Silaffins of diatoms: from applied biotechnology to biomedicine

Material-specific binding peptides empower sustainable innovations in plant health, biocatalysis, medicine and microplastic quantification

Material-binding peptides (MBPs) have emerged as a diverse and innovation-enabling class of peptides in applications such as plant-/human health, immobilization of catalysts, bioactive coatings, accelerated polymer degradation and analytics for micro-/nanoplastics quantification. Progress has been fuelled by recent advancements in protein engineering methodologies and advances in computational and analytical methodologies, which allow the design of, for instance, material-specific MBPs with fine-tuned binding strength for numerous demands in material science applications. A genetic or chemical conjugation of second (biological, chemical or physical property-changing) functionality to MBPs empowers the design of advanced (hybrid) materials, bioactive coatings and analytical tools. In this review, we provide a comprehensive overview comprising naturally occurring MBPs and their function in nature, binding properties of short man-made MBPs (<20 amino acids) mainly obtained from phage-display libraries, and medium-sized binding peptides (20-100 amino acids) that have been reported to bind to metals, polymers or other industrially produced materials. The goal of this review is to provide an in-depth understanding of molecular interactions between materials and material-specific binding peptides, and thereby empower the use of MBPs in material science applications. Protein engineering methodologies and selected examples to tailor MBPs toward applications in agriculture with a focus on plant health, biocatalysis, medicine and environmental monitoring serve as examples of the transformative power of MBPs for various industrial applications. An emphasis will be given to MBPs' role in detecting and quantifying microplastics in high throughput, distinguishing microplastics from other environmental particles, and thereby assisting to close an analytical gap in food safety and monitoring of environmental plastic pollution. In essence, this review aims to provide an overview among researchers from diverse disciplines in respect to material-(specific) binding of MBPs, protein engineering methodologies to tailor their properties to application demands, re-engineering for material science applications using MBPs, and thereby inspire researchers to employ MBPs in their research.

Molecular Driving Forces in the Self-Association of Silaffin Peptide R5 from MD Simulations

The 19-residue silaffin-R5 peptide has been widely studied for its ability to precipitate uniform SiO2 particles through mild temperature and pH pathways, in the absence of any organic solvents. There is consensus that post-translational modification (PTM) of side chains has a large impact on the biomineralization process. Thus, it is imperative to understand the precise mechanisms that dictate the formation of SiO2 from R5 peptide, including the effects of PTM on peptide aggregation and peptide-surface adsorption. In this work, we use molecular dynamics (MD) simulations to study the aggregation of R5 dimer with multiple PTMs, with the presence of different ions in solution. Since this system has strong interactions with deep metastable states, we use parallel bias metadynamics with partitioned families to efficiently sample the different states of the system. We find that peptide aggregation is a prerequisite for biomineralization. We observe that the electrostatic interactions are essential in the R5 dimer aggregation; for wild type R5 that only has positively charged residues, phosphate ions HPO4 2- in the solution form a bridge between two peptides and are essential for peptide aggregation.

Self-Entrapment of Antimicrobial Peptides in Silica Particles for Stable and Effective Antimicrobial Peptide Delivery System

Antimicrobial peptides (AMPs) have emerged as a promising solution to tackle bacterial infections and combat antibiotic resistance. However, their vulnerability to protease degradation and toxicity towards mammalian cells has hindered their clinical application. To overcome these challenges, our study aims to develop a method to enhance the stability and safety of AMPs applicable to effective drug-device combination products. The KR12 antimicrobial peptide was chosen, and in order to further enhance its delivery and efficacy the human immunodeficiency virus TAT protein-derived cell-penetrating peptide (CPP) was fused to form CPP-KR12. A new product, CPP-KR12@Si, was developed by forming silica particles with self-entrapped CPP-KR12 peptide using biomimetic silica precipitability because of its cationic nature. Peptide delivery from CPP-KR12@Si to bacteria and cells was observed at a slightly delivered rate, with improved stability against trypsin treatment and a reduction in cytotoxicity compared to CPP-KR12. Finally, the antimicrobial potential of the CPP-KR12@Si/bone graft substitute (BGS) combination product was demonstrated. CPP-KR12 is coated in the form of submicron-sized particles on the surface of the BGS. Self-entrapped AMP in silica nanoparticles is a safe and effective AMP delivery method that will be useful for developing a drug-device combination product for tissue regeneration.

