Intrinsic and extrinsic determinants of conditional localization of Mms6 to magnetosome organelles inMagnetospirillum magneticumAMB-1

Abstract

Magnetotactic bacteria are a diverse group of microbes that use magnetic particles housed within intracellular lipid-bounded magnetosome organelles to guide navigation along geomagnetic fields. Development of magnetosomes and their magnetic crystals inMagnetospirillum magneticumAMB-1 requires the coordinated action of numerous proteins. Most proteins are thought to localize to magnetosomes during the initial stages of organelle biogenesis regardless of environmental conditions. However, magnetite-shaping protein Mms6 is only found in magnetosomes that contain magnetic particles, suggesting that it might conditionally localize after the formation of magnetosome membranes. The mechanisms for this unusual mode of localization to magnetosomes are unclear. Here, using pulse-chase labeling, we show that Mms6 translated under non-biomineralization conditions translocates to pre-formed magnetosomes when cells are shifted to biomineralizing conditions. Genes essential for magnetite production, namelymamE, mamM,andmamOare necessary for Mms6 localization, whereasmamNinhibits Mms6 localization. MamD localization is also investigated and found to be controlled by similar cellular factors. The membrane localization of Mms6 is dependent on a glycine-leucine repeat region, while the N-terminal domain of Mms6 is necessary for retention in the cytosol and impacts conditional localization to magnetosomes. The N-terminal domain is also sufficient to impart conditional magnetosome localization to MmsF, altering its native constitutive magnetosome localization. Our work illuminates an alternative mode of protein localization to magnetosomes in which Mms6 and MamD are kept in the cytosol by MamN until biomineralization initiates, whereupon they translocate into magnetosome membranes to control the development of growing magnetite crystals.ImportanceMagnetotactic bacteria (MTB) are a diverse group of bacteria that form magnetic nanoparticles surrounded by membranous organelles. MTB are widespread and serve as a model for bacterial organelle formation and biomineralization. Magnetosomes require a specific cohort of proteins to enable magnetite formation, but how those proteins are localized to magnetosome membranes is unclear. Here, we investigate protein localization using pulse-chase microscopy and find a system of protein coordination dependent on biomineralization permissible conditions. In addition, our findings highlight a protein domain that alters the localization behavior of magnetosome proteins. Utilization of this protein domain may provide a synthetic route for conditional functionalization of magnetosomes for biotechnological applications.