The HtrA/DegP family protease MamE is a bifunctional protein with roles in magnetosome protein localization and magnetite biomineralization

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Abstract
Summary: Magnetotactic bacteria contain nanometre‐sized, membrane‐bound organelles, called magnetosomes, which are tasked with the biomineralization of small crystals of the iron oxide magnetite allowing the organism to use geomagnetic field lines for navigation. A key player in this process is the HtrA/DegP family protease MamE. In its absence, Magnetospirillum magneticum str AMB‐1 is able to form magnetosome membranes but not magnetite crystals, a defect previously linked to the mislocalization of magnetosome proteins. In this work we use a directed genetic approach to find that MamE, and another predicted magnetosome‐associated protease, MamO, likely function as proteases in vivo. However, as opposed to the complete loss of mamE where no biomineralization is observed, the protease‐deficient variant of this protein still supports the initiation and formation of small, 20 nm‐sized crystals of magnetite, too small to hold a permanent magnetic dipole moment. This analysis also reveals that MamE is a bifunctional protein with a protease‐independent role in magnetosome protein localization and a protease‐dependent role in maturation of small magnetite crystals. Together, these results imply the existence of a previously unrecognized ‘checkpoint’ in biomineralization where MamE moderates the completion of magnetite formation and thus committal to magneto‐aerotaxis as the organism's dominant mode of navigating the environment.

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