Unravelling the cytoskeletal network that governs mobility and positioning of magnetic organelles in bacteria: novel players and functions.

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Magnetotactic bacteria synthesize unique organelles (magnetosomes) that consist of membrane-bounded magnetite crystals, and they concatenate them against their immanent physical tendency to agglomerate actively into regular chains. The summarized magnetic moments of aligned magnetosomes combined with a well defined subcellular position function most efficiently as an earth magnetic field sensor. Magnetosome formation, chain assembly and positioning in magnetotactic bacteria are genetically controlled to accomplish one of the highest structural levels found in prokaryotic cells. How MTB assemble their magnetosomes into regular chains and how this assemblage becomes positioned, split and equipartitioned to daughter cells are major intriguing questions of MTB cell biology. The discovery of a novel cytoskeletal structure involved in magnetosome assembly, the actin-like magnetosome filament, by our previous work provided a key for fundamental understanding of this process. However, most recent results obtained in the previous funding period suggest that the magnetosome filament may represent just one component of a much more complex cytoskeletal network. In this follow-up proposal, we plan to extend our in-depth understanding of magnetobacterial cell biology and the molecular mechanisms underlying formation and precise localization of highly ordered magnetosome chains in the well-established model Magnetospirillum gryphiswaldense. By genetic and biochemical approaches, we will analyze the interplay of known and new magnetosome-related key players for chain assembly in vivo and in vitro. In addition, we will analyze the mechanism of magnetosome mobility and its coordination with the cell cycle in detail by, e.g. protein interaction assays to reveal connections to conserved cytoskeletal and cell division proteins. The dynamic localization of these proteins in different mutant backgrounds will be followed during cell cycle by advanced fluorescence microscopy including superresolution techniques. In addition, structures formed by cytoskeletal proteins will be analyzed by electron microscopy including advanced electron cryo-tomography. These studies will be complemented by in vitro analyses which aim to understand the interactions of cytoskeletal proteins on molecular level. Our studies will reveal fundamental insights into the roles and evolution of cytoskeletal structures in bacteria and the organization, segregation and subcellular positioning of prokaryotic organelles.