Recent Progress in Diatom Biosilica: A Natural Nanoporous Silica Material as Sustained Release Carrier

A drug delivery system (DDS) is a useful technology that efficiently delivers a target drug to a patient's specific diseased tissue with minimal side effects. DDS is a convergence of several areas of study, comprising pharmacy, medicine, biotechnology, and chemistry fields. In the traditional pharmacological concept, developing drugs for disease treatment has been the primary research field of pharmacology. The significance of DDS in delivering drugs with optimal formulation to target areas to increase bioavailability and minimize side effects has been recently highlighted. In addition, since the burst release found in various DDS platforms can reduce drug delivery efficiency due to unpredictable drug loss, many recent DDS studies have focused on developing carriers with a sustained release. Among various drug carriers, mesoporous silica DDS (MS-DDS) is applied to various drug administration routes, based on its sustained releases, nanosized porous structures, and excellent solubility for poorly soluble drugs. However, the synthesized MS-DDS has caused complications such as toxicity in the body, long-term accumulation, and poor excretion ability owing to acid treatment-centered manufacturing methods. Therefore, biosilica obtained from diatoms, as a natural MS-DDS, has recently emerged as an alternative to synthesized MS-DDS. This natural silica carrier is an optimal DDS platform because culturing diatoms is easy, and the silica can be separated from diatoms using a simple treatment. In this review, we discuss the manufacturing methods and applications to various disease models based on the advantages of biosilica.

Understanding Self-Assembly of Silica-Precipitating Peptides to Control Silica Particle Morphology

The most advanced materials are those found in nature. These evolutionary optimized substances provide highest efficiencies, e.g., in harvesting solar energy or providing extreme stability, and are intrinsically biocompatible. However, the mimicry of biological materials is limited to a few successful applications since there is still a lack of the tools to recreate natural materials. Herein, such means are provided based on a peptide library derived from the silaffin protein R5 that enables rational biomimetic materials design. It is now evident that biomaterials do not form via mechanisms observed in vitro. Instead, the material's function and morphology are predetermined by precursors that self-assemble in solution, often from a combination of protein and salts. These assemblies act as templates for biomaterials. The RRIL peptides used here are a small part of the silica-precipitation machinery in diatoms. By connecting RRIL motifs via varying central bi- or trifunctional residues, a library of stereoisomers is generated, which allows characterization of different template structures in the presence of phosphate ions by combining residue-resolved real-time NMR spectroscopy and molecular dynamics (MD) simulations. Understanding these templates in atomistic detail, the morphology of silica particles is controlled via manipulation of the template precursors.

A living material platform for the biomineralization of biosilica

Nature has a vast array of biomineralization mechanisms. The commonly shared mechanism by many living organisms to form hardened tissues is the nucleation of mineral structures via proteins. Living materials, thanks to synthetic biology, are providing many opportunities to program cells for many functionalities. Here we have demonstrated a living material system for biosilicification. Silaffins are utilized to synthesize silicified cell walls by one of the most abundant organism groups called diatoms. The R5 peptide motif of the silaffins is known for its ability to precipitate silica in ambient conditions. Therefore, various studies have been conducted to implement the silicification activity of R5 in different application areas, such as regenerative medicine and tissue engineering. However, laborious protein purification steps are required prior to silica nanoparticle production in recombinant approaches. In this study, we aimed to engineer an alternative bacterial platform to achieve silicification using released and bacteria-intact forms of R5-attached fluorescent proteins (FP). Hence, we displayed R5-FP hybrids on the cell surface of E. coli via antigen 43 (Ag43) autotransporter system and managed to demonstrate heat-controllable release from the surface. We also showed that the bacteria cells displaying R5-FP can be used in silicification reactions. Lastly, considering the stimulating effect of silica on osteogenic differentiation, we treated human dental pulp stem cells (hDPSCs) with the silica aggregates formed via R5-FP hybrids. Earlier calcium crystal deposition around the hDPSCs was observed. We envision that our platform can serve as a faster and more economical alternative for biosilicification applications, including endodontics.

