Self-assembly of tunable protein suprastructures from recombinant oleosin

A microfluidic platform for the synthesis of polymer and polymer-protein-based protocells

In this study, we demonstrate the fabrication of polymersomes, protein-blended polymersomes, and polymeric microcapsules using droplet microfluidics. Polymersomes with uniform, single bilayers and controlled diameters are assembled from water-in-oil-in-water double-emulsion droplets. This technique relies on adjusting the interfacial energies of the droplet to completely separate the polymer-stabilized inner core from the oil shell. Protein-blended polymersomes are prepared by dissolving protein in the inner and outer phases of polymer-stabilized droplets. Cell-sized polymeric microcapsules are assembled by size reduction in the inner core through osmosis followed by evaporation of the middle phase. All methods are developed and validated using the same glass-capillary microfluidic apparatus. This integrative approach not only demonstrates the versatility of our setup, but also holds significant promise for standardizing and customizing the production of polymer-based artificial cells.

Self-Assembled Recombinant Elastin and Globular Protein Vesicles with Tunable Properties for Diverse Applications

Vesicles are self-assembled structures comprised of a membrane-like exterior surrounding a hollow lumen with applications in drug delivery, artificial cells, and micro-bioreactors. Lipid or polymer vesicles are the most common and are made of lipids or polymers, respectively. They are highly useful structures for many applications but it can be challenging to decorate them with proteins or encapsulate proteins in them, owing to the use of organic solvent in their formation and the large size of proteins relative to lipid or polymer molecules. By utilization of recombinant fusion proteins to make vesicles, specific protein domains can be directly incorporated while also imparting tunability and stability. Protein vesicle assembly relies on the design and use of self-assembling amphiphilic proteins. A specific protein vesicle platform made in purely aqueous conditions of a globular, functional protein fused to a glutamate-rich leucine zipper (ZE) and a thermoresponsive elastin-like polypeptide (ELP) fused to an arginine-rich leucine zipper (ZR) is discussed here. The hydrophobic conformational change of the ELP above its transition temperature drives assembly, and strong ZE/ZR binding enables incorporation of the desired functional protein. Mixing the soluble proteins on ice induces zipper binding, and then warming above the ELP transition temperature (Tt) triggers the transition to and growth of protein-rich coacervates and, finally, reorganization of proteins into vesicles. Vesicle size is tunable based on salt concentration, rate of heating, protein concentration, size of the globular protein, molar ratio of the proteins, and the ELP sequence. Increasing the salt concentration decreases vesicle size by decreasing the Tt, resulting in a shorter coacervation transition stage. Likewise, directly changing the heating rate also changes this time and increasing protein concentration increases coalescence. Increasing globular protein size decreases the size of the vesicle due to steric hindrance. By changing the ELP sequence, which consists of (VPGXG)n, through the guest residue (X) or number of repeats (n), Tt is changed, affecting size. Additionally, the chemical nature of X variation has endowed vesicles with stimuli responsiveness and stability at physiological conditions.Protein vesicles have been used for biocatalysis, biomacromolecular drug delivery, and vaccine applications. Photo-cross-linkable vesicles were used to deliver small molecule cargo to cancer cells in vitro and antigen to immune cells in vivo. pH-responsive vesicles effectively delivered functional protein cargo, including cytochrome C, to the cytosol of cancer cells in vitro, using hydrophobic ion pairing to improve cargo distribution in the vesicles and release. The globular protein used to make the vesicles can be varied to achieve different functions. For example, enzyme vesicles exhibit biocatalysis, and antigen vesicles induce antibody and cellular immune responses after vaccination in mice. Collectively, the development and engineering of the protein vesicle platform has employed amphiphilic self-assembly strategies and rational protein engineering to control physical, chemical, and biological properties for biotechnology and nanomedicine applications.

Molecular Transportation Conversion of Membrane Tension Using a Mechanosensitive Channel in Asymmetric Lipid-Protein Vesicles

Giant lipid vesicles composed of a lipid bilayer form complex membrane structures and enzyme network reactions that can be used to construct well-defined artificial cell models based on microfluidic technologies and synthetic biology. As a different approach to cell-mimicking systems, we formed an asymmetric lipid-amphiphilic protein (oleosin) vesicle containing a lipid and an oleosin monolayer in the outer and inner leaflets, respectively. These asymmetric vesicles enabled the reconstitution and function of β-barrel types of membrane proteins (OmpG) and the fission of vesicles stimulated by lysophospholipids. These applications combine the advantages of the high stability of lipids and oleosin leaflets in asymmetric lipid-oleosin vesicles. In this study, to evaluate the versatility of this asymmetric lipid-oleosin vesicle, the molecular transport of the mechanosensitive channel of large conductance (MscL) with an α-helix was evaluated by changing the tension of the asymmetric vesicle membrane with lysophospholipid. A nanopore of MscL assembled as a pentamer of MscLs transports small molecules of less than 10 kDa by sensing physical stress at the lipid bilayer. The amount and maximum size of the small molecules transported via MscL in the asymmetric lipid-oleosin vesicles were compared to those in the lipid vesicles. We revealed the existence of the C- and N-terminal regions (cytoplasmic side) of MscL on the inner leaflet of the asymmetric lipid-oleosin vesicles using an insertion direction assay. Furthermore, the change in the tension of the lipid-oleosin membrane activated the proteins in these vesicles, inducing their transportation through MscL nanopores. Therefore, asymmetric lipid-oleosin vesicles containing MscL can be used as substrates to study the external environment response of complex artificial cell models.

Unveiling the applications of membrane proteins from oil bodies: leading the way in artificial oil body technology and other biotechnological advancements

Oil bodies (OBs) function as organelles that store lipids in plant seeds. An oil body (OB) is encased by a membrane composed of proteins (e.g., oleosins, caleosins, and steroleosins) and a phospholipid monolayer. The distinctive protein-phospholipid membrane architecture of OBs imparts exceptional stability even in extreme environments, thereby sparking increasing interest in their structure and properties. However, a comprehensive understanding of the structure-activity relationships determining the stability and properties of oil bodies requires a more profound exploration of the associated membrane proteins, an aspect that remains relatively unexplored. In this review, we aim to summarize and discuss the structural attributes, biological functions, and properties of OB membrane proteins. From a commercial perspective, an in-depth understanding of the structural and functional properties of OBs is important for the expansion of their applications by producing artificial oil bodies (AOB). Besides exploring their structural intricacies, we describe various methods that are used for purifying and isolating OB membrane proteins. These insights may provide a foundational framework for the practical utilization of OB membrane proteins in diverse applications within the realm of AOB technology, including biological and probiotic delivery, protein purification, enzyme immobilization, astringency detection, and antibody production.

