Proline-Tyrosine Ring Interactions in Black Widow Dragline Silk Revealed by Solid-State Nuclear Magnetic Resonance and Molecular Dynamics Simulations

Selective one-dimensional 13C-13C spin-diffusion solid-state nuclear magnetic resonance (SSNMR) provides evidence for CH/π ring packing interactions between Pro and Tyr residues in 13C-enriched Latrodectus hesperus dragline silk. The secondary structure of Pro-containing motifs in dragline spider silks consistently points to an elastin-like type II β-turn conformation based on 13C chemical shift analysis. 13C-13C spin diffusion measurements as a function of mixing times allow for the measurement of spatial proximity between the Pro and Tyr rings to be ∼0.5-1 nm, supporting strong Pro-Tyr ring interactions that likely occur through a CH/π mechanism. These results are supported by molecular dynamics (MD) simulations and analysis and reveals new insights into the secondary structure and Pro-Tyr ring stacking interactions for one of nature's toughest biomaterials.

Characterization of the primary structure of the major silk gene, h-fibroin, across caddisfly (Trichoptera) suborders

Larvae of caddisflies (Trichoptera) produce silk to build various underwater structures allowing them to exploit a wide range of aquatic environments. The silk adheres to various substrates underwater and has high tensile strength, extensibility, and toughness and is of interest as a model for biomimetic adhesives. As a step toward understanding how the properties of underwater silk evolved in Trichoptera, we used genomic data to identify full-length sequences and characterize the primary structure of the major silk protein, h-fibroin, across the order. The h-fibroins have conserved termini and basic motif structure with high variation in repeating modules and variation in the percentage of amino acids, mainly proline. This finding might be linked to differences in mechanical properties related to the different silk usage and sets a starting point for future studies to screen and correlate amino acid motifs and other sequence features with quantifiable silk properties.

Predicting mechanical properties of silk from its amino acid sequences via machine learning

The silk fiber is increasingly being sought for its superior mechanical properties, biocompatibility, and eco-friendliness, making it promising as a base material for various applications. One of the characteristics of protein fibers, such as silk, is that their mechanical properties are significantly dependent on the amino acid sequence. Numerous studies have been conducted to determine the specific relationship between the amino acid sequence of silk and its mechanical properties. Still, the relationship between the amino acid sequence of silk and its mechanical properties is yet to be clarified. Other fields have adopted machine learning (ML) to establish a relationship between the inputs, such as the ratio of different input material compositions and the resulting mechanical properties. We have proposed a method to convert the amino acid sequence into numerical values for input and succeeded in predicting the mechanical properties of silk from its amino acid sequences. Our study sheds light on predicting mechanical properties of silk fiber from respective amino acid sequences.

Relating spidroin motif prevalence and periodicity to the mechanical properties of major ampullate spider silks

Spider dragline fibers exhibit incredible mechanical properties, outperforming many synthetic polymers in toughness assays, and possess desirable properties for medical and other human applications. These qualities make dragline fibers popular subjects for biomimetics research. The enormous diversity of spiders presents both an opportunity for the development of new bioinspired materials and a challenge for the identification of fundamental design principles, as the mechanical properties of dragline fibers show both intraspecific and interspecific variations. In this regard, the stress-strain curves of draglines from different species have been shown to be effectively compared by the α* parameter, a value derived from maximum-supercontracted silk fibers. To identify potential molecular mechanisms impacting α* values, here we analyze spider fibroin (spidroin) sequences of the Western black widow (Latrodectus hesperus) and the black and yellow garden spider (Argiope aurantia). This study serves as a primer for investigating the molecular properties of spidroins that underlie species-specific α* values. Initial findings are that while overall motif composition was similar between species, certain motifs and higher level periodicities of glycine-rich region lengths showed variation, notably greater distances between poly-A motifs in A. aurantia sequences. In addition to increased period lengths, A. aurantia spidroins tended to have an increased prevalence of charged and hydrophobic residues. These increases may impact the number and strength of hydrogen bond networks within fibers, which have been implicated in conformational changes and formation of nanocrystals, contributing to the greater extensibility of A. aurantia draglines compared to those of L. hesperus.

Rapid molecular diversification and homogenization of clustered major ampullate silk genes in Argiope garden spiders

The evolutionary diversification of orb-web weaving spiders is closely tied to the mechanical performance of dragline silk. This proteinaceous fiber provides the primary structural framework of orb web architecture, and its extraordinary toughness allows these structures to absorb the high energy of aerial prey impact. The dominant model of dragline silk molecular structure involves the combined function of two highly repetitive, spider-specific, silk genes (spidroins)-MaSp1 and MaSp2. Recent genomic studies, however, have suggested this framework is overly simplistic, and our understanding of how MaSp genes evolve is limited. Here we present a comprehensive analysis of MaSp structural and evolutionary diversity across species of Argiope (garden spiders). This genomic analysis reveals the largest catalog of MaSp genes found in any spider, driven largely by an expansion of MaSp2 genes. The rapid diversification of Argiope MaSp genes, located primarily in a single genomic cluster, is associated with profound changes in silk gene structure. MaSp2 genes, in particular, have evolved complex hierarchically organized repeat units (ensemble repeats) delineated by novel introns that exhibit remarkable evolutionary dynamics. These repetitive introns have arisen independently within the genus, are highly homogenized within a gene, but diverge rapidly between genes. In some cases, these iterated introns are organized in an alternating structure in which every other intron is nearly identical in sequence. We hypothesize that this intron structure has evolved to facilitate homogenization of the coding sequence. We also find evidence of intergenic gene conversion and identify a more diverse array of stereotypical amino acid repeats than previously recognized. Overall, the extreme diversification found among MaSp genes requires changes in the structure-function model of dragline silk performance that focuses on the differential use and interaction among various MaSp paralogs as well as the impact of ensemble repeat structure and different amino acid motifs on mechanical behavior.

