Generation of Multifunctional Magnetic Nanoparticles with Amplified Catalytic Activities by Genetic Expression of Enzyme Arrays on Bacterial Magnetosomes

A Versatile Magnetic Nanoplatform for Plug-and-Play Functionalization: Genetically Programmable Cargo Loading to Bacterial Magnetosomes by SpyCatcher "Click Biology"

Bacterial magnetosomes ("MAGs") represent a promising class of magnetic iron oxide nanoparticles with exceptional material characteristics and high application potential in the biomedical and biotechnological field. For the surface functionalization of MAGs with different protein cargos, their enveloping membrane can be addressed by genetic means. However, the expression of foreign polypeptides as translational fusion to magnetosome membrane proteins is still laborious and lacks versatility as the generated particles are monospecific and thus restricted to predetermined functions. Utilizing the SpyTag-SpyCatcher (ST-SC) bioconjugate system, we here establish a flexible platform for the targeted nanoassembly of multifunctional MAGs that combines the rapidity of chemical coupling (e.g., by cross-linking reactions) and the unmatched selectivity and controllability of in vivo functionalization. MAGs genetically engineered to display either SC- or ST-connectors are shown to efficiently bind a variety of complementary tagged (protein) cargo. Specifically, we cover a broad spectrum of representative functional moieties and foreign cargo (such as enzymes, antibodies, fluorophores, and silica beads) with relevance in biotechnology and biomedicine and demonstrate the interchangeability of the MAGs-adapted ST-SC system. For the controlled generation of artificial shells surrounding the particles, SC-MAGs are effectively coated by protein-corona proteins. The potential of the here-provided toolkit is even more enhanced by using SC-MAGs as an affinity tool for selective protein pulldown in vitro and in vivo. Overall, this innovative technology turns bacterial MAGs into a flexible magnetic nanoscaffold for the targeted plug-and-play display of virtually unlimited additional functionalities, thereby generating a multitude of magnetic hybrid materials that can be used in many applications.

Exploring the host range for genetic transfer of magnetic organelle biosynthesis

Magnetosomes produced by magnetotactic bacteria have great potential for application in biotechnology and medicine due to their unique physicochemical properties and high biocompatibility. Attempts to transfer the genes for magnetosome biosynthesis into non-magnetic organisms have had mixed results. Here we report on a systematic study to identify key components needed for magnetosome biosynthesis after gene transfer. We transfer magnetosome genes to 25 proteobacterial hosts, generating seven new magnetosome-producing strains. We characterize the recombinant magnetosomes produced by these strains and demonstrate that denitrification and anaerobic photosynthesis are linked to the ability to synthesize magnetosomes upon the gene transfer. In addition, we show that the number of magnetosomes synthesized by a foreign host negatively correlates with the guanine-cytosine content difference between the host and the gene donor. Our findings have profound implications for the generation of magnetized living cells and the potential for transgenic biogenic magnetic nanoparticle production.

Comparing the Colloidal Stabilities of Commercial and Biogenic Iron Oxide Nanoparticles That Have Potential In Vitro/In Vivo Applications

For the potential in vitro/in vivo applications of magnetic iron oxide nanoparticles, their stability in different physiological fluids has to be ensured. This important prerequisite includes the preservation of the particles' stability during the envisaged application and, consequently, their invariance with respect to the transfer from storage conditions to cell culture media or even bodily fluids. Here, we investigate the colloidal stabilities of commercial nanoparticles with different coatings as a model system for biogenic iron oxide nanoparticles (magnetosomes) isolated from magnetotactic bacteria. We demonstrate that the stability can be evaluated and quantified by determining the intensity-weighted average of the particle sizes (Z-value) obtained from dynamic light scattering experiments as a simple quality criterion, which can also be used as an indicator for protein corona formation.