Insights into the emulsification mechanism of the surfactant-like protein oleosin

Oleosins are proteins with a unique central hydrophobic hairpin designed to stabilize lipid droplets (oleosomes) in plant seeds. For efficient droplet stabilization, the hydrophobic hairpin with a strong affinity for the apolar droplet core is flanked by hydrophilic arms on each side. This gives oleosins a unique surfactant-like shape making them a very interesting protein. In this study, we tested if isolated oleosins retain their ability to stabilize oil-in-water emulsions, and investigated the underlying stabilization mechanism. Due to their surfactant-like shape, oleosins when dispersed in aqueous buffers associated to micelle-like nanoparticles with a size of ∼33 nm. These micelles, in turn, clustered into larger aggregates of up to 20 µm. Micelle aggregation was more extensive when oleosins lacked charge. During emulsification, oleosin micelles and micelle aggregates dissociated and mostly individual oleosins adsorbed on the oil droplet interface. Oleosins prevented the coalescence of the oil droplets and if sufficiently charged, droplet flocculation as well.

Delivery of external proteins into the cytoplasm using protein capsules modified with IgG on the surface, created from the amphiphilic two helix-bundle protein OLE-ZIP

This study explores a new method for delivering therapeutic proteins into specific cells using OLE-ZIP capsules that present IgG. OLE-ZIP capsules is a spherical caspules prepared from amphihilic dimetic coiled-coil peptide, OLE-ZIP. Upon presenting cetuximab, these capsules showed preferential uptake in A431 cells and increased cytotoxicity when loaded with RNase A.

Hydrophobin Bilayer as Water Impermeable Protein Membrane

One of the most important properties of membranes is their permeability to water and other small molecules. A targeted change in permeability allows the passage of molecules to be controlled. Vesicles made of membranes with low water permeability are preferable for drug delivery, for example, because they are more stable and maintain the drug concentration inside. This study reports on the very low water permeability of pure protein membranes composed of a bilayer of the amphiphilic protein hydrophobin HFBI. Using a droplet interface bilayer setup, we demonstrate that HFBI bilayers are essentially impermeable to water. HFBI bilayers withstand far larger osmotic pressures than lipid membranes. Only by disturbing the packing of the proteins in the HFBI bilayer is a measurable water permeability induced. To investigate possible molecular mechanisms causing the near-zero permeability, we used all-atom molecular dynamics simulations of various HFBI bilayer models. The simulations suggest that the experimental HFBI bilayer permeability is compatible neither with a lateral honeycomb structure, as found for HFBI monolayers, nor with a residual oil layer within the bilayer or with a disordered lateral packing similar to the packing in lipid bilayers. These results suggest that the low permeabilities of HFBI and lipid bilayers rely on different mechanisms. With their extremely low but adaptable permeability and high stability, HFBI membranes could be used as an osmotic pressure-insensitive barrier in situations where lipid membranes fail such as desalination membranes.

Rational design and engineering of polypeptide/protein vesicles for advanced biological applications

Synthetic vesicles have gained considerable popularity in recent years for numerous biological and medical applications. Among the various types of synthetic vesicles, the utilization of polypeptides and/or proteins as fundamental constituents has garnered significant interest for vesicle construction owing to the unique bio-functionalities inherent in rationally designed amino acid sequences. Especially the incorporation of functional proteins onto the vesicle surface facilitates a wide range of advanced biological applications that are not easily attainable with traditional building blocks, such as lipids and polymers. The main goal of this review is to provide a comprehensive overview of the latest advancements in polypeptide/protein vesicles. Moreover, this review encompasses the rational design and engineering strategies employed in the creation of polypeptide/protein vesicles, including the synthesis of building blocks, the modulation of their self-assembly, as well as their diverse applications. Furthermore, this work includes an in-depth discussion of the key challenges and opportunities associated with polypeptide/protein vesicles, providing valuable insights for future research. By offering an up-to-date review of this burgeoning field of polypeptide/protein vesicle research, this review will shed light on the potential applications of these biomaterials.

Improve stability and application of rice oil bodies via surface modification with ferulic acid, (-)-epicatechin, and phytic acid

Rice bran oil bodies (RBOBs) are one of the most exploited functional components from rice bran by-products and are predominantly based on oleosin stabilization. In this study, we explored the effects of different concentrations of added (-)-epicatechin, ferulic acid, and phytic acid on the RBOBs stability. The results revealed that the incorporation of all three natural phytoconstituents could reduce the RBOBs particle size and increase emulsifying properties, demonstrating increasing surface hydrophobicity (p < 0.05), and a good antioxidant effect, which was especially obvious with (-)-epicatechin incorporation. Fourier transform infrared (FT-IR) spectroscopy data demonstrated that these three small molecule substance classes can modify with oleosin on RBOBs surface by covalent and noncovalent effects. Raman spectroscopic analysis illustrated that the vibrational modes of disulphide bonds in oleosin were modified by these three plant natural ingredients. The interactions between the three phytoconstituents and the model protein were investigated by molecular docking experiments.

Cell-sized asymmetric phospholipid-amphiphilic protein vesicles with growth, fission, and molecule transportation

Lipid vesicles, which mimic cell membranes in structure and components, have been used to study the origin of life and artificial cell construction. A different approach to developing cell-mimicking systems focuses on the formation of protein- or polypeptide-based vesicles. However, micro-sized protein vesicles that are similar in membrane dynamics to the cell and that reconstitute membrane proteins are difficult to form. In this study, we generated cell-sized asymmetric phospholipid-amphiphilic protein (oleosin) vesicles that allow the reconstitution of membrane proteins and the growth and fission of vesicles. These vesicles are composed of a lipid membrane on the outer leaflet and an oleosin membrane on the inner leaflet. Further, we elucidated a mechanism for the growth and fission of cell-sized asymmetric phospholipid-oleosin vesicles by feeding phospholipid micelles. Our asymmetric phospholipid-oleosin vesicles with the advantages of the lipid leaflet and the protein leaflet will potentially promote understanding of biochemistry and synthetic biology.

