Segregation of prokaryotic magnetosomes organelles is driven by treadmilling of a dynamic actin-like MamK filament.
BACKGROUND
The navigation of magnetotactic bacteria relies on specific intracellular organelles, the magnetosomes, which are membrane-enclosed crystals of magnetite aligned into a linear chain. The magnetosome chain acts as a cellular compass, aligning the cells in the geomagnetic field in order to search for suitable environmental conditions in chemically stratified water columns and sediments. During cytokinesis, magnetosome chains have to be properly positioned, cleaved and separated in order to be evenly passed into daughter cells. In Magnetospirillum gryphiswaldense, the assembly of the magnetosome chain is controlled by the actin-like MamK, which polymerizes into cytoskeletal filaments that are connected to magnetosomes through the acidic MamJ protein. MamK filaments were speculated to recruit the magnetosome chain to cellular division sites, thus ensuring equal organelle inheritance. However, the underlying mechanism of magnetic organelle segregation has remained largely unknown.


RESULTS
Here, we performed in vivo time-lapse fluorescence imaging to directly track the intracellular movement and dynamics of magnetosome chains as well as photokinetic and ultrastructural analyses of the actin-like cytoskeletal MamK filament. We show that magnetosome chains undergo rapid intracellular repositioning from the new poles towards midcell into the newborn daughter cells, and the driving force for magnetosomes movement is likely provided by the pole-to-midcell treadmilling growth of MamK filaments. We further discovered that splitting and equipartitioning of magnetosome chains occurs with unexpectedly high accuracy, which depends directly on the dynamics of MamK filaments.


CONCLUSION
We propose a novel mechanism for prokaryotic organelle segregation that, similar to the type-II bacterial partitioning system of plasmids, relies on the action of cytomotive actin-like filaments together with specific connectors, which transport the magnetosome cargo in a fashion reminiscent of eukaryotic actin-organelle transport and segregation mechanisms.
['Toro-Nahuelpan|Mauricio|M|0000-0001-5333-3640', 'Müller|Frank D|FD|', 'Klumpp|Stefan|S|', 'Plitzko|Jürgen M|JM|', 'Bramkamp|Marc|M|', 'Schüler|Dirk|D|']
D000199:Actins D001426:Bacterial Proteins D003599:Cytoskeleton D055944:Magnetosomes D042201:Magnetospirillum
Segregation Prokaryote Magnetosomes Organelles MamK Magnetotactic bacteria Partitioning Actin Bacterial cytoskeleton