Evaluation of cell disruption technologies on magnetosome chain length and aggregation behaviour from Magnetospirillum gryphiswaldense MSR-1

Magnetosomes are biologically-derived magnetic nanoparticles (MNPs) naturally produced by magnetotactic bacteria (MTB). Due to their distinctive characteristics, such as narrow size distribution and high biocompatibility, magnetosomes represent an attractive alternative to existing commercially-available chemically-synthesized MNPs. However, to extract magnetosomes from the bacteria, a cell disruption step is required. In this study, a systematic comparison between three disruption techniques (enzymatic treatment, probe sonication and high-pressure homogenization) was carried out to study their effect on the chain length, integrity and aggregation state of magnetosomes isolated from Magnetospirillum gryphiswaldense MSR-1 cells. Experimental results revealed that all three methodologies show high cell disruption yields (>89%). Transmission electron microscopy (TEM), dynamic light scattering (DLS) and, for the first time, nano-flow cytometry (nFCM) were employed to characterize magnetosome preparations after purification. TEM and DLS showed that high-pressure homogenization resulted in optimal conservation of chain integrity, whereas enzymatic treatment caused higher chain cleavage. The data obtained suggest that nFCM is best suited to characterize single membrane-wrapped magnetosomes, which can be particularly useful for applications that require the use of individual magnetosomes. Magnetosomes were also successfully labelled (>90%) with the fluorescent CellMask™ Deep Red membrane stain and analysed by nFCM, demonstrating the promising capacity of this technique as a rapid analytical tool for magnetosome quality assurance. The results of this work contribute to the future development of a robust magnetosome production platform.

Long-Term Stability, Biocompatibility, and Magnetization of Suspensions of Isolated Bacterial Magnetosomes

Magnetosomes are magnetic nanoparticles biosynthesized by magnetotactic bacteria. Due to a genetically strictly controlled biomineralization process, the ensuing magnetosomes have been envisioned as agents for biomedical and clinical applications. In the present work, different stability parameters of magnetosomes isolated from Magnetospirillum gryphiswaldense upon storage in suspension (HEPES buffer, 4 °C, nitrogen atmosphere) for one year in the absence of antibiotics are examined. The magnetic potency, measured by the saturation magnetization of the particle suspension, drops to one-third of its starting value within this year-about ten times slower than at ambient air and room temperature. The particle size distribution, the integrity of the surrounding magnetosome membrane, the colloidal stability, and the biocompatibility turn out to be not severely affected by long-term storage.

Silent gene clusters encode magnetic organelle biosynthesis in a non-magnetotactic phototrophic bacterium

Horizontal gene transfer is a powerful source of innovations in prokaryotes that can affect almost any cellular system, including microbial organelles. The formation of magnetosomes, one of the most sophisticated microbial mineral-containing organelles synthesized by magnetotactic bacteria for magnetic navigation in the environment, was also shown to be a horizontally transferrable trait. However, the mechanisms determining the fate of such genes in new hosts are not well understood, since non-adaptive gene acquisitions are typically rapidly lost and become unavailable for observation. This likely explains why gene clusters encoding magnetosome biosynthesis have never been observed in non-magnetotactic bacteria. Here, we report the first discovery of a horizontally inherited dormant gene clusters encoding biosynthesis of magnetosomes in a non-magnetotactic phototrophic bacterium Rhodovastum atsumiense. We show that these clusters were inactivated through transcriptional silencing and antisense RNA regulation, but retain functionality, as several genes were able to complement the orthologous deletions in a remotely related magnetotactic bacterium. The laboratory transfer of foreign magnetosome genes to R. atsumiense was found to endow the strain with magnetosome biosynthesis, but strong negative selection led to rapid loss of this trait upon subcultivation, highlighting the trait instability in this organism. Our results provide insight into the horizontal dissemination of gene clusters encoding complex prokaryotic organelles and illuminate the potential mechanisms of their genomic preservation in a dormant state.