Chemically Driven Multimodal Locomotion of Active, Flexible Sheets

The inhibitor-promoter feedback loop is a vital component in regulatory pathways that controls functionality in living systems. In this loop, the production of chemical A at one site promotes the production of chemical B at another site, but B inhibits the production of A. In solution, differences in the volumes of the reactants and products of this reaction can generate buoyancy-driven fluid flows, which will deform neighboring soft material. To probe the intrinsic interrelationship among chemistry, hydrodynamics, and fluid-structure interactions, we model a bio-inspired system where a flexible sheet immersed in solution encompasses two spatially separated catalytic patches, which drive the A-B inhibitor-promotor reaction. The convective rolls of fluid generated above the patches can circulate inward or outward depending on the chemical environment. Within the regime displaying chemical oscillations, the dynamic fluid-structure interactions morph the shape of the sheet to periodically "fly", "crawl", or "swim" along the bottom of the confining chamber, revealing an intimate coupling between form and function in this system. The oscillations in the sheet's motion in turn affect the chemical oscillations in the solution. In the regime with non-oscillatory chemistry, the induced flow still morphs the shape of the sheet, but now, the fluid simply translates the sheet along the length of the chamber. The findings reveal the potential for enzymatic reactions in the body to generate hydrodynamic behavior that modifies the shape of neighboring soft tissue, which in turn modifies both the fluid dynamics and the enzymatic reaction. The findings indicate that this non-linear dynamic behavior can be playing a critical role in the functioning of regulatory pathways in living systems.

Recombinant Protein Micelles to Block Transduction by SARS-CoV-2 Pseudovirus

The continuing emergence of variants of the SARS-CoV-2 virus requires the development of modular molecular therapies. Here, we engineered a recombinant amphiphilic protein, oleosin, to spontaneously self-assemble into multivalent micellar nanostructures which can block the Spike S1 protein of SARS-CoV-2 pseudoviruses (PVs). Short recombinant proteins like oleosin can be formulated more easily than antibodies and can be functionalized with precision through genetic engineering. We cloned S1-binding mini-protein genes called LCBx, previously designed by David Baker's laboratory (UW Seattle), to the N-terminus of oleosin, expressing Oleo-LCBx proteins in E. coli. These proteins largely formed 10-100 nm micelles as verified by dynamic light scattering. Two proteins, Oleo-LCB1 and Oleo-LCB3, were seen to completely and irreversibly block transduction by both wild-type and delta variant PVs into 293T-hsACE2 cells at 10 μM. Presented in multivalent micelles, these proteins reduced transduction by PVs down to a functional protein concentration of 5 nM. Additionally, Oleo-LCB1 micelles outperformed corresponding synthetic LCB1 mini-proteins in reducing transduction by PVs. Tunable aqueous solubility of recombinant oleosin allowed incorporation of peptides/mini-proteins at high concentrations within micelles, thus enhancing drug loading. To validate the potential multifunctionality of the micelles, we showed that certain combinations of Oleo-LCB1 and Oleo-LCB3 performed much better than the individual proteins at the same concentration. These micelles, which we showed to be non-toxic to human cells, are thus a promising step toward the design of modular, multifunctional therapeutics that could bind to and inactivate multiple receptors and proteins necessary for the infection of the SARS-CoV-2 virus.

Self-assembling nanocarriers from engineered proteins: Design, functionalization, and application for drug delivery

Self-assembling proteins are valuable building blocks for constructing drug nanocarriers due to their self-assembly behavior, monodispersity, biocompatibility, and biodegradability. Genetic and chemical modifications allow for modular design of protein nanocarriers with effective drug encapsulation, targetability, stimuli responsiveness, and in vivo half-life. Protein nanocarriers have been developed to deliver various therapeutic molecules including small molecules, proteins, and nucleic acids with proven in vitro and in vivo efficacy. This article reviews recent advances in protein nanocarriers that are not derived from natural protein nanostructures, such as protein cages or virus like particles. The protein nanocarriers described here are self-assembled from rationally or de novo designed recombinant proteins, as well as recombinant proteins complexed with other biomolecules, presenting properties that are unique from those of natural protein carriers. Design, functionalization, and therapeutic application of protein nanocarriers will be discussed.

Self-assembly of globular proteins with intrinsically disordered protein polyelectrolytes and block copolymers

Intrinsically disordered polypeptides are a versatile class of materials, combining the biocompatibility of peptides with the disordered structure and diverse phase behaviors of synthetic polymers. Synthetic polyelectrolytes are capable of complex phase behavior when mixed with oppositely charged polyelectrolytes, facilitating nanoparticle formation and bulk phase separation. However, there has been limited exploration of intrinsically disordered protein polyelectrolytes as potential bio-based replacements for synthetic polyelectrolytes. Here, we produce negatively charged, intrinsically disordered polypeptides, capable of high-yield expression in E. coli and use this intrinsically disordered peptide to produce entirely protein-based polyelectrolyte complexes. The complexes display rich phase behavior, showing sensitivity to charge density, salt concentration, temperature, and charge fraction. We characterize this behavior through a combination of turbidity assays, dynamic light scattering, and transmission electron microscopy. The robust expression profile and stimuli-responsive phase behavior of the intrinsically disordered peptides demonstrates their potential as easily producible, biocompatible substitutes for synthetic polyelectrolytes.

Solutal-buoyancy-driven intertwining and rotation of patterned elastic sheets

The intertwining of strands into 3D spirals is ubiquitous in biology, enabling functions from information storage to maintenance of cell structure and directed locomotion. In synthetic systems, entwined fibers can provide superior mechanical properties and act as artificial muscle or structural reinforcements. Unlike structures in nature, the entwinement of synthetic materials typically requires application of an external stimulus, such as mechanical actuation, light, or a magnetic field. Herein, we use computational modeling to design microscale sheets that mimic biology by transducing chemical energy into mechanical action, and thereby self-organize and interlink into 3D spirals, which spontaneously rotate. These flexible sheets are immersed in a fluid-filled microchamber that encompasses an immobilized patch of catalysts on the bottom wall. The sheets themselves can be passive or active (coated with catalyst). Catalytic reactions in the solution generate products that occupy different volumes than the reactants. The resulting density variations exert a force on the fluid (solutal buoyancy force) that causes motion, which in turn drives the interlinking and collective swirling of the sheets. The individual sheets do not rotate; rotation only occurs when the sheets are interlinked. This level of autonomous, coordinated 3D structural organization, intertwining, and rotation is unexpected in synthetic materials systems operating without external controls. Using physical arguments, we identify dimensionless ratios that are useful in scaling these ideas to other systems. These findings are valuable for creating materials that act as "machines", and directing soft matter to undergo self-sustained, multistep assembly that is governed by intrinsic chemical reactions.

