IISER MOHALI

Abstract:
Dynamic membrane remodeling is essential for eukaryotic cell viability and function. Once thought to be unique to eukaryotes, proteins involved in these processes are now recognized in bacteria, suggesting conserved mechanisms. In eukaryotes, the highly conserved endosomal sorting complex required for transport (ESCRT) mediates processes such as cytokinesis and vesicle formation. Recently, proteins of the ESCRT-III superfamily have been identified in bacteria, sharing structural similarities and membrane-remodeling activity with their eukaryotic counterparts. While both form oligomers, bacterial ESCRT-III proteins typically assemble into homooligomers, whereas eukaryotic versions form heterooligomeric complexes.
The oligomeric structure of IM30 exhibits a remarkable plasticity, which has also been observed with eukaryotic ESCRT-III proteins. In addition to barrels, we have observed the formation of rod structures in solution, as well as carpets and spirals on membranes. Upon binding to solid-supported membranes, IM30 barrels disassemble into smaller oligomers, which involves partial unfolding of the monomers. In fact, the oligomeric assembly induces/stabilizes α-helical regions, and in IM30*, an IM30 variant defective in oligomerization, only a helical hairpin formed by the helices α1-3 retains its ordered structure while the remaining regions are disordered. Under stress conditions, IM30 has been observed to form enigmatic puncta within cells, a behavior that has also been observed for other bacterial ESCRT-III proteins. We now revealed that IM30 can form liquid-like condensates both in vitro and in vivo upon disassembly of its oligomeric structures. We find that IM30 phase separates in vitro under physiologically relevant conditions, and the formation of these condensates does not require IM30 proteins to adopt a structured oligomeric form. Instead, a coiled-coil domain conserved in all ESCRT-III proteins drives phase separation, while, interestingly, an intrinsically disordered region plays a minor role in this process. Our findings suggest that condensate formation by an ESCRT-III superfamily member represents a previously undescribed cellular stress response that likely involves condensate interactions with membranes. This work provides new insights into the structural plasticity and functional versatility of ESCRT-III proteins involved in membrane maintenance and remodeling.