Emerging enzyme surface display systems for waste resource recovery

The current century marks an inflection point for human progress, as the developed world increasingly comes to recognize that the ecological and socioeconomic impacts of resource extraction must be balanced with more sustainable modes of growth that are less reliant on non-renewable sources of energy and materials. This has opened a window of opportunity for cross-sector development of biotechnologies that harness the metabolic problem-solving power of microbial communities. In this context, recovery has emerged as an organizing principal to create value from industrial and municipal waste streams, and the search is on for new enzymes and platforms that can be used for waste resource recovery at scale. Enzyme surface display on cells or functionalized materials has emerged as a promising platform for waste valorization. Typically, surface display involves the use of substrate binding or catalytic domains of interest translationally fused with extracellular membrane proteins in a microbial chassis. Novel display systems with improved performance features include S-layer display with increased protein density, spore display with increased resistance to harsh conditions, and intracellular inclusions including DNA-free cells or nanoparticles with improved social licence for in situ applications. Combining these display systems with advances in bioprinting, electrospinning and high-throughput functional screening have potential to transform outmoded extractive paradigms into 'trans-metabolic" processes for remediation and waste resource recovery within an emerging circular bioeconomy.

Large-Scale Cultivation of Magnetotactic Bacteria and the Optimism for Sustainable and Cheap Approaches in Nanotechnology

Magnetotactic bacteria (MTB), a diverse group of marine and freshwater microorganisms, have attracted the scientific community's attention since their discovery. These bacteria biomineralize ferrimagnetic nanocrystals, the magnetosomes, or biological magnetic nanoparticles (BMNs), in a single or multiple chain(s) within the cell. As a result, cells experience an optimized magnetic dipolar moment responsible for a passive alignment along the lines of the geomagnetic field. Advances in MTB cultivation and BMN isolation have contributed to the expansion of the biotechnological potential of MTB in recent decades. Several studies with mass-cultured MTB expanded the possibilities of using purified nanocrystals and whole cells in nano- and biotechnology. Freshwater MTB were primarily investigated in scaling up processes for the production of BMNs. However, marine MTB have the potential to overcome freshwater species applications due to the putative high efficiency of their BMNs in capturing molecules. Regarding the use of MTB or BMNs in different approaches, the application of BMNs in biomedicine remains the focus of most studies, but their application is not restricted to this field. In recent years, environment monitoring and recovery, engineering applications, wastewater treatment, and industrial processes have benefited from MTB-based biotechnologies. This review explores the advances in MTB large-scale cultivation and the consequent development of innovative tools or processes.

A Magnetosome-Based Platform for Flow Biocatalysis

Biocatalysis in flow reactor systems is of increasing importance for the transformation of the chemical industry. However, the necessary immobilization of biocatalysts remains a challenge. We here demonstrate that biogenic magnetic nanoparticles, so-called magnetosomes, represent an attractive alternative for the development of nanoscale particle formulations to enable high and stable conversion rates in biocatalytic flow processes. In addition to their intriguing material characteristics, such as high crystallinity, stable magnetic moments, and narrow particle size distribution, magnetosomes offer the unbeatable advantage over chemically synthesized nanoparticles that foreign protein "cargo" can be immobilized on the enveloping membrane via genetic engineering and thus, stably presented on the particle surface. To exploit these advantages, we develop a modular connector system in which abundant magnetosome membrane anchors are genetically fused with SpyCatcher coupling groups, allowing efficient covalent coupling with complementary SpyTag-functionalized proteins. The versatility of this approach is demonstrated by immobilizing a dimeric phenolic acid decarboxylase to SpyCatcher magnetosomes. The functionalized magnetosomes outperform similarly functionalized commercial particles by exhibiting stable substrate conversion during a 60 h period, with an average space-time yield of 49.2 mmol L^-1 h^-1. Overall, our results demonstrate that SpyCatcher magnetosomes significantly expand the genetic toolbox for particle surface functionalization and increase their application potential as nano-biocatalysts.

Protein scaffolds: A tool for multi-enzyme assembly

The synthesis of complex molecules using multiple enzymes simultaneously in one reaction vessel has rapidly emerged as a new frontier in the field of bioprocess technology. However, operating different enzymes together in a single vessel limits their operational performance which needs to be addressed. With this respect, scaffolding proteins play an immense role in bringing different enzymes together in a specific manner. The scaffolding improves the catalytic performance, enzyme stability and provides an optimal micro-environment for biochemical reactions. This review describes the components of protein scaffolds, different ways of constructing a protein scaffold-based multi-enzyme complex, and their effects on enzyme kinetics. Moreover, different conjugation strategies viz; dockerin-cohesin interaction, SpyTag-SpyCatcher system, peptide linker-based ligation, affibody, and sortase-mediated ligation are discussed in detail. Various analytical and characterization tools that have enabled the development of these scaffolding strategies are also reviewed. Such mega-enzyme complexes promise wider applications in the field of biotechnology and bioengineering.