Heterogeneous Synthetic Vesicles toward Artificial Cells: Engineering Structure and Composition of Membranes for Multimodal Functionalities

The desire to develop artificial cells to imitate living cells in synthetic vesicle platforms has continuously increased over the past few decades. In particular, heterogeneous synthetic vesicles made from two or more building blocks have attracted attention for artificial cell applications based on their multifunctional modules with asymmetric structures. In addition to the traditional liposomes or polymersomes, polypeptides and proteins have recently been highlighted as potential building blocks to construct artificial cells owing to their specific biological functionalities. Incorporating one or more functionally folded, globular protein into synthetic vesicles enables more cell-like functions mediated by proteins. This Review highlights the recent research about synthetic vesicles toward artificial cell models, from traditional synthetic vesicles to protein-assembled vesicles with asymmetric structures. We aim to provide fundamental and practical insights into applying knowledge on molecular self-assembly to the bottom-up construction of artificial cell platforms with heterogeneous building blocks.

Enzyme-catalyzed cascade reactions on multienzyme proteinosomes

In this research, to mimic the structures and the functionalities of the organelles in living cells multienzyme proteinosomes with β-galactosidase (β-gal), glucose oxidase (GOx) and horseradish peroxidase (HRP) on the surfaces are fabricated by hydrophobic-interaction induced self-assembly approach. To investigate the mechanism of the formation of proteinosomes, poly(di(ethylene glycol) methyl ether methacrylate) (PDEGMA) and bovine serum albumin are employed in a model system and the study demonstrates that the hydrophobic interaction between the dehydrated polymer chains and the hydrophobic patches on the proteins plays a key role in the fabrication of the proteinosomes. Based on the model system, multienzyme proteinosomes with β-gal, GOx and HRP on the surfaces are fabricated through hydrophobic interaction between PDEGMA and enzyme molecules. Enzyme-catalyzed cascade reactions are performed on the surfaces of the proteinosomes, and the immobilized enzymes show higher bioactivities than the "free" enzymes, due to the direct transfer of the product as a substrate from one enzyme molecule to another. This research provides a unique method for the synthesis of multienzyme proteinosomes with improved bioactivities, and the biofunctional structures will find promising applications in medical and biological science.

Self-assembling systems comprising intrinsically disordered protein polymers like elastin-like recombinamers

Despite lacking cooperatively folded structures under native conditions, numerous intrinsically disordered proteins (IDPs) nevertheless have great functional importance. These IDPs are hybrids containing both ordered and intrinsically disordered protein regions (IDPRs), the structure of which is highly flexible in this unfolded state. The conformational flexibility of these disordered systems favors transitions between disordered and ordered states triggered by intrinsic and extrinsic factors, folding into different dynamic molecular assemblies to enable proper protein functions. Indeed, prokaryotic enzymes present less disorder than eukaryotic enzymes, thus showing that this disorder is related to functional and structural complexity. Protein-based polymers that mimic these IDPs include the so-called elastin-like polypeptides (ELPs), which are inspired by the composition of natural elastin. Elastin-like recombinamers (ELRs) are ELPs produced using recombinant techniques and which can therefore be tailored for a specific application. One of the most widely used and studied characteristic structures in this field is the pentapeptide (VPGXG)_n . The structural disorder in ELRs probably arises due to the high content of proline and glycine in the ELR backbone, because both these amino acids help to keep the polypeptide structure of elastomers disordered and hydrated. Moreover, the recombinant nature of these systems means that different sequences can be designed, including bioactive domains, to obtain specific structures for each application. Some of these structures, along with their applications as IDPs that self-assemble into functional vesicles or micelles from diblock copolymer ELRs, will be studied in the following sections. The incorporation of additional order- and disorder-promoting peptide/protein domains, such as α-helical coils or β-strands, in the ELR sequence, and their influence on self-assembly, will also be reviewed. In addition, chemically cross-linked systems with controllable order-disorder balance, and their role in biomineralization, will be discussed. Finally, we will review different multivalent IDPs-based coatings and films for different biomedical applications, such as spatially controlled cell adhesion, osseointegration, or biomaterial-associated infection (BAI).

Hierarchical Nanomaterials Assembled from Peptoids and Other Sequence-Defined Synthetic Polymers

In nature, the self-assembly of sequence-specific biopolymers into hierarchical structures plays an essential role in the construction of functional biomaterials. To develop synthetic materials that can mimic and surpass the function of these natural counterparts, various sequence-defined bio- and biomimetic polymers have been developed and exploited as building blocks for hierarchical self-assembly. This review summarizes the recent advances in the molecular self-assembly of hierarchical nanomaterials based on peptoids (or poly-N-substituted glycines) and other sequence-defined synthetic polymers. Modern techniques to monitor the assembly mechanisms and characterize the physicochemical properties of these self-assembly systems are highlighted. In addition, discussions about their potential applications in biomedical sciences and renewable energy are also included. This review aims to highlight essential features of sequence-defined synthetic polymers (e.g., high stability and protein-like high-information content) and how these unique features enable the construction of robust biomimetic functional materials with high programmability and predictability, with an emphasis on peptoids and their self-assembled nanomaterials.

Photocrosslinked, Tunable Protein Vesicles for Drug Delivery Applications

Recombinant proteins have emerged as promising building blocks for vesicle self-assembly because of their versatility through genetic manipulation and biocompatibility. Vesicles composed of thermally responsive elastin-like polypeptide (ELP) fusion proteins encapsulate cargo during assembly. However, vesicle stability in physiological environments remains a significant challenge for biofunctional applications. Here, incorporation of an unnatural amino acid, para-azido phenylalanine, into the ELP domain is reported to enable photocrosslinking of protein vesicles and tuning of vesicle size and swelling. The size of the vesicles can be tuned by changing ELP hydrophobicity and ionic strength. Protein vesicles are assessed for their ability to encapsulate doxorubicin and dually deliver doxorubicin and fluorescent protein in vitro as a proof of concept. The resulting photocrosslinkable vesicles made from full-sized, functional proteins show high potential in drug delivery applications, especially for small molecule/protein combination therapies or targeted therapies.

Rapid Chemical Synthesis of Self-Assembling Semi-Synthetic Proteins

The design of well-defined monodispersed self-assembling semi-synthetic proteins is emerging as a promising research avenue. These proteins hold great potential to be used as scaffolds for various protein nanotechnology applications. Currently, there are very few chemical methods reported; however, they suffer from elaborate multistep organic synthesis. Herein, we report a new chemical methodology for the rapid synthesis of a diverse set of semi-synthetic protein families, which include protein amphiphiles, facially amphiphilic protein-dendron conjugates, and pH-sensitive protein-dendron conjugates. This chemical method holds great potential to access a wide variety of semi-synthetic proteins in a short time.

