The Dual Role of Mamb in Magnetosome Membrane Assembly and Magnetite Biomineralization

Noa Keren-Khadmy kenoa@post.bgu.ac.il ^1,2,3 René Uebe ⁴ Natalie Zeytuni ^1,2,3 Emanuel Katzmann ⁵ Yotam Navon ^2,6 Geula Davidov ^1,2,3 Ronit Bitton ^2,6 Jürgen M. Plitzko ⁵ Dirk Schüler ⁴ Raz Zarivach ^1,2,3

¹Department of Life Sciences, Ben-Gurion University of the Negev, Beer-Sheva, Israel
²The National Institute for Biotechnology in the Negev, Ben-Gurion University of the Negev, Beer-Sheva, Israel
³Ilse Katz Institute for Nanoscale Science & Technology, Ben-Gurion University of the Negev, Beer-Sheva, Israel
⁴Department of Microbiology, University of Bayreuth, Bayreuth, Germany
⁵Department of Molecular Structural Biology, Max Planck Institute of Biochemistry, Martinsried, Germany
⁶Department of Chemical Engineering, Ben-Gurion University of the Negev, Beer-Sheva, Israel

Magnetospirillum gryphiswaldense MSR-1 synthesizes membrane-enclosed magnetite (Fe₃O₄) nanoparticles, called magnetosomes, for magnetic navigation. Formation of these unique organelles involves a complex process comprising several key steps which are governed by specific magnetosome-associated proteins. One of the most important proteins in this process is the cation diffusion facilitator (CDF) family member MamB, which has been implicated in magnetosome-directed iron transport. While deletion mutagenesis studies revealed that MamB is essential for the formation of magnetosome membrane vesicles however, its precise role remains elusive. In this study, we employed a multi-disciplinary approach to define the role of MamB during magnetosome formation. Using site-directed mutagenesis complemented by structural analyses, fluorescence microscopy and cryo-electron tomography, we show that MamB is an active magnetosome-directed transporter serving two distinct, yet essential functions. First, MamB initiates magnetosome vesicle formation in a transport-independent process, probably by serving as a landmark protein. Second, MamB transport activity is required for magnetite nucleation. Furthermore, by determining the crystal structure of the MamB cytosolic C-terminal domain, we also provide mechanistic insight into transport regulation. Additionally, we present evidence that magnetosome vesicle growth and chain formation are independent of magnetite nucleation and magnetic interactions, respectively. Together, our data provide novel insight into the role of the key bifunctional magnetosome protein MamB, and the early steps of magnetosome formation.