The cyanobacterial ESCRT-III protein IM30 forms biomolecular condensates at physiologically relevant conditions

Abstract

IM30, the inner membrane-associated protein of 30 kDa, conserved in cyanobacteria and chloroplasts, is a member of the ESCRT-III superfamily of membrane remodeling proteins. Like other ESCRT-III proteins, IM30 forms higher-order oligomeric structures, although the mechanisms regulating its assembly and disassembly remain poorly understood. A hallmark of ESCRT-III protein monomers is the presence of at least five α-helices, with the long helices α1 and α2/3 forming a helical hairpin that constitutes the structural core of all superfamily members. In contrast to eukaryotic ESCRT-III subunits, helices α0 and α4-α6 of Synechocystis IM30 unfold upon oligomer disassembly. Given that intrinsically disordered proteins often form biomolecular condensates via liquid-liquid phase separation and IM30 has previously been observed to form puncta structures in vivo under membrane stress, we here investigated whether IM30 has the ability to form biomolecular condensates in vitro. We demonstrate that IM30 forms condensates under physiologically relevant conditions of salt and protein concentrations, suggesting a functional link between the now observed condensate formation and membrane dynamics. Condensate formation is driven by the polyampholyte nature of IM30, yielding condensates that can be dissolved by both high and low salt concentrations. In living cyanobacterial cells, we observed puncta structures under salt stress, which we now link to the formation of condensates. We propose that condensates serve as transient hubs, locally concentrating IM30 monomers under stress conditions without requiring energy-intensive disassembly of preformed oligomers. Thus, condensate formation may represent a crucial early step in IM30-mediated stress response in bacteria and chloroplasts.