Photonic Nano-/Microstructured Diatom Based Biosilica in Metal Modification and Removal-A Review

The siliceous exoskeletal shells of diatoms, commonly known as frustules, have drawn attention because of their photoluminescence property and high volume to surface area. Photonic biosilica can also enhance the plasmonic sensitivity of nanoparticles. Because of this, researchers have studied the effectiveness of various metal particles after combining with biosilica. Additionally, naturally occurring diatom-based biosilica has excellent adsorption and absorption capabilities, which have already been exploited for wastewater treatment. Moreover, the nanoporous, ultra-hydrophilic frustules can easily accumulate more molecules on their surfaces. As a consequence, it becomes easier to conjugate noble metals with silica, making them more stable and effective. The main focus of this review is to agglomerate the utility of biocompatible diatom frustules, which is a no-cost natural resource of biosilica, in metal modification and removal.

Bio-Doped Microbial Nanosilica as Optosensing Biomaterial for Visual Quantitation of Nitrite in Cured Meats

A microbial optosensor for nitrite was constructed based on biomimetic silica nanoparticles, which were doped with R5, a polypeptide component of silaffin, as a robust biosilica immobilization matrix entrapped with Raoultella planticola and NAD(P)H cofactor during the in vitro biosilicification process of silica nanoparticles. Ruthenium(II)(bipy)2(phenanthroline-benzoylthiourea), the chromophoric pH probe, was physically adsorbed on the resulting biogenic nanosilica. Optical quantitation of the nitrite concentration was performed via reflectance transduction of the bio-doped microbial nanosilica at a maximum reflectance of 608 nm, due to the deprotonation of phen-BT ligands in the ruthenium complex, while the intracellular enzyme expression system catalyzed the enzymatic reduction of nitrite. Reflectance enhancement of the microbial optosensor was linearly proportional to the nitrite concentration from 1−100 mg L−1, with a 0.25 mg L−1 limit of detection and a rapid response time of 4 min. The proposed microbial optosensor showed good stability of >2 weeks, great repeatability for 5 repetitive assays (relative standard deviation, (RSD) = 0.2−1.4%), high reproducibility (RSD = 2.5%), and a negligible response to common interferents found in processed meats, such as NO3−, NH4+, K+, Ca2+, and Mg2+ ions, was observed. The microbial biosensor demonstrated an excellent capacity to provide an accurate estimation of nitrite in several cured meat samples via validation using a standard UV-vis spectrophotometric Griess assay.

BMP2-Mediated Silica Deposition: An Effective Strategy for Bone Mineralization

The combined use of an osteogenic factor, such as bone morphogenetic protein 2 (BMP2), with a bone scaffold was quite functional for the reconstruction of bone defects. Although many studies using BMP2 have been done, there is still a need to develop an efficient way to apply BMP2 in the bone scaffold. Here, we reported an interesting fact that BMP2 has a silica deposition ability in the presence of silicic acid and proposed that such an ability of BMP2 can effectively immobilize and transport itself by a kind of coprecipitation of BMP2 with a silica matrix. The presence of BMP2 in the resulting silica was proved by SEM and EDS and was visualized by FITC-labeled BMP2. The delivery efficacy of BMP2 of silica-entrapped BMP2 on osteoblast differentiation and mineralization using MC3T3 E1 preosteoblast cells was evaluated in vitro. The coprecipitated BMP2 with silica exhibited osteogenesis at a low concentration that was insufficient to give an osteoinductive signal as the free form. Expectedly, the silica-entrapped BMP2 exhibited thermal stability over free BMP2. When applied to bone graft substitution, e.g., hydroxyapatite granules (HA), silica-entrapped BMP 2 laden HA (BMP2@Si/HA) showed sustained BMP2 release, whereas free BMP2 adsorbed HA by a simple dipping method (BMP2/HA) displayed a burst release of BMP2 at an initial time. In the rat critical-size calvarial defect model, BMP2@Si/HA showed better bone regeneration than BMP2/HA by about 10%. The BMP2/silica hybrid deposited on a carrier surface via BMP2-mediated silica precipitation demonstrated an increase in the loading efficiency, a decrease in the burst release of BMP2, and an increase in bone regeneration. Taken together, the coprecipitated BMP2 with a silica matrix has the advantages of not only being able to immobilize BMP2 efficiently without compromising its function but also serving as a stable carrier for BMP2 delivery.

