates of highly cohesive FG-Nups that 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. Nup condensation is enhanced by stress and advancing age, and overexpression of a single FG-Nup in post-mitotic neurons is sufficient to induce ectopic condensation and organismal paralysis. Our results suggest that Nup foci are non-essential, “accidental”, and potentially toxic condensates whose assembly is actively suppressed in healthy cells.