From magnetosome membrane invagination to transport - the dual role of MamB in magnetosome formation

Magnetotactic bacteria (MTB) are a unique group of prokaryotesthat synthesize specialized organelles, magnetosomes, for magnetic navigation. In the MTB model, Magnetospirillum gryphiswaldense MSR-1, magnetosomes consist of membrane-enclosed magnetite (Fe 3O4 ) nanoparticles that are aligned into a single coherent chain by dedicated cytoskeletal structures. The formation of these unique organelles is a complex process that comprises several key steps that are governed by magnetosome-associated proteins. One of the most important proteins during this process is the cation diffusion facilitator (CDF) family member, MamB. MamB was shown to be required for magnetosome membrane invagination from the cytoplasmic membrane and was implicated in magnetosome-directed iron transport but its precise role remained elusive. In this study, we employed a multidisciplinary approach to define MamB’s role during magnetosome formation. Using site-directed mutagenesis complemented with structural analyses, fluorescence microscopy, and cryoelectrontomography we show for the first time that MamB is an active magnetosome-directed transporter with two distinct, essential functions. First, MamB is required for magnetosome vesicle formation. Second, MamB’s transport activity is essential for magnetite nucleation but not required for vesicle formation. Furthermore, we determined the crystallographic structure of MamB cytosolic terminal domain and showed that it shares similarities with the cation diffusion facilitator protein family.