Defined interactions are crucial for the structure and function of the Synechocystis IM30 protein
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Abstract
Biogenesis and maintenance of the thylakoid membranes in cyanobacteria and chloroplasts is a vital process. Nevertheless, the mechanism of the biogenesis has stayed a mystery until today. The IM30 protein appears to play an important role in this process, as has been shown via depletion experiments. Recently, the ability of IM30 to bind to negatively charged lipids and to fuse liposomes in presence of Mg2+ has been demonstrated. This discovery indicates a capability of IM30 to fuse the cytoplasmic membrane in cyanobacteria or the inner envelope in chloroplasts with the constructive thylakoid membrane in order to create a transfer point for lipids and/or proteins. Additionally, connections between different thylakoid membranes through IM30 are thinkable.
In the framework of this thesis, interactions of IM30 leading to membrane fusion and the fusion mechanism itself were investigated. Here, the interplay of IM30 and Mg2+ as well as negatively charged membranes were separately examined via in vitro approaches and finally combined in order to have a closer look at the IM30-mediated membrane fusion.
It was observed that Mg2+ binds directly to IM30 and induces a structural alteration in the protein, which results in stabilization against different stressors and an increased tendency of IM30 to form double-rings. The IM30-membrane interaction was shown to depend on the electrostatic interaction with anionic lipids and is enhanced in presence of Mg2+. Furthermore, lower-ordered IM30 oligomerization states seem to have an increased membrane binding affinity. This circumstance potentially causes the dissociation of IM30 ring oligomers on membranes in absence of Mg2+. Since the presence of the ring structure is probably crucial for the IM30 function, dissociation of rings might be prevented by Mg2+. Stabilized by Mg2+, an IM30 double-ring might be able to fuse two membranes. While doing this, the double-ring seems to dissociate and most probably needs to be reactivated prior to another fusion.
The results of this thesis provide hints concerning the in vivo function of IM30, which can now be investigated further in a more target-oriented manner.