The Complex Transcriptional Landscape of Magnetosome Gene Clusters in Magnetospirillum gryphiswaldense

Magnetosomes are complex membrane organelles synthesized by magnetotactic bacteria (MTB) for navigation in the Earth’s magnetic field. In the alphaproteobacterium Magnetospirillum gryphiswaldense, all steps of magnetosome formation are tightly controlled by >30 specific genes arranged in several gene clusters. However, the transcriptional organization of the magnetosome gene clusters has remained poorly understood. Here, by applying Cappable-seq and whole-transcriptome shotgun RNA sequencing, we show that mamGFDCop and feoAB1op are transcribed as single transcriptional units, whereas multiple transcription start sites (TSS) are present in mms6op, mamXYop, and the long (>16 kb) mamABop. Using a bioluminescence reporter assay and promoter knockouts, we demonstrate that most of the identified TSS originate from biologically meaningful promoters which mediate production of multiple transcripts and are functionally relevant for proper magnetosome biosynthesis. In addition, we identified a strong promoter in a large intergenic region within mamXYop, which likely drives transcription of a noncoding RNA important for gene expression in this operon. In summary, our data suggest a more complex transcriptional architecture of the magnetosome operons than previously recognized, which is largely conserved in other magnetotactic Magnetospirillum species and, thus, is likely fundamental for magnetosome biosynthesis in these organisms.

IMPORTANCE Magnetosomes have emerged as a model system to study prokaryotic organelles and a source of biocompatible magnetic nanoparticles for various biomedical applications. However, the lack of knowledge about the transcriptional organization of magnetosome gene clusters has severely impeded the engineering, manipulation, and transfer of this highly complex biosynthetic pathway into other organisms. Here, we provide a high-resolution image of the previously unappreciated transcriptional landscape of the magnetosome operons. Our findings are important for further unraveling the complex genetic framework of magnetosome biosynthesis. In addition, they will facilitate the rational reengineering of magnetic bacteria for improved bioproduction of tunable magnetic nanoparticles, as well as transplantation of magnetosome biosynthesis into foreign hosts by synthetic biology approaches. Overall, our study exemplifies how a genetically complex pathway is orchestrated at the transcriptional level to ensure the balanced expression of the numerous constituents required for the proper assembly of one of the most intricate prokaryotic organelles.

High-Yield Production, Characterization, and Functionalization of Recombinant Magnetosomes in the Synthetic Bacterium Rhodospirillum rubrum "magneticum"

Recently, the photosynthetic Rhodospirillum rubrum has been endowed with the ability of magnetosome biosynthesis by transfer and expression of biosynthetic gene clusters from the magnetotactic bacterium Magnetospirillum gryphiswaldense. However, the growth conditions for efficient magnetite biomineralization in the synthetic R. rubrum "magneticum", as well as the particles themselves (i.e., structure and composition), have so far not been fully characterized. In this study, different cultivation strategies, particularly the influence of temperature and light intensity, are systematically investigated to achieve optimal magnetosome biosynthesis. Reduced temperatures ≤16 °C and gradual increase in light intensities favor magnetite biomineralization at high rates, suggesting that magnetosome formation might utilize cellular processes, cofactors, and/or pathways that are linked to photosynthetic growth. Magnetosome yields of up to 13.6 mg magnetite per liter cell culture are obtained upon photoheterotrophic large-scale cultivation. Furthermore, it is shown that even more complex, i.e., oligomeric, catalytically active functional moieties like enzyme proteins can be efficiently expressed on the magnetosome surface, thereby enabling the in vivo functionalization by genetic engineering. In summary, it is demonstrated that the synthetic R. rubrum "magneticum" is a suitable host for high-yield magnetosome biosynthesis and the sustainable production of genetically engineered, bioconjugated magnetosomes.

