Stress-specific NONO interactomes reveal a key role of Hsp70 chaperone activity in regulation of paraspeckle formation.

Paraspeckles are stress-induced nuclear RNA-protein condensates that assemble on the long non-coding RNA NEAT1. Their increased formation under certain cellular circumstances has gained growing interest due to their association with serious human diseases, such as neurodegenerative disorders and cancer. The biological functions of paraspeckles still appear obscure, but increasing evidence suggests that they contribute to regulation of gene expression by recruiting specific proteins and RNA molecules. Here, we have characterized and compared two stress-enriched interactomes of the essential paraspeckle protein NONO in both wild-type and paraspeckle-deficient NEAT1 knockout cells. We identified Hsp70 as part of stress-enriched NONO complexes in wild-type but not in NEAT1-depleted cells. We show that proteotoxic stress-induced paraspeckle formation and NEAT1 expression are strictly dependent on Hsp70 chaperone activity. Our data demonstrate that both NONO and Hsp70 transiently translocate to the nucleolus during heat shock and that paraspeckle formation during recovery follows Hsp70-dependent relocation of NONO from the nucleolus to the nucleoplasm. Taken together, we demonstrate an important role of Hsp70 in paraspeckle assembly and identify a possible link between the nuclear protein quality control system and paraspeckles.