The biogenesis of membranous cell organelles relies on extensive membrane remodeling and precise spatio-temporal targeting of proteins. While these processes are well characterized in eukaryotes, their molecular-level understanding in prokaryotes, which typically form organelles of lower complexity, still remains limited.
Here we analyzed the biogenesis of magnetosomes, membrane-enclosed magnetite nanoparticles used for magnetic navigation, as a model system for prokaryotic organelle formation. Using phenotypic assessments, proteome profiling, protein interaction studies, and fluorescence microscopy in Magnetospirillum gryphiswaldense, we identified three so-far uncharacterized proteins of the HOTT protein family that are crucial for the late stages of magnetosome membrane biogenesis. Since we also identified a remote homology between the HOTT protein family and chloroplast protein translocation systems, our findings suggest a novel mechanism of organelle biogenesis in prokaryotes. Moreover, analyses of subcellular localization patterns of GFP-labelled HOTT proteins in different mutant backgrounds are not consistent with the currently accepted model of magnetosome biogenesis. Thus, we propose a new spatiotemporal model for magnetosome biogenesis.