Screening for the interacting partners of the proteins MamK & MamJ by two-hybrid genomic DNA library of Magnetospirillum magneticum AMB-1

Biomedical applications of magnetosomes: State of the art and perspectives

Magnetosomes, synthesized by magnetotactic bacteria (MTB), have been used in nano- and biotechnological applications, owing to their unique properties such as superparamagnetism, uniform size distribution, excellent bioavailability, and easily modifiable functional groups. In this review, we first discuss the mechanisms of magnetosome formation and describe various modification methods. Subsequently, we focus on presenting the biomedical advancements of bacterial magnetosomes in biomedical imaging, drug delivery, anticancer therapy, biosensor. Finally, we discuss future applications and challenges. This review summarizes the application of magnetosomes in the biomedical field, highlighting the latest advancements and exploring the future development of magnetosomes.

Magnetosomes: biogenic iron nanoparticles produced by environmental bacteria

The scientific community's interest in magnetotactic bacteria has increased substantially in recent decades. These prokaryotes have the particularity of synthesizing nanomagnets, called magnetosomes. The majority of research is based on several scientific questions. Where do magnetotactic bacteria live, what are their characteristics, and why are they magnetic? What are the molecular phenomena of magnetosome biomineralization and what are the physical characteristics of magnetosomes? In addition to scientific curiosity to better understand these stunning organisms, there are biotechnological opportunities to consider. Magnetotactic bacteria, as well as magnetosomes, are used in medical applications, for example cancer treatment, or in environmental ones, for example bioremediation. In this mini-review, we investigated all the aspects mentioned above and summarized the currently available knowledge.

Genomic study of a novel magnetotactic Alphaproteobacteria uncovers the multiple ancestry of magnetotaxis

Ecological and evolutionary processes involved in magnetotactic bacteria (MTB) adaptation to their environment have been a matter of debate for many years. Ongoing efforts for their characterization are progressively contributing to understand these processes, including the genetic and molecular mechanisms responsible for biomineralization. Despite numerous culture-independent MTB characterizations, essentially within the Proteobacteria phylum, only few species have been isolated in culture because of their complex growth conditions. Here, we report a newly cultivated magnetotactic, microaerophilic and chemoorganoheterotrophic bacterium isolated from the Mediterranean Sea in Marseille, France: Candidatus Terasakiella magnetica strain PR-1 that belongs to an Alphaproteobacteria genus with no magnetotactic relative. By comparing the morphology and the whole genome shotgun sequence of this MTB with those of closer relatives, we brought further evidence that the apparent vertical ancestry of magnetosome genes suggested by previous studies within Alphaproteobacteria hides a more complex evolutionary history involving horizontal gene transfers and/or duplication events before and after the emergence of Magnetospirillum, Magnetovibrio and Magnetospira genera. A genome-scale comparative genomics analysis identified several additional candidate functions and genes that could be specifically associated to MTB lifestyle in this class of bacteria.

Cryo-EM structure of the bacterial actin AlfA reveals unique assembly and ATP-binding interactions and the absence of a conserved subdomain

Bacterial actins are an evolutionarily diverse family of ATP-dependent filaments built from protomers with a conserved structural fold. Actin-based segregation systems are encoded on many bacterial plasmids and function to partition plasmids into daughter cells. The bacterial actin AlfA segregates plasmids by a mechanism distinct from other partition systems, dependent on its unique dynamic properties. Here, we report the near-atomic resolution electron cryo-microscopy structure of the AlfA filament, which reveals a strikingly divergent filament architecture resulting from the loss of a subdomain conserved in all other actins and a mode of ATP binding. Its unusual assembly interfaces and nucleotide interactions provide insight into AlfA dynamics, and expand the range of evolutionary variation accessible to actin quaternary structure.

Magnetosome biogenesis in magnetotactic bacteria

Magnetotactic bacteria derive their magnetic orientation from magnetosomes, which are unique organelles that contain nanometre-sized crystals of magnetic iron minerals. Although these organelles have evident potential for exciting biotechnological applications, a lack of genetically tractable magnetotactic bacteria had hampered the development of such tools; however, in the past decade, genetic studies using two model Magnetospirillum species have revealed much about the mechanisms of magnetosome biogenesis. In this Review, we highlight these new insights and place the molecular mechanisms of magnetosome biogenesis in the context of the complex cell biology of Magnetospirillum spp. Furthermore, we discuss the diverse properties of magnetosome biogenesis in other species of magnetotactic bacteria and consider the value of genetically 'magnetizing' non-magnetotactic bacteria. Finally, we discuss future prospects for this highly interdisciplinary and rapidly advancing field.

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

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.

IMPORTANCE

Magnetotactic bacteria synthesize magnetite (Fe3O4) 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.

Structure of the magnetosome-associated actin-like MamK filament at subnanometer resolution

Magnetotactic bacteria possess cellular compartments called magnetosomes that sense magnetic fields. Alignment of magnetosomes in the bacterial cell is necessary for their function, and this is achieved through anchoring of magnetosomes to filaments composed of the protein MamK. MamK is an actin homolog that polymerizes upon ATP binding. Here, we report the structure of the MamK filament at ∼6.5 Å, obtained by cryo-Electron Microscopy. This structure confirms our previously reported double-stranded, nonstaggered architecture, and reveals the molecular basis for filament formation. While MamK is closest in sequence to the bacterial actin MreB, the longitudinal contacts along each MamK strand most closely resemble those of eukaryotic actin. In contrast, the cross-strand interface, with a surprisingly limited set of contacts, is novel among actin homologs and gives rise to the nonstaggered architecture.

From invagination to navigation: The story of magnetosome-associated proteins in magnetotactic bacteria

Magnetotactic bacteria (MTB) are a group of Gram-negative microorganisms that are able to sense and change their orientation in accordance with the geomagnetic field. This unique capability is due to the presence of a special suborganelle called the magnetosome, composed of either a magnetite or gregite crystal surrounded by a lipid membrane. MTB were first detected in 1975 and since then numerous efforts have been made to clarify the special mechanism of magnetosome formation at the molecular level. Magnetosome formation can be divided into several steps, beginning with vesicle invagination from the cell membrane, through protein sorting, followed by the combined steps of iron transportation, biomineralization, and the alignment of magnetosomes into a chain. The magnetosome-chain enables the sensing of the magnetic field, and thus, allows the MTB to navigate. It is known that magnetosome formation is tightly controlled by a distinctive set of magnetosome-associated proteins that are encoded mainly in a genomically conserved region within MTB called the magnetosome island (MAI). Most of these proteins were shown to have an impact on the magnetism of MTB. Here, we describe the process in which the magnetosome is formed with an emphasis on the different proteins that participate in each stage of the magnetosome formation scheme.

Bacterial actins and their diversity

For many years, bacteria were considered rather simple organisms, but the dogmatic notion that subcellular organization is a eukaryotic trait has been overthrown for more than a decade. The discovery of homologues of the eukaryotic cytoskeletal proteins actin, tubulin, and intermediate filaments in bacteria has been instrumental in changing this view. Over the past few years, we have gained an incredible level of insight into the diverse family of bacterial actins and their molecular workings. Here we review the functional, biochemical, and structural features of the most well-studied bacterial actins.