Quarta, N.; Debrich, K.; Hellmann, N.; Ge, X.; Argudo, P. G.; Bhandari, T. R.; Bonn, M.; Girard, M.; Parekh, S. H.; Liu, L.-N.; Schneider, D.

Biomolecular condensates are well characterized in eukaryotes, but their role in bacteria remains largely elusive. In the cyanobacteriumn Synechocystis sp. PCC 6803, the protein IM30, a member of the ESCRT-III superfamily of membrane remodeling proteins, forms stress-induced puncta across diverse environmental challenges, indicating a general adaptive response. Live-cell imaging reveals that IM30 is uniformly distributed throughout the cytoplasm at low concentrations but assembles into puncta upon exceeding a critical saturation threshold, a hallmark of liquid-liquid phase separation. Crucially, stress triggers puncta formation even below this threshold, suggesting stress lowers the phase-separation barrier. Super-resolution microscopy confirms spherical, condensate-like morphologies, while FRAP demonstrates rapid fluorescence recovery, consistent with with a fluid interior and dynamic exchange between puncta and the cytosol. Cellular IM30 levels exceed the in vitro determined critical concentration, placing the protein in a supersaturated state primed for condensation. Domain mapping identifies the structured 1-3 helical hairpin as the minimal phase separation driver; in contrast, the disordered 4-6 segment alone cannot phase-separate. Phase separation occurs within a physiologically relevant pH range (4.5-6.5), matching the localized acidification of the cyanobacterial cytoplasm at damaged thylakoid membranes. This directly links membrane stress, pH changes, and IM30 recruitment. Collectively, these findings establish IM30 puncta as bona fide, stress-responsive biomolecular condensates that function as rapid stress sensors and effectors, providing a mechanistic framework for phase separation and condensate formation by bacterial ESCRT-III proteins during environmental adaptation.