A Janus-Faced IM30 Ring Involved in Thylakoid Membrane Fusion Is Assembled from IM30 Tetramers

Biogenesis and dynamics of thylakoid membranes likely involves membrane fusion events. Membrane attachment of the inner membrane-associated protein of 30 kDa (IM30) affects the structure of the lipid bilayer, finally resulting in membrane fusion. Yet, how IM30 triggers membrane fusion is largely unclear. IM30 monomers pre-assemble into stable tetrameric building blocks, which further align to form oligomeric ring structures, and differently sized IM30 rings bind to membranes. Based on a 3D reconstruction of IM30 rings, we locate the IM30 loop 2 region at the bottom of the ring and show intact membrane binding but missing fusogenic activity of loop 2 mutants. However, helix 7, which has recently been shown to mediate membrane binding, was located at the oppossite, top side of IM30 rings. We propose that a two-sided IM30 ring complex connects two opposing membranes, finally resulting in membrane fusion. Thus, IM30-mediated membrane fusion requires a Janus-faced IM30 ring. [Display omitted] •IM30 monomers assemble into stable tetrameric building blocks prior to ring formation•3D reconstructions of different IM30 rings are presented•Helix 7 and loop 2 are located at opposite sides of IM30 rings•IM30-mediated membrane fusion requires a Janus-faced IM30 ring The IM30 protein is a membrane fusion protein that forms large oligomeric rings from tetrameric building blocks. Saur et al. present 3D reconstructions of the Janus-faced IM30 ring, which illustrates membrane binding properties as well as the membrane fusion activity of IM30.