Factors deciding the assembly and thermostability of the DmrB cage

Dihydromethanopterin reductase (DmrB), is a naturally occurring cage protein found in various archaeal and a few bacterial species. It exists as 24mer with cubic geometry where 8 trimeric subunits are present at the corners of each cube. Each trimer is made up of three monomeric units and six FMN, where two molecules of FMN are present at the interface of each monomer. DmrB is involved in the conversion of dihydromethanopterin to tetrahydromethanopterin using FMN as a redox equivalent. In the present study, we have used spectroscopic and biochemical techniques along with complementary bio-informatic work to understand the assembly principles of the DmrB. Our results show a concentration dependant self-assembly of DmrB which is mediated by ionic interactions. The co-factor FMN stabilizes and preserves the secondary and quaternary structure of DmrB against thermal insult, indicating that the higher order assembly of DmrB is very thermostable. Our work provides an interesting piece of information regarding the role of the co-factors in the thermostability of these classes of cage proteins. The understanding of the assembly and disassembly of this thermostable cage would enable the downstream usage of this system in various nano-biotechnological applications.

Chemical pumps and flexible sheets spontaneously form self-regulating oscillators in solution

The synchronization of self-oscillating systems is vital to various biological functions, from the coordinated contraction of heart muscle to the self-organization of slime molds. Through modeling, we design bioinspired materials systems that spontaneously form shape-changing self-oscillators, which communicate to synchronize both their temporal and spatial behavior. Here, catalytic reactions at the bottom of a fluid-filled chamber and on mobile, flexible sheets generate the energy to "pump" the surrounding fluid, which also transports the immersed sheets. The sheets exert a force on the fluid that modifies the flow, which in turn affects the shape and movement of the flexible sheets. This feedback enables a single coated (active) and even an uncoated (passive) sheet to undergo self-oscillation, displaying different oscillatory modes with increases in the catalytic reaction rate. Two sheets (active or passive) introduce excluded volume, steric interactions. This distinctive combination of the hydrodynamic, fluid-structure, and steric interactions causes the sheets to form coupled oscillators, whose motion is synchronized in time and space. We develop a heuristic model that rationalizes this behavior. These coupled self-oscillators exhibit rich and tunable phase dynamics, which depends on the sheets' initial placement, coverage by catalyst and relative size. Moreover, through variations in the reactant concentration, the system can switch between the different oscillatory modes. This breadth of dynamic behavior expands the functionality of the coupled oscillators, enabling soft robots to display a variety of self-sustained, self-regulating moves.

Polymerization-induced proteinosome formation

In recent years, the fabrication of well-organized proteinosomes has been a popular topic due to the potential applications of the structures in materials science and nanotechnology. A big challenge in the fabrication of proteinosomes is to maintain the structures and the functionalities of proteins on the proteinosomes. In this research, a new concept of polymerization-induced formation of proteinosomes is proposed. In thermal dispersion polymerization of N-isopropyl acrylamide (NIPAM) in the presence of bovine serum albumin (BSA), the growing PNIPAM chains experience phase transition from hydrated coils to dehydrated globules, and the dehydrated PNIPAM chains have hydrophobic interaction with BSA, leading to the formation of hollow proteinosomes. Kinetics studies indicate that there is a transition from the homogeneous polymerization of NIPAM in solution to the heterogeneous polymerization in the proteinosomes. Transmission electron microscopy, atomic force microscopy, confocal laser scanning microscopy and dynamic light scattering all demonstrate the formation of hollow structures. The results of circular dichroism spectroscopy indicate that the secondary structure of BSA remains unchanged in the polymerization process. The formation of proteinosomes is reversible. Upon cooling of the solution to a temperature below the phase transition temperature of PNIPAM, the proteinosomes are dissociated due to the absence of the hydrophobic interaction. The proteinosomes can be used in the encapsulation of hydrophilic compounds in aqueous solution. In this research, not only BSA but also ovalbumin (OVA) is used as a model protein for the fabrication of proteinosomes by the polymerization-induced approach.

In search of a novel chassis material for synthetic cells: emergence of synthetic peptide compartment

Giant lipid vesicles have been used extensively as a synthetic cell model to recapitulate various life-like processes, including in vitro protein synthesis, DNA replication, and cytoskeleton organization. Cell-sized lipid vesicles are mechanically fragile in nature and prone to rupture due to osmotic stress, which limits their usability. Recently, peptide vesicles have been introduced as a synthetic cell model that would potentially overcome the aforementioned limitations. Peptide vesicles are robust, reasonably more stable than lipid vesicles and can withstand harsh conditions including pH, thermal, and osmotic variations. This mini-review summarizes the current state-of-the-art in the design, engineering, and realization of peptide-based chassis materials, including both experimental and computational work. We present an outlook for simulation-aided and data-driven design and experimental realization of engineered and multifunctional synthetic cells.

Engineering spatiotemporal organization and dynamics in synthetic cells

Constructing synthetic cells has recently become an appealing area of research. Decades of research in biochemistry and cell biology have amassed detailed part lists of components involved in various cellular processes. Nevertheless, recreating any cellular process in vitro in cell-sized compartments remains ambitious and challenging. Two broad features or principles are key to the development of synthetic cells-compartmentalization and self-organization/spatiotemporal dynamics. In this review article, we discuss the current state of the art and research trends in the engineering of synthetic cell membranes, development of internal compartmentalization, reconstitution of self-organizing dynamics, and integration of activities across scales of space and time. We also identify some research areas that could play a major role in advancing the impact and utility of engineered synthetic cells. This article is categorized under: Biology-Inspired Nanomaterials > Lipid-Based Structures Biology-Inspired Nanomaterials > Protein and Virus-Based Structures.

Programmed and Sequential Disassembly of Multi-responsive Supramolecular Protein Nanoassemblies: A Detailed Mechanistic Investigation

The rational design of a multi-responsive protein-based supramolecular system that can predictably respond to more than one stimulus remains an essential but highly challenging goal in biomolecular engineering. Herein, we report a novel chemical method for the construction of multi-responsive supramolecular nanoassemblies using custom-designed facially amphiphilic monodisperse protein-dendron bioconjugates. The macromolecular synthons contain a globular hydrophilic protein domain site-specifically conjugated to photo-responsive hydrophobic benzyl-ether dendrons of different generations through oligo(ethylene glycol) linkers of defined length. The size of the protein nanoassemblies can be systematically tuned by choosing an appropriate dendron or linker of defined length. Exposure of protein nanoassemblies to light results in partial rather than complete disassembly of the complex. The newly formed protein nanoparticle no longer responds to light but could be disassembled into constitutive monomers under acidic conditions or by further treatment with a small molecule. More interestingly, the distribution ratio of the assembled versus disassembled states of protein nanoassemblies after photochemical reaction does not depend on dendron generation, the nature of the linker functionality or the identity of the protein, but is heavily influenced by the linker length. In sum, this work discloses a new chemical method for the rational design of a monodisperse multi-responsive protein-based supramolecular system with exquisite control over the disassembly process.