Outstanding Questions on the Beneficial Role of Silicon in Crop Plants

Silicon (Si) is widely accepted as a beneficial element for plants. Despite the substantial progress made in understanding Si transport mechanisms and modes of action in plants, several questions remain unanswered. In this review, we discuss such outstanding questions and issues commonly encountered by biologists studying the role of Si in plants in relation to Si bioavailability. In recent years, advances in our understanding of the role of Si-solubilizing bacteria and the efficacy of Si nanoparticles have been made. However, there are many unknown aspects associated with structural and functional features of Si transporters, Si loading into the xylem, and the role of specialized cells like silica cells and compounds preventing Si polymerization in plant tissues. In addition, despite several 1,000 reports showing the positive effects of Si in high as well as low Si-accumulating plant species, the exact roles of Si at the molecular level are yet to be understood. Some evidence suggests that Si regulates hormonal pathways and nutrient uptake, thereby explaining various observed benefits of Si uptake. However, how Si modulates hormonal pathways or improves nutrient uptake remains to be explained. Finally, we summarize the knowledge gaps that will provide a roadmap for further research on plant silicon biology, leading to an exploration of the benefits of Si uptake to enhance crop production.

Biomimetic and bioinspired silicifications: Recent advances for biomaterial design and applications

The rational design and controllable synthesis of functional silica-based materials have gained increased interest in a variety of biomedical and biotechnological applications due to their unique properties. The current review shows that marine organisms, such as siliceous sponges and diatoms, could be the inspiration for the fabrication of advanced biohybrid materials. Several biomolecules were involved in the molecular mechanism of biosilicification in vivo. Mimicking their behavior, functional silica-based biomaterials have been generated via biomimetic and bioinspired silicification in vitro. Additionally, several advanced technologies were developed for in vitro and in vivo immobilization of biomolecules with potential applications in biocatalysis, biosensors, bioimaging, and immunoassays. A thin silica layer could coat a single living cell or virus as a protective shell offering new opportunities in biotechnology and nanomedicine fields. Promising nanotechnologies have been developed for drug encapsulation and delivery in a targeted and controlled manner, in particular for poorly soluble hydrophobic drugs. Moreover, biomimetic silica, as a morphogenetically active biocompatible material, has been utilized in the field of bone regeneration and in the development of biomedical implantable devices. STATEMENT OF SIGNIFICANCE: In nature, silica-based biomaterials, such as diatom frustules and sponge spicules, with high mechanical and physical properties were created under biocompatible conditions. The fundamental knowledge underlying the molecular mechanisms of biosilica formation could inspire engineers and chemists to design novel hybrid biomaterials using molecular biomimetic strategies. The production of such biohybrid materials brings the biosilicification field closer to practical applications. This review starts with the biosilicification process of sponges and diatoms with recently updated researches. Then, this article covers recent advances in the design of silica-based biomaterials and their potential applications in the fields of biotechnology and nanomedicine, highlighting several promising technologies for encapsulation of functional proteins and living cells, drug delivery and the preparation of scaffolds for bone regeneration.

GPCR Genes as Activators of Surface Colonization Pathways in a Model Marine Diatom

Surface colonization allows diatoms, a dominant group of phytoplankton in oceans, to adapt to harsh marine environments while mediating biofoulings to human-made underwater facilities. The regulatory pathways underlying diatom surface colonization, which involves morphotype switching in some species, remain mostly unknown. Here, we describe the identification of 61 signaling genes, including G-protein-coupled receptors (GPCRs) and protein kinases, which are differentially regulated during surface colonization in the model diatom species, Phaeodactylum tricornutum. We show that the transformation of P. tricornutum with constructs expressing individual GPCR genes induces cells to adopt the surface colonization morphology. P. tricornutum cells transformed to express GPCR1A display 30% more resistance to UV light exposure than their non-biofouling wild-type counterparts, consistent with increased silicification of cell walls associated with the oval biofouling morphotype. Our results provide a mechanistic definition of morphological shifts during surface colonization and identify candidate target proteins for the screening of eco-friendly, anti-biofouling molecules.

