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

Abstract

Nucleoporins (Nups) form the selective permeability barrier that separates the nucleoplasm from the cytoplasm. In addition to their localization at the nuclear envelope, Nups have been observed in cytoplasmic foci in many cell types. We have investigated the origin of Nup foci using C. elegans oocytes, which naturally accumulate high concentrations of Nups. We find that the foci derive from condensation of highly cohesive FG-Nups, which are maintained at concentrations right above the solubility limit in oocytes. Nup solubility is enhanced by chaperone activity and posttranslational modifications that also promote nuclear pore channel fluidity and pore disassembly during mitosis. Oocyte Nup foci dissolve during M phase and are not essential for embryonic viability. Overexpression of the highly cohesive FG-Nup, Nup98, in post-mitotic neurons leads to uncontrolled Nup aggregation and organismal paralysis, underscoring the importance of mechanisms that maintain Nup solubility in the cytoplasm.

Molecular determinants of Karyopherin-β2 chaperone and disaggregation activity

“…Indeed, the N-terminal HEAT repeats of a related NIR, Kapβ1, make contacts with FG-rich Nups (107). Moreover, a related nuclear-export factor, CRM1, may prevent FG-rich Nup condensation in the cytoplasm (111). Regardless, H8-E can still disaggregate FUS fibrils, albeit less efficiently than FL Kapβ2, indicating that residues beyond H1-H7 can also break contacts between FUS PrLDs that preserve fibril integrity.…”

RNA granules: functional compartments or incidental condensates?

“…Protein modifications can also modulate solubility. Nucleoporins stockpiled in oocytes depend on phosphorylation and sugar modifications to remain soluble and limit the formation of potentially toxic condensates ( Thomas et al 2022 ). Phosphorylation by the DYRK kinase MBK-2 is essential to accelerate P granule dynamics and ensure that their polarized dissolution and condensation occur sufficiently fast to keep up with embryonic cell divisions ( Folkmann et al 2021 ).…”

“…In Drosophila oocytes, nucleoporin condensates have been proposed to function as preassembly sites for nuclear pore complexes stored in membrane structures called annulate lamellae ( Hampoelz et al 2019 ). A study in C. elegans , however, suggests that nucleoporin condensates are dispensable for viability and correspond to minor condensation by-products of a large stockpile of highly cohesive FG-Nups maintained near c sat in oocytes for use later during embryogenesis ( Thomas et al 2022 ). Whether RNAs localize to condensates to promote, or as a consequence of, binding between encoded proteins during translation remains to be determined.…”

“…Care should also be taken to avoid fixation or microscopy conditions that enhance or suppress condensation ( Uebel and Phillips 2019 ; Elaswad et al 2022a ; Irgen-Gioro et al 2022 ). The relatively small volume of RNA granules compared with the rest of the cell may mean that, even when prominent by microscopy, RNA granules may only account for a small proportion of molecules, as demonstrated for stress granules, P-bodies, and nucleoporin condensates ( Khong et al 2017 ; Xing et al 2020 ; Thomas et al 2022 ). Such a finding may help rule out models that require a significant proportion of RNPs to localize to the condensates (such as inhibitory/titration models) but does not necessarily rule out other functions.…”

“…Mutations that disrupt incidental condensates could be used to guide the identification of binding sites in proteins and/or RNAs that mediate RNP complex assembly in vivo. Incidental condensates could also serve as useful markers of stress responses and aging, as exemplified by stress granules that arise under stress conditions that block translation initiation ( Kedersha and Anderson 2002 ; Kedersha et al 2005 ) and cytoplasmic nucleoporin condensates, whose assembly is enhanced by heat shock and organismal aging ( Patterson et al 2011 ; Thomas et al 2022 ). Because cells use energy-consuming mechanisms to counter condensation (Box 2), incidental condensates may also prove useful markers to identify quiescent cells with lower ATP production.…”