IM30 triggers membrane fusion in cyanobacteria and chloroplasts

Abstract

The thylakoid membrane of chloroplasts and cyanobacteria is a unique internal membrane system harbouring the complexes of the photosynthetic electron transfer chain. Despite their apparent importance, little is known about the biogenesis and maintenance of thylakoid membranes. Although membrane fusion events are essential for the formation of thylakoid membranes, proteins involved in membrane fusion have yet to be identified in photosynthetic cells or organelles. Here we show that IM30, a conserved chloroplast and cyanobacterial protein of approximately 30 kDa binds as an oligomeric ring in a well-defined geometry specifically to membranes containing anionic lipids. Triggered by Mg 2 þ , membrane binding causes destabilization and eventually results in membrane fusion. We propose that IM30 establishes contacts between internal membrane sites and promotes fusion to enable regulated exchange of proteins and/or lipids in cyanobacteria and chloroplasts.

Organization into Higher Ordered Ring Structures Counteracts Membrane Binding of IM30, a Protein Associated with Inner Membranes in Chloroplasts and Cyanobacteria

“…1, the addition of increasing IM30 concentrations did not affect the melting behavior and the phase transition of DMPC liposomes, confirming that IM30 does not bind to the zwitterionic PC head groups (Fig. 1A), in perfect agreement with previous findings (10). In contrast, a shift in the melting curves became obvious when increasing IM30 amounts were added to DMPG liposomes (Fig.…”

“…Because DOPG has a phase transition temperature (T m ) of ϳ18°C, DMPG was chosen in these experiments, which has a T m of ϳ23°C (20). DMPC, which has the same fatty acid composition as DMPG, was used as a control, because IM30 is not supposed to interact with the zwitterionic lipid head groups (10).…”

“…Nevertheless, although diverse physiological functions were proposed for IM30 in the past two decades, the exact protein function remained elusive for a long time. Only recently, IM30 has been identified to be able to reorganize lipid bilayer structures and to mediate membrane fusion in chloroplasts and cyanobacteria (10). Similar to its bacterial homolog, the PspA (phage shock protein A), which is involved in a bacterial stress response in situations provoking membrane stress (11), IM30 specifically interacts with negatively charged membrane surfaces, finally triggering membrane fusion (10,12).…”

“…Only recently, IM30 has been identified to be able to reorganize lipid bilayer structures and to mediate membrane fusion in chloroplasts and cyanobacteria (10). Similar to its bacterial homolog, the PspA (phage shock protein A), which is involved in a bacterial stress response in situations provoking membrane stress (11), IM30 specifically interacts with negatively charged membrane surfaces, finally triggering membrane fusion (10,12). IM30 does not exhibit membrane spanning helices, as is typical for a canonical transmembrane protein (2,3), and thus, it appears to peripherally bind to the cytoplasmic surface of the inner envelope/cytoplasmic membrane and TMs (5,13).…”

“…It was unclear for a long time whether these IM30 rings have any physiological meaning or just form to "store" IM30. Because ring formation, as well as membrane binding, was disturbed when the first 21 N-terminal amino acids of the A. thaliana IM30 were deleted, it was con-cluded that formation of a ring structure is essential for lipid interaction (12), and it was indeed recently shown by electron microscopy that IM30 rings bind to PG-containing membranes (10). However, because IM30 forms diverse ring structures with different amounts of IM30 protomers (5), it appears to be unlikely that formation of a defined ring structure is prerequisite for membrane binding and membrane remodeling.…”