Complete Sequences of the Velvet Worm Slime Proteins Reveal that Slime Formation is Enabled by Disulfide Bonds and Intrinsically Disordered Regions

Exploring Stimuli-Responsive Natural Processes for the Fabrication of High-Performance Materials

Climate change and environmental pollution have underscored the urgency for more sustainable alternatives in synthetic polymer production. Nature's repertoire of biopolymers with excellent multifaceted properties alongside biodegradability could inspire next-generation innovative green polymer fabrication routes. Stimuli-induced processing, driven by changes in environmental factors, such as pH, ionic strength, and mechanical forces, plays a crucial role in natural polymeric self-assembly process. This perspective aims to close the gap in understanding biopolymer formation by highlighting the essential role of stimuli triggers in facilitating the bottom-up fabrication, allowing for the formation of intricate hierarchical structures. In particular, this perspective will delve into the stimuli-responsive processing of high-performance biopolymers produced by mussels, caddisflies, velvet worms, sharks, whelks, and squids, which are known for their robust mechanical properties, durability, and wet adhesion capabilities. Finally, we provide an overview of current advancements and challenges in understanding stimuli-induced natural formation pathways and their translation to biomimetic materials.

Proton conductivity of the protein-based velvet worm slime

The properties of complex bodily fluids are linked to their biological functions through natural selection. Velvet worms capture their prey by ensnaring them with a proteinaceous fluid (slime). We examined the electrical conductivity of slime and found that dry slime is an insulator. However, its conductivity can increase by up to 106 times in its hydrated state, which can be further increased by an order in magnitude under acidic hydration (pH ≈ 2.3). The transient current measured using ion-blocking electrodes showed a continuous decay for up to 7 h, revealing slime's nature as a proton conducting material. Slime undergoes a spontaneous fibrilization process producing high aspect ratio ≈ 105 fibers that exhibit an average conductivity ≈2.4 ± 1.1 mS cm-1. These findings enhance our understanding of slime as a natural biopolymer and provide molecular-level guidelines to rationally design biomaterials that may be employed as hygroscopic conductors.

Engineering of a Fluorescent Protein for a Sensing of an Intrinsically Disordered Protein through Transition in the Chromophore State

Intrinsically disordered proteins (IDPs) not only play important roles in biological processes but are also linked with the pathogenesis of various human diseases. Specific and reliable sensing of IDPs is crucial for exploring their roles but remains elusive due to structural plasticity. Here, we present the development of a new type of fluorescent protein for the ratiometric sensing and tracking of an IDP. A β-strand of green fluorescent protein (GFP) was truncated, and the resulting GFP was further engineered to undergo the transition in the absorption maximum upon binding of a target motif within amyloid-β (Aβ) as a model IDP through rational design and directed evolution. Spectroscopic and structural analyses of the engineered truncated GFP demonstrated that a shift in the absorption maximum is driven by the change in the chromophore state from an anionic (460 nm) state into a neutral (390 nm) state as the Aβ binds, allowing a ratiometric detection of Aβ. The utility of the developed GFP was shown by the efficient and specific detection of an Aβ and the tracking of its conformational change and localization in astrocytes.

Peculiar Phosphonate Modifications of Velvet Worm Slime Revealed by Advanced Nuclear Magnetic Resonance and Mass Spectrometry

Nature is rich with examples of highly specialized biological materials produced by organisms for functions, including defense, hunting, and protection. Along these lines, velvet worms (Onychophora) expel a protein-based slime used for hunting and defense that upon shearing and dehydration forms fibers as stiff as thermoplastics. These fibers can dissolve back into their precursor proteins in water, after which they can be drawn into new fibers, providing biological inspiration to design recyclable materials. Elevated phosphorus content in velvet worm slime was previously observed and putatively ascribed to protein phosphorylation. Here, we show instead that phosphorus is primarily present as phosphonate moieties in the slime of distantly related velvet worm species. Using high-resolution nuclear magnetic resonance (NMR), natural abundance dynamic nuclear polarization (DNP), and mass spectrometry (MS), we demonstrate that 2-aminoethyl phosphonate (2-AEP) is associated with glycans linked to large slime proteins, while transcriptomic analyses confirm the expression of 2-AEP synthesizing enzymes in slime glands. The evolutionary conservation of this rare protein modification suggests an essential functional role of phosphonates in velvet worm slime and should stimulate further study of the function of this unusual chemical modification in nature.