Tuning the Structural Integrity and Mechanical Properties of Globular Protein Vesicles by Blending Crosslinkable and NonCrosslinkable Building Blocks

Vesicles made from functionally folded, globular proteins that perform specific biological activities, such as catalysis, sensing, or therapeutics, show potential applications as artificial cells, microbioreactors, or protein drug delivery vehicles. The mechanical properties of vesicle membranes, including the elastic modulus and hardness, play a critical role in dictating the stability and shape transformation of the vesicles under external stimuli triggers. Herein, we have developed a strategy to tune the mechanical properties and integrity of globular protein vesicle (GPV) membranes of which building molecules are recombinant fusion protein complexes: a mCherry fused with an acidic leucine zipper (mCherry-Z_E) and a basic leucine zipper fused with an elastin-like polypeptide (Z_R-ELP). To control the mechanical properties of GPVs, we introduced a nonstandard amino acid (para-azidophenylalanine (pAzF)) into the ELP domains (ELP-X), which enabled the creation of crosslinked vesicles under ultraviolet (UV) irradiation. Crosslinked GPVs made from mCherry-Z_E/Z_R-ELP-X complexes presented higher stability than noncrosslinked GPVs under hypotonic osmotic stress. The degree of swelling of GPVs increased as less crosslinking was achieved in the vesicle membranes, which resulted in the disassembly of GPVs into membraneless coacervates. Nanoindentation by atomic force microscopy (AFM) confirmed that the stiffness and Young's elastic modulus of GPVs increase as the blending molar ratio of Z_R-ELP-X to Z_R-ELP increases to make vesicles. The results obtained in this study suggest a rational design to make GPVs with tunable mechanical properties for target applications by simply varying the blending ratio of Z_R-ELP and Z_R-ELP-X in the vesicle self-assembly.

Electrostatic assisted fabrication and dissociation of multi-component proteinosomes

Self-assembly of proteins into well-organized proteinosomes has attracted great interest due to the potential medical and biological applications of the structures. Herein, a new concept of electrostatic assisted fabrication of proteinosomes is proposed. The self-assembly is performed by using multi-step dialysis approach, where negatively charged bovine serum albumin-poly(N-isopropylacrylamide) (BSA-PNIPAM) bioconjugate and positively charged enzyme (lysozyme or trypsin) are initially dissolved in phosphate buffer (PB) solution at a high salt concentration, and subsequently the protein solution is dialyzed against PB solutions at low salt concentrations, resulting in the formation of biofunctional proteinosomes. Transmission electron microscopy (TEM), cryo-TEM and light scattering results all demonstrate the formation of hollow structures. The wall of a proteinosome is composed of BSA and enzyme (lysozyme or trypsin), and PNIPAM chains of the bioconjugate are in the corona stabilizing the structure. In comparison with the native enzymes, the enzyme molecules in the assemblies basically retain their bioactivities. The proteinosomes formed by BSA-PNIPAM and lysozyme can be dissociated in the presence of trypsin, and those self-assembled by BSA-PNIPAM and trypsin are able to be self-hydrolyzed, resulting in the dissociation of the structures in aqueous solution. The size and morphology changes of the proteinosomes in the hydrolysis are studied.

Protein Vesicles Self-Assembled from Functional Globular Proteins with Different Charge and Size

Protein vesicles can be synthesized by mixing two fusion proteins: an elastin-like polypeptide (ELP) fused to an arginine-rich leucine zipper (Z_R) with a globular, soluble protein fused to a glutamate-rich leucine zipper (Z_E). Currently, only fluorescent proteins have been incorporated into vesicles; however, for protein vesicles to be useful for biocatalysis, drug delivery, or biosensing, vesicles must assemble from functional proteins that span an array of properties and functionalities. In this work, the globular protein was systematically changed to determine the effects of the surface charge and size on the self-assembly of protein vesicles. The formation of microphases, which included vesicles, coacervates, and hybrid structures, was monitored at different assembly conditions to determine the phase space for each globular protein. The results show that the protein surface charge has a small effect on vesicle self-assembly. However, increasing the size of the globular protein decreases the vesicle size and increases the stability at lower Z_E/Z_R molar ratios. The phase diagrams created can be used as guidelines to incorporate new functional proteins into vesicles. Furthermore, this work reports catalytically active enzyme vesicles, demonstrating the potential for the application of vesicles as biocatalysts or biosensors.

Bioinspired Protein-Based Assembling: Toward Advanced Life-Like Behaviors

The ability of living organisms to perform structure, energy, and information-related processes for molecular self-assembly through compartmentalization and chemical transformation can possibly be mimicked via artificial cell models. Recent progress in the development of various types of functional microcompartmentalized ensembles that can imitate rudimentary aspects of living cells has refocused attention on the important question of how inanimate systems can transition into living matter. Hence, herein, the most recent advances in the construction of protein-bounded microcompartments (proteinosomes), which have been exploited as a versatile synthetic chassis for integrating a wide range of functional components and biochemical machineries, are critically summarized. The techniques developed for fabricating various types of proteinosomes are discussed, focusing on the significance of how chemical information, substance transportation, enzymatic-reaction-based metabolism, and self-organization can be integrated and recursively exploited in constructed ensembles. Therefore, proteinosomes capable of exhibiting gene-directed protein synthesis, modulated membrane permeability, spatially confined membrane-gated catalytic reaction, internalized cytoskeletal-like matrix assembly, on-demand compartmentalization, and predatory-like chemical communication in artificial cell communities are specially highlighted. These developments are expected to bridge the gap between materials science and life science, and offer a theoretical foundation for developing life-inspired assembled materials toward various applications.