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The model diatom Phaeodactylum tricornutum shifts morphology to form biofilms

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G-protein-coupled receptors (GPCRs) can modulate diatom surface colonization

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GPCR1A expression can induce biofouling morphotype and UV resistance

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Identified genes and pathways can serve as targets for anti-biofouling discoveries

Phytolith Formation in Plants: From Soil to Cell

Silica is deposited extra- and intracellularly in plants in solid form, as phytoliths. Phytoliths have emerged as accepted taxonomic tools and proxies for reconstructing ancient flora, agricultural economies, environment, and climate. The discovery of silicon transporter genes has aided in the understanding of the mechanism of silicon transport and deposition within the plant body and reconstructing plant phylogeny that is based on the ability of plants to accumulate silica. However, a precise understanding of the process of silica deposition and the formation of phytoliths is still an enigma and the information regarding the proteins that are involved in plant biosilicification is still scarce. With the observation of various shapes and morphologies of phytoliths, it is essential to understand which factors control this mechanism. During the last two decades, significant research has been done in this regard and silicon research has expanded as an Earth-life science superdiscipline. We review and integrate the recent knowledge and concepts on the uptake and transport of silica and its deposition as phytoliths in plants. We also discuss how different factors define the shape, size, and chemistry of the phytoliths and how biosilicification evolved in plants. The role of channel-type and efflux silicon transporters, proline-rich proteins, and siliplant1 protein in transport and deposition of silica is presented. The role of phytoliths against biotic and abiotic stress, as mechanical barriers, and their use as taxonomic tools and proxies, is highlighted.

Gene to diagnostic: Self immobilizing protein for silica microparticle biosensor, modelled with sarcosine oxidase

A rational design approach is proposed for a multifunctional enzyme reagent for point-of-care diagnostics. The biomaterial reduces downstream isolation steps and eliminates immobilization coupling chemicals for integration in a diagnostic platform. Fusion constructs combined the central functional assay protein (e.g. monomeric sarcosine oxidase, mSOx, horseradish peroxidase, HRP), a visualizing protein (e.g. mCherry) and an in-built immobilization peptide (e.g. R5). Monitoring protein expression in E.coli was facilitated by following the increase in mCherry fluorescence, which could be matched to a color card, indicating when good protein expression has occurred. The R5 peptide (SSKKSGSYSGSKGSKRRIL) provided inbuilt affinity for silica and an immobilization capability for a silica based diagnostic, without requiring additional chemical coupling reagents. Silica particles extracted from beach sand were used to collect protein from crude protein extract with 85-95% selective uptake. The silica immobilized R5 proteins were stable for more than 2 months at room temperature. The K_m for the silica-R5₂-mCh-mSOx-R5-6H was 16.5 ± 0.9 mM (compared with 16.5 ± 0.4 mM, 16.3 ± 0.3 mM, and 16.1 ± 0.4 mM for R5₂-mCh-mSOx-R5-6H, mSOx-R5-6H and mSOx-6H respectively in solution). The use of the "silica-enzymes" in sarcosine and peroxide assays was shown, and a design using particle sedimentation through the sample was examined. Using shadowgraphy and particle image velocimetry the particle trajectory through the sample was mapped and an hourglass design with a narrow waist shown to give good control of particle position. The hourglass biosensor was demonstrated for sarcosine assay in the clinically useful range of 2.5-10 μM in both a dynamic and end point measurement regime.

