The dimerization mechanism of the N-terminal domain of spider silk proteins is conserved despite extensive sequence divergence

A Minispidroin Guides the Molecular Design for Cellular Condensation Mechanisms in S. cerevisiae

Structural engineering of molecules for condensation is an emerging technique within synthetic biology. Liquid-liquid phase separation of biomolecules leading to condensation is a central step in the assembly of biological materials into their functional forms. Intracellular condensates can also function within cells in a regulatory manner to facilitate reaction pathways and to compartmentalize interactions. We need to develop a strong understanding of how to design molecules for condensates and how their in vivo-in vitro properties are related. The spider silk protein NT2RepCT undergoes condensation during its fiber-forming process. Using parallel in vivo and in vitro characterization, in this study, we mapped the effects of intracellular conditions for NT2RepCT and its several structural variants. We found that intracellular conditions may suppress to some extent condensation whereas molecular crowding affects both condensate properties and their formation. Intracellular characterization of protein condensation allowed experiments on pH effects and solubilization to be performed within yeast cells. The growth of intracellular NT2RepCT condensates was restricted, and Ostwald ripening was not observed in yeast cells, in contrast to earlier observations in E. coli. Our results lead the way to using intracellular condensation to screen for properties of molecular assembly. For characterizing different structural variants, intracellular functional characterization can eliminate the need for time-consuming batch purification and in vitro condensation. Therefore, we suggest that the in vivo-in vitro understanding will become useful in, e.g., high-throughput screening for molecular functions and in strategies for designing tunable intracellular condensates.

Disabling spidroin N-terminal homologs' reverse reaction unveils why its intermolecular disulfide bonds have not evolved for 380 million years

Spiders, ubiquitous predators known for their powerful silks, rely on spidroins that self-assemble from high-concentration solutions stored in silk glands, which are mediated by the NT and CT domains. CT homodimers containing intermolecular disulfide bonds enhance silk performance, promoting spider survival and reproduction. However, no NT capable of forming such disulfide bonds has been identified. Our study reveals that NT homodimers with sulfur substitution can form under alkaline conditions, shedding light on why spiders have not evolved intermolecular disulfide bonds in the NT module during their 380 million years of evolution. This discovery significantly advances our comprehension of spider evolution and silk spinning mechanisms, while also providing novel insights into protein storage, assembly, as well as the mechanisms and therapeutic strategies for neurodegenerative diseases associated with protein aggregation.

Factors Influencing Properties of Spider Silk Coatings and Their Interactions within a Biological Environment

Biomaterials are an indispensable part of biomedical research. However, although many materials display suitable application-specific properties, they provide only poor biocompatibility when implanted into a human/animal body leading to inflammation and rejection reactions. Coatings made of spider silk proteins are promising alternatives for various applications since they are biocompatible, non-toxic and anti-inflammatory. Nevertheless, the biological response toward a spider silk coating cannot be generalized. The properties of spider silk coatings are influenced by many factors, including silk source, solvent, the substrate to be coated, pre- and post-treatments and the processing technique. All these factors consequently affect the biological response of the environment and the putative application of the appropriate silk coating. Here, we summarize recently identified factors to be considered before spider silk processing as well as physicochemical characterization methods. Furthermore, we highlight important results of biological evaluations to emphasize the importance of adjustability and adaption to a specific application. Finally, we provide an experimental matrix of parameters to be considered for a specific application and a guided biological response as exemplarily tested with two different fibroblast cell lines.

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.

Classification and functional characterization of spidroin genes in a wandering spider, Pardosa pseudoannulata

Spiders impress us with their sophisticated use of silk and the stunningly distinct silk proteins (spidroins) in each spider species. Understanding how silks and spidroins function and evolve within the spider world is one profound interest to expand our knowledge on spider evolution. Spidroins are characterized with the divergent repeat core region flanked with the relatively conserved N- and C-terminus. The structure and number of the repeats contribute to the unique mechanical properties of the spidroin and the silk. Spidroins have been intensively studied in web-weaver spiders, but information regarding their diversity in wandering spiders remains scarce. Here, twenty spidroin genes were identified in the pond wolf spider, Pardosa pseudoannulata, belonging to the retrolateral tibial apophysis (RTA) clade. These spidroins were categorized into four classes, including twelve ampullate spidroin (AmpSp), four aciniform spidroin (AcSp), one tubuliform spidroin (TuSp), one pyriform spidroin (PiSp), and two spidroin-like proteins. Multiple copies of the AmpSp and AcSp genes were tandemly arranged in a cluster within the genome, and the N-terminal domains and repetitive sequences of the proximately located spidroins were highly similar, suggesting that the spidroin genes diversified via tandem duplication. Only four types of morphologically distinct silk glands were found in P. pseudoannulata, namely Ma, Mi, Ac, and Pi glands, consistent with the glandular affiliation hypothesis that spidroins co-evolved with glandular specialization to fit species-specific needs. Expression profiling revealed that the single tubuliform spidroin (TuSp) gene was highly expressed in gravid females and two AcSp genes displayed synchronous expression. Knock-down of the TuSp gene via RNAi resulted in fragile and cracked eggsacs and prolonged the female pre-oviposition period, validating its importance in spider reproduction. The genome-scale characterization and functional study of spidroin genes allows associating the presence of specific spidroins with silk utility in P. pseudoannulata and will expand our knowledge of spider evolution.

1000 spider silkomes: Linking sequences to silk physical properties

Spider silks are among the toughest known materials and thus provide models for renewable, biodegradable, and sustainable biopolymers. However, the entirety of their diversity still remains elusive, and silks that exceed the performance limits of industrial fibers are constantly being found. We obtained transcriptome assemblies from 1098 species of spiders to comprehensively catalog silk gene sequences and measured the mechanical, thermal, structural, and hydration properties of the dragline silks of 446 species. The combination of these silk protein genotype-phenotype data revealed essential contributions of multicomponent structures with major ampullate spidroin 1 to 3 paralogs in high-performance dragline silks and numerous amino acid motifs contributing to each of the measured properties. We hope that our global sampling, comprehensive testing, integrated analysis, and open data will provide a solid starting point for future biomaterial designs.

Solution Structure of Tubuliform Spidroin N-Terminal Domain and Implications for pH Dependent Dimerization

The spidroin N-terminal domain (NT) is responsible for high solubility and pH-dependent assembly of spider silk proteins during storage and fiber formation, respectively. It forms a monomeric five-helix bundle at neutral pH and dimerizes at lowered pH, thereby firmly interconnecting the spidroins. Mechanistic studies with the NTs from major ampullate, minor ampullate, and flagelliform spidroins (MaSp, MiSp, and FlSp) have shown that the pH dependency is conserved between different silk types, although the residues that mediate this process can differ. Here we study the tubuliform spidroin (TuSp) NT from Argiope argentata, which lacks several well conserved residues involved in the dimerization of other NTs. We solve its structure at low pH revealing an antiparallel dimer of two five-α-helix bundles, which contrasts with a previously determined Nephila antipodiana TuSp NT monomer structure. Further, we study a set of mutants and find that the residues participating in the protonation events during dimerization are different from MaSp and MiSp NT. Charge reversal of one of these residues (R117 in TuSp) results in significantly altered electrostatic interactions between monomer subunits. Altogether, the structure and mutant studies suggest that TuSp NT monomers assemble by elimination of intramolecular repulsive charge interactions, which could lead to slight tilting of α-helices.