Engineering the Architecture of Elastin-Like Polypeptides: From Unimers to Hierarchical Self-Assembly

Well-defined tunable nanostructures formed through the hierarchical self-assembly of peptide building blocks have drawn significant attention due to their potential applications in biomedical science. Artificial protein polymers derived from elastin-like polypeptides (ELPs), which are based on the repeating sequence of tropoelastin (the water-soluble precursor to elastin), provide a promising platform for creating nanostructures due to their biocompatibility, ease of synthesis, and customizable architecture. By designing the sequence and composition of ELPs at the gene level, their physicochemical properties can be controlled to a degree that is unmatched by synthetic polymers. A variety of ELP-based nanostructures are designed, inspired by the self-assembly of elastin and other proteins in biological systems. The choice of building blocks determines not only the physical properties of the nanostructures, but also their self-assembly into architectures ranging from spherical micelles to elongated nanofibers. This review focuses on the molecular determinants of ELP and ELP-hybrid self-assembly and formation of spherical, rod-like, worm-like, fibrillar, and vesicle architectures. A brief discussion of the potential biomedical applications of these supramolecular assemblies is also included.

Biopolymer Microparticles Prepared by Microfluidics for Biomedical Applications

Biopolymers are macromolecules that are derived from natural sources and have attractive properties for a plethora of biomedical applications due to their biocompatibility, biodegradability, low antigenicity, and high bioactivity. Microfluidics has emerged as a powerful approach for fabricating polymeric microparticles (MPs) with designed structures and compositions through precise manipulation of multiphasic flows at the microscale. The synergistic combination of materials chemistry afforded by biopolymers and precision provided by microfluidic capabilities make it possible to design engineered biopolymer-based MPs with well-defined physicochemical properties that are capable of enabling an efficient delivery of therapeutics, 3D culture of cells, and sensing of biomolecules. Here, an overview of microfluidic approaches is provided for the design and fabrication of functional MPs from three classes of biopolymers including polysaccharides, proteins, and microbial polymers, and their advances for biomedical applications are highlighted. An outlook into the future research on microfluidically-produced biopolymer MPs for biomedical applications is also provided.

Magnifying the Structural Components of Biomembranes: A Prototype for the Study of the Self-Assembly of Giant Lipids

How biomembranes are self-organized to perform their functions remains a pivotal issue in biological and chemical science. Understanding the self-assembly principles of lipid-like molecules hence becomes crucial. Herein, we report the mesostructural evolution of amphiphilic sphere-rod conjugates (giant lipids), and study the roles of geometric parameters (head-tail ratio and cross-sectional area) during this course. As a prototype system, giant lipids resemble natural lipidic molecules by capturing their essential features. The self-assembly behavior of two categories of giant lipids (I-shape and T-shape, a total of 8 molecules) is demonstrated. A rich variety of mesostructures is constructed in solution state and their molecular packing models are rationally understood. Giant lipids recast the phase behavior of natural lipids to a certain degree and the abundant self-assembled morphologies reveal distinct physiochemical behaviors when geometric parameters deviate from natural analogues.

Synthetic Protein Scaffolding at Biological Membranes

Protein scaffolding is a natural phenomenon whereby proteins colocalize into macromolecular complexes via specific protein-protein interactions. In the case of metabolic enzymes, protein scaffolding drives metabolic flux through specific pathways by colocalizing enzyme active sites. Synthetic protein scaffolding is increasingly used as a mechanism to improve product specificity and yields in metabolic engineering projects. To date, synthetic scaffolding has focused primarily on soluble enzyme systems, but many metabolic pathways for high-value secondary metabolites depend on membrane-bound enzymes. The compositional diversity of biological membranes and general challenges associated with modifying membrane proteins complicate scaffolding with membrane-requiring enzymes. Several recent studies have introduced new approaches to protein scaffolding at membrane surfaces, with notable success in improving product yields from specific metabolic pathways.

Engineering the Echogenic Properties of Microfluidic Microbubbles Using Mixtures of Recombinant Protein and Amphiphilic Copolymers

Microbubbles are used as ultrasound contrast agents in medical diagnosis and also have shown great promise in ultrasound-mediated therapy. However, short lifetime and broad size distribution of microbubbles limit their applications in therapy and imaging. Moreover, it is challenging to tailor the echogenic response of microbubbles to make them suitable for specific applications. To overcome these challenges, we use microfluidic flow-focusing to prepare monodisperse microbubbles with a mixture of a recombinant amphiphilic protein, oleosin, and a synthetic amphiphilic copolymer, Pluronic. We show that these microbubbles have superior uniformity and stability under ultrasonic stimulation compared to commercial agents. We also demonstrate that by using different Pluronics, the echogenic response of the microbubbles can be tailored. Our work shows the versatility of using the combination of microfluidics and protein/copolymer mixtures as a method of engineering microbubbles. This tunability could potentially be important and powerful in producing microbubble agents for theranostic applications.

Self-Assembly Toolbox of Tailored Supramolecular Architectures Based on an Amphiphilic Protein Library

The molecular structuring of complex architectures and the enclosure of space are essential requirements for technical and living systems. Self-assembly of supramolecular structures with desired shape, size, and stability remains challenging since it requires precise regulation of physicochemical and conformational properties of the components. Here a general platform for controlled self-assembly of tailored amphiphilic elastin-like proteins into desired supramolecular protein assemblies ranging from spherical coacervates over molecularly defined twisted fibers to stable unilamellar vesicles is introduced. The described assembly protocols efficiently yield protein membrane-based compartments (PMBC) with adjustable size, stability, and net surface charge. PMBCs demonstrate membrane fusion and phase separation behavior and are able to encapsulate structurally and chemically diverse cargo molecules ranging from small molecules to naturally folded proteins. The ability to engineer tailored supramolecular architectures with defined fusion behavior, tunable properties, and encapsulated cargo paves the road for novel drug delivery systems, the design of artificial cells, and confined catalytic nanofactories.

Synthetic Biology for Multiscale Designed Biomimetic Assemblies: From Designed Self-Assembling Biopolymers to Bacterial Bioprinting

Nature is based on complex self-assembling systems that span from the nanoscale to the macroscale. We have already begun to design biomimetic systems with properties that have not evolved in nature, based on designed molecular interactions and regulation of biological systems. Synthetic biology is based on the principle of modularity, repurposing diverse building modules to design new types of molecular and cellular assemblies. While we are currently able to use techniques from synthetic biology to design self-assembling molecules and re-engineer functional cells, we still need to use guided assembly to construct biological assemblies at the macroscale. We review the recent strategies for designing biological systems ranging from molecular assemblies based on self-assembly of (poly)peptides to the guided assembly of patterned bacteria, spanning 7 orders of magnitude.

