The assembly of osmoDroplet reveals dehydration sensing by helix transition and association
Abstract
Hyperosmotic stress triggers cellular dehydration and macromolecular crowding, leading to rapid biomolecular condensation. However, the individual roles of dehydration and crowding have been unclear, and the design principles of osmosensors remain poorly understood. Here, we identify a conserved sensor module comprising an osmo-sensing α-helix adjacent to an intrinsically disordered region (IDR). Using helices from Arabidopsis SEUSS and rice DRG9, we demonstrate that tandem repeats (≥2) self-assemble into a stable core that nucleates IDR condensation into ‘osmoDroplets’ in yeast under hyperosmotic stress. This assembly is tunable by helix copy number and IDR properties. The helices are largely unstructured under isotonic conditions but fold into stable α-helices upon hyperosmotic stress. Circular dichroism shows that dehydration specifically triggers this folding. Mutagenesis reveals that hydrophilic residues are essential for osmo-sensing, mediating changes in secondary structure and condensate formation. Simulations indicate that hyperosmolarity induces helical transition by reducing water-residue hydrogen bonds, compensated by intramolecular bonding. This versatile helix-IDR architecture functions across plant lineages and IDRs, defining a module where dehydration-driven folding directly transduces water loss into condensate assembly. Our work elucidates fundamental principles for engineering synthetic biological sensors and provides a practical tool for assessing the phase-separation capability of any given IDR.
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The authors declare no competing interests to disclose.
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