Biofunctionalization of Nano Channels by Direct In-Pore Solid-Phase Peptide Synthesis

Diatom biosilica are highly complex inorganic/organic hybrid materials. To get deeper insights on their structure at a molecular level, model systems that mimic the complex natural compounds were synthesized and characterized. A simple and efficient peptide immobilization strategy was developed, which uses a well-ordered porous silica material as a support and commercially available Fmoc-amino acids, similar to the known solid-phase peptide synthesis. As an example, Fmoc-glycine and Fmoc-phenylalanine are immobilized on the silica support. The success of functionalization was investigated by ¹³ C CP MAS and ²⁹ Si CP MAS solid-state NMR. Thermogravimetric analysis (TGA) and elemental analysis (EA) were performed to quantify the functionalization. Changes of the specific surface area, pore volume, and pore diameters in all modification steps were studied by Brunauer-Emmett-Teller based nitrogen adsorption-desorption measurements (BET). The combination of the analytical methods provided high grafting densities of 2.1±0.2 molecules/nm² on the surface. Furthermore, they allowed for monitoring chemical changes on the pore surface and changes of the pore properties of the material during the different functionalization steps. This universal approach is suitable for the selective synthesis of pores with tunable surface-peptide functionalization, with applications to the synthesis of a big variety of silica-peptide model systems, which in the future may lead to a deeper understanding of complex biological systems.

Multiple Routes to Smart Nanostructured Materials from Diatom Microalgae: A Chemical Perspective

Diatoms are unicellular photosynthetic microalgae, ubiquitously diffused in both marine and freshwater environments, which exist worldwide with more than 100 000 species, each with different morphologies and dimensions, but typically ranging from 10 to 200 µm. A special feature of diatoms is their production of siliceous micro- to nanoporous cell walls, the frustules, whose hierarchical organization of silica layers produces extraordinarily intricate pore patterns. Due to the high surface area, mechanical resistance, unique optical features, and biocompatibility, a number of applications of diatom frustules have been investigated in photonics, sensing, optoelectronics, biomedicine, and energy conversion and storage. Current progress in diatom-based nanotechnology relies primarily on the availability of various strategies to isolate frustules, retaining their morphological features, and modify their chemical composition for applications that are not restricted to those of the bare biosilica produced by diatoms. Chemical or biological methods that decorate, integrate, convert, or mimic diatoms' biosilica shells while preserving their structural features represent powerful tools in developing scalable, low-cost routes to a wide variety of nanostructured smart materials. Here, the different approaches to chemical modification as the basis for the description of applications relating to the different materials thus obtained are presented.

From Protein Features to Sensing Surfaces

Proteins play a major role in biosensors in which they provide catalytic activity and specificity in molecular recognition. However, the immobilization process is far from straightforward as it often affects the protein functionality. Extensive interaction of the protein with the surface or significant surface crowding can lead to changes in the mobility and conformation of the protein structure. This review will provide insights as to how an analysis of the physico-chemical features of the protein surface before the immobilization process can help to identify the optimal immobilization approach. Such an analysis can help to preserve the functionality of the protein when on a biosensor surface.

Diatom Frustule Morphogenesis and Function: a Multidisciplinary Survey

Diatoms represent the major component of phytoplankton and are responsible for about 20-25% of global primary production. Hundreds of millions of years of evolution led to tens of thousands of species differing in dimensions and morphologies. In particular, diatom porous silica cell walls, the frustules, are characterized by an extraordinary, species-specific diversity. It is of great interest, among the marine biologists and geneticists community, to shed light on the origin and evolutionary advantage of this variability of dimensions, geometries and pore distributions. In the present article the main reported data related to frustule morphogenesis and functionalities with contributions from fundamental biology, genetics, mathematics, geometry and physics are reviewed.

Linear and orthogonal peptide templating of silicified protein fibres

Biomineralisation is essential for biology. Specialist proteins use peptide motifs that catalyse mineral deposition into nano-to-microscale inorganic materials. Unlike in native proteins, the motifs incorporated into self-assembled fibres can persistently propagate on the microscopic scale enabling empirically defined silica nanostructures. Herein we show that the two main modes of motif templating - linear and orthogonal - in self-assembling, fibre-forming peptide sequences effectively silicify protein fibres. We show that the mere charge and morphology of protein fibres are not sufficient for silica deposition, but it is the synergy between fibrillogenesis and silica-specific motifs regularly spaced in fibres that ensures silica templating, regardless of the relative orientation of the motifs.