Enhanced Cell Killing by Paclitaxel-Loaded Recombinant Protein Micelles Bearing Integrin-Binding and Cell-Penetrating Peptides

Peptide ligands are effective and specific vectors that can target cell surface receptors, and have shown great potential for targeting drug delivery vehicles. Often, materials used as drug delivery matrices are chemically synthesized and difficult to functionalize, which compromises their development as smart drug carriers. Here, we assemble carriers from a recombinant protein as a novel approach to overcome these limitations. We have previously shown that oleosin, a natural surfactant protein, can be engineered to self-assemble into spherical micelles, and that functionalizing oleosin with RGDS can increase cellular uptake in integrin bearing cells. Here, we investigated whether we could further enhance cellular by incorporating either a RGDS synergy peptide PHSRN or a cell-penetrating Tat peptide derived from human immunodeficiency virus (HIV). The resulting modified oleosins self-assemble into spherical micelles in aqueous environments. Spherical micelles made from oleosin can effectively encapsulate the hydrophobic chemotherapeutic drug paclitaxel (PX). After 15 hours, 350 nM PX loaded oleosin micelles equipped with both RGDS and Tat increased cell killing by twofold compared to free paclitaxel, and 1.2-fold compared to micelles made from RGD-oleosin alone. Micelles equipped with PHSRN alone does not facilitate cell killing compared to free paclitaxel, whereas micelles equipped with both PHSRN and RGDS increased cell killing by 1.1 fold compared to micelles with RGDS alone in 15 hours. Therefore, incorporating multiple motifs into oleosin is an approach for candidate for making a versatile drug delivery carrier.

Diblock copolymers enhance folding of a mechanosensitive membrane protein during cell-free expression

Membrane protein folding is a critical step that underlies proper cellular function as well as the design of technologies like vesicle-based biosensors and artificial cells. Membrane composition is known to play a role in membrane protein folding; however, the specific mechanical properties of membranes that govern protein folding remain unclear. Using a highly elastic nonnatural amphiphile, we highlight the importance of a membrane mechanical property, membrane elasticity, on the spontaneous insertion and folding of a model α-helical membrane protein. Through this study, we gain a deeper understanding of cellular membrane protein folding and offer a potential approach to improve the production of membrane proteins through optimizing the mechanical properties of synthetic scaffolds present in cell-free reactions.

The expression and integration of membrane proteins into vesicle membranes is a critical step in the design of cell-mimetic biosensors, bioreactors, and artificial cells. While membrane proteins have been integrated into a variety of nonnatural membranes, the effects of the chemical and physical properties of these vesicle membranes on protein behavior remain largely unknown. Nonnatural amphiphiles, such as diblock copolymers, provide an interface that can be synthetically controlled to better investigate this relationship. Here, we focus on the initial step in a membrane protein’s life cycle: expression and folding. We observe improvements in both the folding and overall production of a model mechanosensitive channel protein, the mechanosensitive channel of large conductance, during cell-free reactions when vesicles containing diblock copolymers are present. By systematically tuning the membrane composition of vesicles through incorporation of a poly(ethylene oxide)-b-poly(butadiene) diblock copolymer, we show that membrane protein folding and production can be improved over that observed in traditional lipid vesicles. We then reproduce this effect with an alternate membrane-elasticizing molecule, C₁₂E₈. Our results suggest that global membrane physical properties, specifically available membrane surface area and the membrane area expansion modulus, significantly influence the folding and yield of a membrane protein. Furthermore, our results set the stage for explorations into how nonnatural membrane amphiphiles can be used to both study and enhance the production of biological membrane proteins.

Biomolecular Assemblies: Moving from Observation to Predictive Design

Biomolecular assembly is a key driving force in nearly all life processes, providing structure, information storage, and communication within cells and at the whole organism level. These assembly processes rely on precise interactions between functional groups on nucleic acids, proteins, carbohydrates, and small molecules, and can be fine-tuned to span a range of time, length, and complexity scales. Recognizing the power of these motifs, researchers have sought to emulate and engineer biomolecular assemblies in the laboratory, with goals ranging from modulating cellular function to the creation of new polymeric materials. In most cases, engineering efforts are inspired or informed by understanding the structure and properties of naturally occurring assemblies, which has in turn fueled the development of predictive models that enable computational design of novel assemblies. This Review will focus on selected examples of protein assemblies, highlighting the story arc from initial discovery of an assembly, through initial engineering attempts, toward the ultimate goal of predictive design. The aim of this Review is to highlight areas where significant progress has been made, as well as to outline remaining challenges, as solving these challenges will be the key that unlocks the full power of biomolecules for advances in technology and medicine.

Redox sensitive protein droplets from recombinant oleosin

Protein engineering enables the creation of materials with designer functionality and tailored responsiveness. Here, we design a protein with two control motifs for its phase separation into micron sized liquid droplets - one driven by a hydrophobic domain and the other by oxidation of a disulfide bond. Our work is based on the plant surfactant protein, oleosin, which has a hydrophobic domain but no cysteines. Oleosin phase separates to form liquid droplets below a critical temperature akin to many naturally occurring membrane-less organelles. Sequence mutations are made to introduce a cysteine residue into oleosin. The addition of a cysteine causes phase separation at a lower concentration and increases the phase transition temperature. Adding a reducing agent to phase-separated, cysteine-containing oleosin rapidly dissolves the droplets. The transition temperature is tuned by varying the location of the cysteine or by blending the parent cysteine-less molecule with the cysteine-containing mutant. This provides a novel way to control protein droplet formation and dissolution. We envision this work having applications as a system for the release of a protein or drug with engineered sensitivity to reducing conditions and as a mimic of membrane-less organelles in synthetic protocells.

Adhesion Properties of Freestanding Hydrophobin Bilayers

Hydrophobins are a family of small-sized proteins featuring a distinct hydrophobic patch on the protein's surface, rendering them amphiphilic. This particularity allows hydrophobins to self-assemble into monolayers at any hydrophilic/hydrophobic interface. Moreover, stable pure protein bilayers can be created from two interfacial hydrophobin monolayers by contacting either their hydrophobic or their hydrophilic sides. In this study, this is achieved via a microfluidic approach, in which also the bilayers' adhesion energy can be determined. This enables us to study the origin of the adhesion of hydrophobic and hydrophilic core bilayers made from the class II hydrophobins HFBI and HFBII. Using different fluid media in this setup and introducing genetically modified variants of the HFBI molecule, the different force contributions to the adhesion of the bilayer sheets are studied. It was found that in the hydrophilic contact situation, the adhesive interaction was higher than that in the hydrophobic contact situation and could be even enhanced by reducing the contributions of electrostatic interactions. This effect indicates that the van der Waals interaction is the dominant contribution that explains the stability of the observed bilayers.