Membrane interaction of cyanobacterial and chloroplast ESCRT-III proteins.

More than three decades ago, the inner membrane-associated protein of 30 kDa (IM30), also known as Vipp1, was identified in pea chloroplasts to bind to the chloroplast inner envelope membrane. IM30/Vipp1 is a membrane-associated and soluble stromal protein and is proposed to mediate vesicle formation. Furthermore, it is linked to membrane stabilization processes, as also discussed for its bacterial homolog PspA. More recently, the structures of cyanobacterial IM30/Vipp1 and PspA proteins were resolved, revealing that these proteins belong to the endosomal sorting complex required for transport III (ESCRT-III) superfamily. ESCRT-IIIs are known for their central roles in diverse membrane remodeling activities in eukaryotic cells. This discovery, together with recent in vitro studies, now enables a molecular understanding of IM30/Vipp1 structure and activity in cyanobacteria and chloroplasts, an organelle of cyanobacterial origin. Here, we discuss membrane binding of IM30/Vipp1 in chloroplasts/cyanobacteria, contrasting them with eukaryotic ESCRT-III function. Recent analyses have identified two key regions in IM30/Vipp1 that mediate initial membrane binding, as well as the formation of spiral, barrel, and/or rod structures on membrane surfaces, eventually facilitating membrane internalizations and tubulation. The potential roles of these membrane-bound structures in the remodeling of chloroplasts and cyanobacterial membranes are discussed.