Magnetoskeleton : Elucidating cytoskeletal elements involved in magnetosome chain assembly and segregation of Magnetospirillum gryphiswaldense

Abstract

Magnetotactic bacteria synthesize dedicated magnetic organelles termed magnetosomes, which are membrane-enveloped magnetic nanocrystals. To serve as an efficient magnetoreceptor, magnetosomes are organized into a single, midcell positioned and tightly spaced chain that aligns the cell to the Earth’s magnetic field, facilitating taxis towards a suitable environment. In the model organism Magnetospirillum gryphiswaldense, the magnetosome chain is assembled by concerted action of the actin-like MamK filament and the adaptor protein MamJ, which links the magnetosomes to the MamK cytoskeleton. To pass on the selective advantage of magnetotaxis to ensure cell fitness, during cytokinesis, the magnetosome chain is regularly found traversing the cell division site, where it becomes equipartitioned. Subsequently, the newly divided chains must undergo a hypothesized dynamic relocalization to midcell, which has been attributed to MamK. Yet, the underling molecular mechanisms of magnetosomes segregation, putative dynamics and coordination with the cell cycle have remained elusive. In this thesis, in vivo time-lapse imaging and photokinetic approaches revealed that a directed MamK filament treadmilling is responsible for a rapid and dynamic pole-to-midcell repositioning of magnetosome chains in recently divided cells. In addition, it was found that magnetosomes equal partitioning is highly precise and directly dependent on an intact MamK filament dynamics. Thus, MamK dynamics and its interplay with MamJ provide an active driving force for magnetosomes movement. Peculiarly, loss of MamK did not abolish magnetosome chain configuration, but merely caused fragmented chains, revealing an additional yet unrecognized determinant for magnetosomes organization. In this work, a subcellular analysis by cryo-electron tomography (cryo-ET) showed that the magnetosome chain is confined to the positive curvature of the cytoplasmic membrane of the helical cell (i.e., the geodetic axis). Interestingly, the chain geodetic position became lost in absence of the transmembrane protein MamY. Moreover, double deletion of mamY and mamK entirely abolished chain configuration, suggesting MamY as a novel and essential scaffolding element for magnetosomes organization and positioning. This notion was substantiated by construction of an artificial magnetosome chain on MamY, circumventing both MamK and MamJ functions. PALM and 3D-SIM analyses demonstrated that MamY is highly enriched along the geodetic axis, likely in a polymerized state. These results evidence that MamY acts as a landmark of positive cell curvature and positions a linear magnetoreceptor inside a helical cell, aligning the motility axis and magnetosome chain magnetic-dipole moment. Therefore, MamY is proposed as a new member of the hereby-termed magnetoskeleton, which comprises essential proteins for magnetosomes spatial organization. Since coordination of magnetosome dynamics with cell cycle is yet poorly understood, the role of two major cell cycle regulators, PopZ and MipZ, was analyzed in…