Multivalent proteins rapidly and reversibly phase-separate upon osmotic cell volume  change
Processing bodies (PBs) and stress granules (SGs) are prominent examples of subcellular,  membrane-less compartments that are observed under physiological and  stress conditions, respectively. We observe that the trimeric PB protein DCP1A rapidly  (within ~10 s) phase-separates in mammalian cells during hyperosmotic stress and  dissolves upon isosmotic rescue (over ~100 s) with minimal impact on cell viability  even after multiple cycles of osmotic perturbation. Strikingly, this rapid intracellular  hyperosmotic phase separation (HOPS) correlates with the degree of cell volume  compression, distinct from SG assembly, and is exhibited broadly by homo-multimeric  (valency = 2) proteins across several cell types. Notably, HOPS sequesters pre-mRNA  cleavage factor components from actively transcribing genomic loci, providing a  mechanism for hyperosmolarity-induced global impairment of transcription termination.  Together, our data suggest that the multimeric proteome rapidly responds to changes  in hydration and molecular crowding, revealing an unexpected mode of globally  programmed phase separation and sequestration that adapts the cell to volume  change. Overall design: Bru-seq of U2OS cells stably overexpressing GFP-DCP1a were performed under isotonic (150 mM Na+) or hypertonic (300 mM Na+) conditions to assess nascent transcripts