Chitosan-Coated Diatom Silica as Hemostatic Agent for Hemorrhage Control

Uncontrolled hemorrhage leads to high death risk both in military and civilian trauma. Current hemostatic agents still have various limitations and side effects. In this study, natural diatom silica obtained from diatomite and diatom culture was purified and developed for hemorrhage control. To improve the biocompatibility and hemostatic performance of diatom silica, a series of chitosan-coated diatom (CS-diatom) was developed. The composition of CS-diatom prepared was optimized by in vitro hemocompatibility and blood coagulation evaluation for that prepared with 0.5%, 1%, 3%, and 5% chitosan. The results demonstrated that the CS-diatom prepared with 1% chitosan exhibited favorable biocompatibility (hemolysis ratio < 5%, no cytotoxicity to MEFs), great fluid absorbility (24.39 ± 1.53 times the weight of liquid), and desirable hemostasis effect (351 ± 14.73 s at 5 mg/mL, 248 ± 32.42s at 10 mg/mL). Further blood coagulation mechanism study indicated that CS-diatom could provide an ideal interface to induce erythrocyte absorption and aggregation, along with activating the intrinsic coagulation pathway and thus accelerated blood coagulation. Benefitting from the multiple hemostatic performances, CS-diatom showed the shortest clotting time (98.34 ± 26.54 s) and lowest blood loss (0.31 ± 0.11 g) in rat-tail amputation model compare to diatomite and diatom as well as gauze and commercial QuikClot zeolite. The results evidenced that the CS-diatom was a safe and effective hemostatic agent and provided a new understanding of nonsynthetic mesoporous materials for hemorrhage control.

Biomimetic fabrication of biotinylated peptide nanostructures upon diatom scaffold; a plausible model for sustainable energy

This study demonstrates the interaction of biotin–Trp–Trp peptide with diatoms. Such interaction afforded unique nanoarray over the diatom frustules thus can be used as most parsimonious model for the production of renewable energy.

Silaffins in Silica Biomineralization and Biomimetic Silica Precipitation

Biomineralization processes leading to complex solid structures of inorganic material in biological systems are constantly gaining attention in biotechnology and biomedical research. An outstanding example for biomineral morphogenesis is the formation of highly elaborate, nano-patterned silica shells by diatoms. Among the organic macromolecules that have been closely linked to the tightly controlled precipitation of silica in diatoms, silaffins play an extraordinary role. These peptides typically occur as complex posttranslationally modified variants and are directly involved in the silica deposition process in diatoms. However, even in vitro silaffin-based peptides alone, with and without posttranslational modifications, can efficiently mediate biomimetic silica precipitation leading to silica material with different properties as well as with encapsulated cargo molecules of a large size range. In this review, the biomineralization process of silica in diatoms is summarized with a specific focus on silaffins and their in vitro silica precipitation properties. Applications in the area of bio- and nanotechnology as well as in diagnostics and therapy are discussed.

Affinity purification of recombinant proteins using a novel silica-binding peptide as a fusion tag

We recently reported that silica is deposited on the coat of Bacillus cereus spores as a layer of nanometer-sized particles (Hirota et al. 2010 J Bacteriol 192: 111-116). Gene disruption analysis revealed that the spore coat protein CotB1 mediates the accumulation of silica (our unpublished results). Here, we report that B. cereus CotB1 (171 amino acids [aa]) and its C-terminal 14-aa region (corresponding to residues 158-171, designated CotB1p) show strong affinity for silica particles, with dissociation constants at pH 8.0 of 2.09 and 1.24 nM, respectively. Using CotB1 and CotB1p as silica-binding tags, we developed a silica-based affinity purification method in which silica particles are used as an adsorbent for CotB1/CotB1p fusion proteins. Small ubiquitin-like modifier (SUMO) technology was employed to release the target proteins from the adsorbed fusion proteins. SUMO-protease-mediated site-specific cleavage at the C-terminus of the fused SUMO sequence released the tagless target proteins into the liquid phase while leaving the tag region still bound to the solid phase. Using the fluorescent protein mCherry as a model, our purification method achieved 85 % recovery, with a purity of 95 % and yields of 0.60 ± 0.06 and 1.13 ± 0.13 mg per 10-mL bacterial culture for the CotB1-SUMO-mCherry and CotB1p-SUMO-mCherry fusions, respectively. CotB1p, a short 14-aa peptide, which demonstrates high affinity for silica, could be a promising fusion tag for both affinity purification and enzyme immobilization on silica supports.