Magnetotactic Bacteria and Magnetosomes: Basic Properties and Applications

Magnetotactic bacteria (MTB) belong to several phyla. This class of microorganisms exhibits the ability of magneto-aerotaxis. MTB synthesize biominerals in organelle-like structures called magnetosomes, which contain single-domain crystals of magnetite (Fe3O4) or greigite (Fe3S4) characterized by a high degree of structural and compositional perfection. Magnetosomes from dead MTB could be preserved in sediments (called fossil magnetosomes or magnetofossils). Under certain conditions, magnetofossils are capable of retaining their remanence for millions of years. This accounts for the growing interest in MTB and magnetofossils in paleo- and rock magnetism and in a wider field of biogeoscience. At the same time, high biocompatibility of magnetosomes makes possible their potential use in biomedical applications, including magnetic resonance imaging, hyperthermia, magnetically guided drug delivery, and immunomagnetic analysis. In this review, we attempt to summarize the current state of the art in the field of MTB research and applications.

Induction of Axonal Outgrowth in Mouse Hippocampal Neurons via Bacterial Magnetosomes

Magnetosomes are membrane-enclosed iron oxide crystals biosynthesized by magnetotactic bacteria. As the biomineralization of bacterial magnetosomes can be genetically controlled, they have become promising nanomaterials for bionanotechnological applications. In the present paper, we explore a novel application of magnetosomes as nanotool for manipulating axonal outgrowth via stretch-growth (SG). SG refers to the process of stimulation of axonal outgrowth through the application of mechanical forces. Thanks to their superior magnetic properties, magnetosomes have been used to magnetize mouse hippocampal neurons in order to stretch axons under the application of magnetic fields. We found that magnetosomes are avidly internalized by cells. They adhere to the cell membrane, are quickly internalized, and slowly degrade after a few days from the internalization process. Our data show that bacterial magnetosomes are more efficient than synthetic iron oxide nanoparticles in stimulating axonal outgrowth via SG.

Identification and elimination of genomic regions irrelevant for magnetosome biosynthesis by large-scale deletion in Magnetospirillum gryphiswaldense

BACKGROUND

Magnetosome formation in the alphaproteobacterium Magnetospirillum gryphiswaldense is controlled by more than 30 known mam and mms genes clustered within a large genomic region, the 'magnetosome island' (MAI), which also harbors numerous mobile genetic elements, repeats, and genetic junk. Because of the inherent genetic instability of the MAI caused by neighboring gene content, the elimination of these regions and their substitution by a compact, minimal magnetosome expression cassette would be important for future analysis and engineering. In addition, the role of the MAI boundaries and adjacent regions are still unclear, and recent studies indicated that further auxiliary determinants for magnetosome biosynthesis are encoded outside the MAI. However, techniques for large-scale genome editing of magnetic bacteria are still limited, and the full complement of genes controlling magnetosome formation has remained uncertain.

RESULTS

Here we demonstrate that an allelic replacement method based on homologous recombination can be applied for large-scale genome editing in M. gryphiswaldense. By analysis of 24 deletion mutants covering about 167 kb of non-redundant genome content, we identified genes and regions inside and outside the MAI irrelevant for magnetosome biosynthesis. A contiguous stretch of ~ 100 kb, including the scattered mam and mms6 operons, could be functionally substituted by a compact and contiguous ~ 38 kb cassette comprising all essential biosynthetic gene clusters, but devoid of interspersing irrelevant or problematic gene content.

CONCLUSIONS

Our results further delineate the genetic complement for magnetosome biosynthesis and will be useful for future large-scale genome editing and genetic engineering of magnetosome biosynthesis.

Towards a 'chassis' for bacterial magnetosome biosynthesis: genome streamlining of Magnetospirillum gryphiswaldense by multiple deletions

BACKGROUND

Because of its tractability and straightforward cultivation, the magnetic bacterium Magnetospirillum gryphiswaldense has emerged as a model for the analysis of magnetosome biosynthesis and bioproduction. However, its future use as platform for synthetic biology and biotechnology will require methods for large-scale genome editing and streamlining.

