Formation of Magnetosome

The formation of magnetosomes in magnetotactic bacteria is called biomineralization, of which the specific mechanisms are still unclear at present, however. By genetic methods such as T-DNA insertion, researchers construct different loss-of-function mutants of several species of magnetotactic bacteria to screen out the genes that control biomineralizations. As yet, more than 40 different genes have been identified that encode magnetosome-associated proteins and/or that yield magnetosomes-related phenotypes in AMB-1 and MSR-1 mutants. Although several auxiliary, non-essential functions are contributed by general metabolic and regulatory cellular pathways that are not specific to magnetosome biogenesis, all specific functions in magnetosomes biogenesis are associated with a set of about 30 mam (magnetosome membrane) and mms (magnetic particle-membrane specific) genes that are clustered in a single chromosomal region, the genomic magnetosome island. The magnetosome island extends across about 100kb, which is equivalent to approximately 2% of a typical MSR-1 genome and shares characteristics with other bacterial genomic island (such as putative insertion sites near tRNA genes, distinct levels of GC enrichment, and a large number of mobile genetic elements)^[3][4][5].

The formation of the magnetosomes can be divided into 4 main steps^[6].

1.Formation of magnetosome membrane. Compartmentalization of magnetosome biogenesis by the magnetosome membrane provides the growing magnetite crystal with a specialized ‘nano-reactor’ in which the iron, redox and pH environments of bio‑mineralization can be strictly regulated. The magnetosome membrane was shown to comprise a nm proteinaceous phospholipid bilayer that is invaginated from the cytoplasmic membrane. The presence and interaction of several magnetosome proteins are required to induce membrane curvature and vesicle formation. It has been suggested that MamB might be a landmark protein for the formation of protein interaction networks that induce membrane curvature.

2.Targeting proteins to the magnetosome membrane. This step is tither before, coincidently or following the invagination of magnetosome membrane. All Mam and Mms proteins that have been studied in detail have been found to associate with purified magnetosomes, though the quantity that is present in purified magnetosome preparations varies for each other. Furthermore, localization to the magnetosome membrane has been confirmed for many of these proteins by immunolabelling or fluorescence tagging in intact cells, however, how proteins are specifically targeted to the magnetosome membrane is not currently known, and no motifs that encode sorting signals to the magnetosome membrane have been identified.

3.Biomineralization of magnetite crystals. Following the formation of magnetosome membrane, a process of biomineralization occurs in which large amounts of iron are accumulated in magnetosome membrane vesicles. Currently, most evidence supports the notion that iron accumulation in magnetosome membrane vesicles occurs subsequently to iron uptake by the cell. For example, in MSR, loos of the GTP-dependent Fe²⁺ transporters FeoB1 and FoeB2, which are probably distributed over the entire cytoplasmic membrane and not specific to the entire magnetosome membrane, caused a strong impairment of biomineralization. Then, the absorbed iron ions are utilized to grow magnetite within a narrow redox range as it is mixed-valence iron oxide. Nucleation of magnetite crystals occurs when, under optimal conditions (>pH 7 and low redox potential), soluble Fe²⁺ and Fe³⁺ start to form crystalline iron phases.

4.Assembly of magnetosome chains. Magnetosomes are aligned into linear chains in which magnetic moments of individual particles sum up to maximize the total magnetic responses of the cell. Depending on the growth conditions, magnetosome chains in MSR-1 can contain more than 100 magnetosomes. Magnetosome chain assembly results from the coordinated interplay of magnetic interactions, which occur naturally between magnetite crystals, and active assembly mechanisms, which are mediated by proteins. Cryoelectron tomograms of MSR and AMB revealed the existence of bundles of cytoskeletal filaments that traverse the cell in close proximity to magnetosome chains. These filaments are formed by MamK, which is an actin-like protein that is mostly restricted to magnetotactic bacteria, in which it is highly conserved, and encoded by a gene in the mamAB operon. Rather than simply providing a rigid scaffold for chain assembly, MamK has been suggested to have an active role in the process that may involve the generation of mechanical forces through polymerization and depolymerization, similarly to other cytomotive cytoskeletal filaments.

Each step is regulated by magnetosome island precisely, and the difference between different species of magnetotactic bacteria are the shape of crystals they produce^{[8][9][10][11]}.Among the proteins that encoded by magnetosome island, MamC, which is also named Mms13, though is not a key protein that regulates the 4 steps above, yet covers most area on the surface of the magnetosome membrane. Details of the loading process of MamC is still unclear. Structure-function studies showed that the MamC has 2 transmembrane domains, leading its N- and C-terminal are both localized to the cytoplasmic side. On the inner side of the magnetosomes, MamC has a magnetite-interaction loop structure that connect the Fe₃O₄ directly to regulate the shape of the magnetite crystals ^[13][14]. By taking advantages of these features, MamC has the potential to be an anchor molecule to link nucleic acid and proteins by surface display technology^[15].

References

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