Comparative Subcellular Localization Analysis of Magnetosome Proteins Reveals a Unique Localization Behavior of Mms6 Protein onto Magnetite Crystals

Abstract

The magnetosome is an organelle specialized for inorganic magnetite crystal synthesis in magnetotactic bacteria. The complex mechanism of magnetosome formation is regulated by magnetosome proteins in a stepwise manner. Protein localization is a key step for magnetosome development; however, a global study of magnetosome protein localization remains to be conducted. Here, we comparatively analyzed the subcellular localization of a series of green fluorescent protein (GFP)-tagged magnetosome proteins. The protein localizations were categorized into 5 groups (short-length linear, middle-length linear, long-length linear, cell membrane, and intracellular dispersing), which were related to the protein functions. Mms6, which regulates magnetite crystal growth, localized along magnetosome chain structures under magnetite-forming (microaerobic) conditions but was dispersed in the cell under nonforming (aerobic) conditions. Correlative fluorescence and electron microscopy analyses revealed that Mms6 preferentially localized to magnetosomes enclosing magnetite crystals. We suggest that a highly organized spatial regulation mechanism controls magnetosome protein localization during magnetosome formation in magnetotactic bacteria. IMPORTANCEMagnetotactic bacteria synthesize magnetite (Fe 3 O 4 ) nanocrystals in a prokaryotic organelle called the magnetosome. This organelle is formed using various magnetosome proteins in multiple steps, including vesicle formation, magnetosome alignment, and magnetite crystal formation, to provide compartmentalized nanospaces for the regulation of iron concentrations and redox conditions, enabling the synthesis of a morphologically controlled magnetite crystal. Thus, to rationalize the complex organelle development, the localization of magnetosome proteins is considered to be highly regulated; however, the mechanisms remain largely unknown. Here, we performed comparative localization analysis of magnetosome proteins that revealed the presence of a spatial regulation mechanism within the linear structure of magnetosomes. This discovery provides evidence of a highly regulated protein localization mechanism for this bacterial organelle development. P rotein localization at appropriate positions within a cell is an essential mechanism for the effective performance of the diverse biological reactions that occur within the restricted intracellular area in both eukaryotes and prokaryotes. In various prokaryotes, intracellular compartments can be created to provide the domains required for highly specialized reactions. Whereas some of these compartments are completely proteinaceous (e.g., carboxysomes, metabolosomes, and ferritin) (1-3), others contain molecular components similar to those in cell membranes, including lipids and proteins (e.g., nucleoids, polyhydroxybutyrate, and spores) (4, 5). Such compartmentalized organelles are recognized to be formed within bacteria through multiple processes involving the spatial regulation of protein localization, but the details of this regulatory m...

Magnetotactic Bacteria Accumulate a Large Pool of Iron Distinct from Their Magnetite Crystals

“…These results are in good agreement with the work of Aron and coworkers ( 25 ). AMB-1 produces fragmented chains of magnetite, with magnetosome vesicles spreading along the cell’s long axis from pole to pole ( 26 , 27 ). Unlike other Magnetospirillum strains such as MSR-1, apparent gaps between magnetite crystals can be observed from electron microscopy in AMB-1 ( Fig.…”

Novel Protein Mg2046 Regulates Magnetosome Synthesis in Magnetospirillum gryphiswaldense MSR-1 by Modulating a Proper Redox Status

“…Mms6 was proposed to bind iron ions to control magnetic particle size (Arakaki et al, 2003; Tanaka et al, 2011; Wang et al, 2012). Recent Mms6 structure prediction suggests that its C-terminus is hydrophilic and enriched in acidic amino acid residues, such that Mms6 can aggregate spontaneously on the surface of solution or solid phase to form polymers of varying sizes, which determine magnetic particle size (Arakaki et al, 2016; Staniland and Rawlings, 2016). Other studies suggest that self-assembled MmsF controls magnetite nanoparticle formation in vitro (Murat et al, 2011), and is a key regulator of magnetite biomineralization (Rawlings et al, 2014).…”

Enhanced Tubulation of Liposome Containing Cardiolipin by MamY Protein from Magnetotactic Bacteria

“…Based on the protein domain analysis of MamY by Pfam, p4, and p7 are expected to have a similarity with caspase recruitment domain, involving in apoptotic signaling and requiring CL as a binding platform to recruit apoptotic factors . In addition, based on the proteomic and genomic studies in magnetotactic bacteria, dozens of proteins including filamentous MamK protein and functionally unclear proteins have been suggested to play important roles for magnetosome formation . In eukaryotic cells, as BAR protein regulates the intracellular vesicle formation cooperated with filamentous actin protein and with various other proteins, MamY protein may also function to form the magnetosome vesicle cooperatively with other magnetosome proteins.…”

“…[67] In addition, based on the proteomic and genomic studies in magnetotactic bacteria, dozens of proteins including filamentous MamK protein and functionally unclear proteins have been suggested to play important roles for magnetosome formation. [36,39,40,68,69] In eukaryotic cells, as BAR protein regulates the intracellular vesicle formation cooperated with filamentous actin protein and with various other proteins, [70] MamY protein may also function to form the magnetosome vesicle cooperatively with other magnetosome proteins. By the utilization of these proteins with MamY, liposome tubulation efficiency may be further improve.…”