Abstract
The inner membrane-associated protein of 30 kDa (IM30) is crucial for the development and maintenance of the thylakoid membrane system in chloroplasts and cyanobacteria. While its exact physiological function still is under debate, it has recently been suggested that IM30 has (at least) a dual function, and the protein is involved in stabilization of the thylakoid membrane as well as in Mg2+-dependent membrane fusion. IM30 binds to negatively charged membrane lipids, preferentially at stressed membrane regions where protons potentially leak out from the thylakoid lumen into the chloroplast stroma or the cyanobacterial cytoplasm, respectively. Here we show in vitro that IM30 membrane binding, as well as membrane fusion, is strongly increased in acidic environments. This enhanced activity involves a rearrangement of the protein structure. We suggest that this acid-induced transition is part of a mechanism that allows IM30 to (i) sense sites of proton leakage at the thylakoid membrane, to (ii) preferentially bind there, and to (iii) seal leaky membrane regions via membrane fusion processes.
Highlights
inner membrane-associated protein of 30 kDa (IM30), the Inner Membrane-associated protein of 30 kDa, is conserved in almost all oxygenic photosynthetic organisms, involving cyanobacteria as well as algae and higher plants [1]
IM30 has evolved via gene duplication from its bacterial ancestor PspA [2], which is suggested to be involved in membrane protection/maintenance in bacteria [3]
We propose that the structural rearrangements of IM30 induced by an acidified environment enable the protein to sense and bind to defective thylakoid membrane (TM) regions where protons leak out of the TM lumen into the cytoplasm, which increases the membrane fusion capabilities of IM30
Summary
IM30, the Inner Membrane-associated protein of 30 kDa, is conserved in almost all oxygenic photosynthetic organisms, involving cyanobacteria as well as algae and higher plants [1]. Members of the IM30/PspA family, including IM30, PspA, and LiaH, the PspA homolog found in Bacillus and Listeria species, are known to spontaneously assemble into large, homo-oligomeric ring structures with molecular masses of at least 1 MDa [5,9,10,11,12,13,14,15,16] Such ring structures have a diameter of 20 nm and a height of 8–11 nm in the case of E. coli PspA [5], and a diameter of 25 nm in the case of the B. subtilis LiaH [16]. PCC 6803 (from here on: Synechocystis), the currently best-studied IM30 protein, forms rings with diameters of 24–33 nm, which, have a constant height of 13–15 nm [14]
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