RESULTS

We established an approach for combinatory genome reduction and generated a library of strains in which up to 16 regions including large gene clusters, mobile genetic elements and phage-related genes were sequentially removed, equivalent to ~ 227.6 kb and nearly 5.5% of the genome. Finally, the fragmented genomic magnetosome island was replaced by a compact cassette comprising all key magnetosome biosynthetic gene clusters. The prospective 'chassis' revealed wild type-like cell growth and magnetosome biosynthesis under optimal conditions, as well as slightly improved resilience and increased genetic stability.

CONCLUSION

We provide first proof-of-principle for the feasibility of multiple genome reduction and large-scale engineering of magnetotactic bacteria. The library of deletions will be valuable for turning M. gryphiswaldense into a microbial cell factory for synthetic biology and production of magnetic nanoparticles.

A Compass To Boost Navigation: Cell Biology of Bacterial Magnetotaxis

Magnetotactic bacteria are aquatic or sediment-dwelling microorganisms able to take advantage of the Earth's magnetic field for directed motility. The source of this amazing trait is magnetosomes, unique organelles used to synthesize single nanometer-sized crystals of magnetic iron minerals that are queued up to build an intracellular compass. Most of these microorganisms cannot be cultivated under controlled conditions, much less genetically engineered, with only few exceptions. However, two of the genetically amenable Magnetospirillum species have emerged as tractable model organisms to study magnetosome formation and magnetotaxis. Recently, much has been revealed about the process of magnetosome biogenesis and dedicated structures for magnetosome dynamics and positioning, which suggest an unexpected cellular intricacy of these organisms. In this minireview, we summarize new insights and place the molecular mechanisms of magnetosome formation in the context of the complex cell biology of Magnetospirillum spp. First, we provide an overview on magnetosome vesicle synthesis and magnetite biomineralization, followed by a discussion of the perceptions of dynamic organelle positioning and its biological implications, which highlight that magnetotactic bacteria have evolved sophisticated mechanisms to construct, incorporate, and inherit a unique navigational device. Finally, we discuss the impact of magnetotaxis on motility and its interconnection with chemotaxis, showing that magnetotactic bacteria are outstandingly adapted to lifestyle and habitat.

A Versatile Toolkit for Controllable and Highly Selective Multifunctionalization of Bacterial Magnetic Nanoparticles

Their unique material characteristics, i.e. high crystallinity, strong magnetization, uniform shape and size, and the ability to engineer the enveloping membrane in vivo make bacterial magnetosomes highly interesting for many biomedical and biotechnological applications. In this study, a versatile toolkit is developed for the multifunctionalization of magnetic nanoparticles in the magnetotactic bacterium Magnetospirillum gryphiswaldense, and the use of several abundant magnetosome membrane proteins as anchors for functional moieties is explored. High-level magnetosome display of cargo proteins enables the generation of engineered nanoparticles with several genetically encoded functionalities, including a core-shell structure, magnetization, two different catalytic activities, fluorescence and the presence of a versatile connector that allows the incorporation into a hydrogel-based matrix by specific coupling reactions. The resulting reusable magnetic composite demonstrates the high potential of synthetic biology for the production of multifunctional nanomaterials, turning the magnetosome surface into a platform for specific versatile display of functional moieties.

Bioengineered Polyhydroxyalkanoates as Immobilized Enzyme Scaffolds for Industrial Applications

Enzymes function as biocatalysts and are extensively exploited in industrial applications. Immobilization of enzymes using support materials has been shown to improve enzyme properties, including stability and functionality in extreme conditions and recyclability in biocatalytic processing. This review focuses on the recent advances utilizing the design space of in vivo self-assembled polyhydroxyalkanoate (PHA) particles as biocatalyst immobilization scaffolds. Self-assembly of biologically active enzyme-coated PHA particles is a one-step in vivo production process, which avoids the costly and laborious in vitro chemical cross-linking of purified enzymes to separately produced support materials. The homogeneous orientation of enzymes densely coating PHA particles enhances the accessibility of catalytic sites, improving enzyme function. The PHA particle technology has been developed into a remarkable scaffolding platform for the design of cost-effective designer biocatalysts amenable toward robust industrial bioprocessing. In this review, the PHA particle technology will be compared to other biological supramolecular assembly-based technologies suitable for in vivo enzyme immobilization. Recent progress in the fabrication of biological particulate scaffolds using enzymes of industrial interest will be summarized. Additionally, we outline innovative approaches to overcome limitations of in vivo assembled PHA particles to enable fine-tuned immobilization of multiple enzymes to enhance performance in multi-step cascade reactions, such as those used in continuous flow bioprocessing.

