Multivalent proteins rapidly and reversibly phase-separate upon osmotic cell volume change [ChIP-seq]

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: ChIPseq of U2OS cells stably overexpressing GFP-DCP1a were perfromed under isotonic (150 mM Na+) conditions or hypertonic (300 mM Na+) conditions to assess chromatin occupancy of factors involved in RNA metabolism. Treatment, 30 min.