Polyanionic C-Terminal IDR-Dependent Phase Separation of DrHsp17 Contributes to Acid Stress Protection in Deinococcus radiodurans

The extremophilic bacterium Deinococcus radiodurans is a promising chassis for bioremediating acidic radioactive environments, yet how it protects intracellular proteins against acid stress remains poorly understood. Here, we investigate the atypical small heat shock protein DrHsp17 to elucidate its physiological role and biophysical properties under acid stress. Our results show that DrHsp17 contributes to cell survival under severe acidic conditions, which induce liquid–liquid phase separation (LLPS) of DrHsp17 to form dynamic condensate droplets both in vivo and in vitro. The phase-separation behavior of DrHsp17 depends on its polyanionic C-terminal intrinsically disordered region (IDR). Notably, LLPS is associated with enhanced chaperone activity of DrHsp17 and reduced intracellular protein aggregation. These findings indicate polyanionic IDR-driven phase separation of the DrHsp17 as a novel mechanism involved in bacterial acid stress-sensing and resistance, with the potential for engineering D. radiodurans cells for bioremediation under acid stress.