Bacterial Magnetosomes as Novel Platform for the Presentation of Immunostimulatory, Membrane-Bound Ligands in Cellular Biotechnology

Cell-cell interactions involving specific membrane proteins are critical triggers in cellular development. Ex vivo strategies to mimic these effects currently use soluble proteins or (recombinant) presenter cells, albeit with mixed results. A promising alternative are bacterial magnetosomes, which can be selectively transformed into cell-free membrane-protein presenters by genetic engineering. In this study, the human CD40 Ligand (CD40L), a key ligand for B cell activation, is expressed on the particle surface. Functionality is demonstrated on sensor cells expressing the human CD40 receptor. Binding of CD40L magnetosomes to these cells triggers a signaling cascade leading to the secretion of embryonic alkaline phosphatase. Concomitantly, the CD40-CD40L interaction is strong enough to allow cell recovery by magnetic sorting. Overall, this study demonstrates the potential of magnetosomes as promising cell-free tools for cellular biotechnology, based on the display of membrane-bound target molecules, thereby creating a biomimetic interaction.

Precise Assembly of Genetically Functionalized Magnetosomes and Tobacco Mosaic Virus Particles Generates a Magnetic Biocomposite

Magnetosomes represent magnetic nanoparticles with unprecedented characteristics. Both their crystal morphology and the composition of the enveloping membrane can be manipulated by genetic means, allowing the display of functional moieties on the particle surface. In this study, we explore the generation of a new biomaterial assembly by coupling magnetosomes with tobacco mosaic virus (TMV) particles, both functionalized with complementary recognition sites. TMV consists of single-stranded RNA encapsidated by more than 2100 coat proteins, which enable chemical modification via functional groups. Incubation of EmGFP- or biotin-decorated TMV particles with magnetosomes genetically functionalized with GFP-binding nanobodies or streptavidin, respectively, results in the formation of magnetic, mesoscopic, strand-like biocomposites. TMV facilitates the agglomeration of magnetosomes by providing a scaffold. The size of the TMV-magnetosome mesostrands can be adjusted by varying the TMV-magnetosome particle ratios. The versatility of this novel material combination is furthermore demonstrated by coupling magnetosomes and terminal, 5'-functionalized TMV particles with high molecular precision, which results in "drumstick"-like TMV-magnetosome complexes. In summary, our approaches provide promising strategies for the generation of new biomaterial assemblies that could be used as scaffold for the introduction of further functionalities, and we foresee a broad application potential in the biomedical and biotechnological field.

In Vivo Coating of Bacterial Magnetic Nanoparticles by Magnetosome Expression of Spider Silk-Inspired Peptides

Magnetosomes are natural magnetic nanoparticles with exceptional properties that are synthesized in magnetotactic bacteria by a highly regulated biomineralization process. Their usability in many applications could be further improved by encapsulation in biocompatible polymers. In this study, we explored the production of spider silk-inspired peptides on magnetosomes of the alphaproteobacterium Magnetospirillum gryphiswaldense. Genetic fusion of different silk sequence-like variants to abundant magnetosome membrane proteins enhanced magnetite biomineralization and caused the formation of a proteinaceous capsule, which increased the colloidal stability of isolated particles. Furthermore, we show that spider silk peptides fused to a magnetosome membrane protein can be used as seeds for silk fibril growth on the magnetosome surface. In summary, we demonstrate that the combination of two different biogenic materials generates a genetically encoded hybrid composite with engineerable new properties and enhanced potential for various applications.