The Internal Structure of the Velvet Worm Projectile Slime: A Small-Angle Scattering Study

For prey capture and defense, velvet worms eject an adhesive slime which has been established as a model system for recyclable complex liquids. Triggered by mechanical agitation, the liquid bio-adhesive rapidly transitions into solid fibers. In order to understand this mechanoresponsive behavior, here, the nanostructural organization of slime components are studied using small-angle scattering with neutrons and X-rays. The scattering intensities are successfully described with a three-component model accounting for proteins of two dominant molecular weight fractions and nanoscale globules. In contrast to the previous assumption that high molecular weight proteins-the presumed building blocks of the fiber core-are contained in the nanoglobules, it is found that the majority of slime proteins exist freely in solution. Only less than 10% of the slime proteins are contained in the nanoglobules, necessitating a reassessment of their function in fiber formation. Comparing scattering data of slime re-hydrated with light and heavy water reveals that the majority of lipids in slime are contained in the nanoglobules with homogeneous distribution. Vibrating mechanical impact under exclusion of air neither leads to formation of fibers nor alters the bulk structure of slime significantly, suggesting that interfacial phenomena and directional shearing are required for fiber formation.

Protein-Based Biological Materials: Molecular Design and Artificial Production

Polymeric materials produced from fossil fuels have been intimately linked to the development of industrial activities in the 20th century and, consequently, to the transformation of our way of living. While this has brought many benefits, the fabrication and disposal of these materials is bringing enormous sustainable challenges. Thus, materials that are produced in a more sustainable fashion and whose degradation products are harmless to the environment are urgently needed. Natural biopolymers─which can compete with and sometimes surpass the performance of synthetic polymers─provide a great source of inspiration. They are made of natural chemicals, under benign environmental conditions, and their degradation products are harmless. Before these materials can be synthetically replicated, it is essential to elucidate their chemical design and biofabrication. For protein-based materials, this means obtaining the complete sequences of the proteinaceous building blocks, a task that historically took decades of research. Thus, we start this review with a historical perspective on early efforts to obtain the primary sequences of load-bearing proteins, followed by the latest developments in sequencing and proteomic technologies that have greatly accelerated sequencing of extracellular proteins. Next, four main classes of protein materials are presented, namely fibrous materials, bioelastomers exhibiting high reversible deformability, hard bulk materials, and biological adhesives. In each class, we focus on the design at the primary and secondary structure levels and discuss their interplays with the mechanical response. We finally discuss earlier and the latest research to artificially produce protein-based materials using biotechnology and synthetic biology, including current developments by start-up companies to scale-up the production of proteinaceous materials in an economically viable manner.

Biological Materials Processing: Time-Tested Tricks for Sustainable Fiber Fabrication

There is an urgent need to improve the sustainability of the materials we produce and use. Here, we explore what humans can learn from nature about how to sustainably fabricate polymeric fibers with excellent material properties by reviewing the physical and chemical aspects of materials processing distilled from diverse model systems, including spider silk, mussel byssus, velvet worm slime, hagfish slime, and mistletoe viscin. We identify common and divergent strategies, highlighting the potential for bioinspired design and technology transfer. Despite the diversity of the biopolymeric fibers surveyed, we identify several common strategies across multiple systems, including: (1) use of stimuli-responsive biomolecular building blocks, (2) use of concentrated fluid precursor phases (e.g., coacervates and liquid crystals) stored under controlled chemical conditions, and (3) use of chemical (pH, salt concentration, redox chemistry) and physical (mechanical shear, extensional flow) stimuli to trigger the transition from fluid precursor to solid material. Importantly, because these materials largely form and function outside of the body of the organisms, these principles can more easily be transferred for bioinspired design in synthetic systems. We end the review by discussing ongoing efforts and challenges to mimic biological model systems, with a particular focus on artificial spider silks and mussel-inspired materials.

Modulation of Mechanical Properties of Short Bioinspired Peptide Materials by Single Amino-Acid Mutations

The occurrence of modular peptide repeats in load-bearing (structural) proteins is common in nature, with distinctive peptide sequences that often remain conserved across different phylogenetic lineages. These highly conserved peptide sequences endow specific mechanical properties to the material, such as toughness or elasticity. Here, using bioinformatic tools and phylogenetic analysis, we have identified the GX8 peptide with the sequence GLYGGYGX (where X can be any residue) in a wide range of organisms. By simple mutation of the X residue, we demonstrate that GX8 can be self-assembled into various supramolecular structures, exhibiting vastly different physicochemical and viscoelastic properties, from liquid-like coacervate microdroplets to hydrogels to stiff solid materials. A combination of spectroscopic, electron microscopy, mechanical, and molecular dynamics studies is employed to obtain insights into molecular scale interactions driving self-assembly of GX8 peptides, underscoring that π-π stacking and hydrophobic interactions are the drivers of peptide self-assembly, whereas the X residue determines the extent of hydrogen bonding that regulates the macroscopic mechanical response. This study highlights the ability of single amino-acid polymorphism to tune the supramolecular assembly and bulk material properties of GX8 peptides, enabling us to cover a broad range of potential biomedical applications such as hydrogels for tissue engineering or coacervates for drug delivery.