Complexity of Spider Dragline Silk

The tiny spider makes dragline silk fibers with unbeatable toughness, all under the most innocuous conditions. Scientists have persistently tried to emulate its natural silk spinning process using recombinant proteins with a view toward creating a new wave of smart materials, yet most efforts have fallen short of attaining the native fiber’s excellent mechanical properties. One reason for these shortcomings may be that artificial spider silk systems tend to be overly simplified and may not sufficiently take into account the true complexity of the underlying protein sequences and of the multidimensional aspects of the natural self-assembly process that give rise to the hierarchically structured fibers. Here, we discuss recent findings regarding the material constituents of spider dragline silk, including novel spidroin subtypes, nonspidroin proteins, and possible involvement of post-translational modifications, which together suggest a complexity that transcends the two-component MaSp1/MaSp2 system. We subsequently consider insights into the spidroin domain functions, structures, and overall mechanisms for the rapid transition from disordered soluble protein into a highly organized fiber, including the possibility of viewing spider silk self-assembly through a framework relevant to biomolecular condensates. Finally, we consider the concept of “biomimetics” as it applies to artificial spider silk production with a focus on key practical aspects of design and evaluation that may hopefully inform efforts to more closely reproduce the remarkable structure and function of the native silk fiber using artificial methods.

Identification and quantification of fatty acids in hunting web of adult Steatoda grossa (Theridiidae) female spiders

This is the first study on composition of fatty acids in hunting web of Steatoda grossa (Theridiidae) spiders and one of only four similar studies ever made. Its main contribution is a discovery that fatty acids not only cover an outside of the web fibers, but they are even more abundantly represented in the fibers’ inner structure. Although little attention has been so far attributed to the contents of fatty acids in spider silks, one has to remember that their biocompatibility combined with an extraordinary tensile strength make them a worth investigating template for material bioengineering studies.

Correlation between protein secondary structure and mechanical performance for the ultra-tough dragline silk of Darwin's bark spider

The spider major ampullate (MA) silk exhibits high tensile strength and extensibility and is typically a blend of MaSp1 and MaSp2 proteins with the latter comprising glycine-proline-glycine-glycine-X repeating motifs that promote extensibility and supercontraction. The MA silk from Darwin's bark spider (Caerostris darwini) is estimated to be two to three times tougher than the MA silk from other spider species. Previous research suggests that a unique MaSp4 protein incorporates proline into a novel glycine-proline-glycine-proline motif and may explain C. darwini MA silk's extraordinary toughness. However, no direct correlation has been made between the silk's molecular structure and its mechanical properties for C. darwini. Here, we correlate the relative protein secondary structure composition of MA silk from C. darwini and four other spider species with mechanical properties before and after supercontraction to understand the effect of the additional MaSp4 protein. Our results demonstrate that C. darwini MA silk possesses a unique protein composition with a lower ratio of helices (31%) and β-sheets (20%) than other species. Before supercontraction, toughness, modulus and tensile strength correlate with percentages of β-sheets, unordered or random coiled regions and β-turns. However, after supercontraction, only modulus and strain at break correlate with percentages of β-sheets and β-turns. Our study highlights that additional information including crystal size and crystal and chain orientation is necessary to build a complete structure-property correlation model.

Chemical Synthesis of Silk-Mimetic Polymers

Silk is a naturally occurring high-performance material that can surpass man-made polymers in toughness and strength. The remarkable mechanical properties of silk result from the primary sequence of silk fibroin, which bears semblance to a linear segmented copolymer with alternating rigid ("crystalline") and flexible ("amorphous") blocks. Silk-mimetic polymers are therefore of great emerging interest, as they can potentially exhibit the advantageous features of natural silk while possessing synthetic flexibility as well as non-natural compositions. This review describes the relationships between primary sequence and material properties in natural silk fibroin and furthermore discusses chemical approaches towards the synthesis of silk-mimetic polymers. In particular, step-growth polymerization, controlled radical polymerization, and copolymerization with naturally derived silk fibroin are presented as strategies for synthesizing silk-mimetic polymers with varying molecular weights and degrees of sequence control. Strategies for improving macromolecular solubility during polymerization are also highlighted. Lastly, the relationships between synthetic approach, supramolecular structure, and bulk material properties are explored in this review, with the aim of providing an informative perspective on the challenges facing chemical synthesis of silk-mimetic polymers with desirable properties.

Effect of long-term cadmium and copper intoxication on the efficiency of ampullate silk glands in false black widow Steatoda grossa (Theridiidae) spiders

The aim of the study was to compare cellular effects of xenobiotic cadmium and biogenic copper in ampullate silk glands of false black widow Steatoda grossa spider after long-term exposure via ingestion under laboratory conditions. Both the level of selected detoxification parameters (glutathione S-transferase, catalase, and the level of total antioxidant capacity) and degree of genotoxic changes (comet assay) were determined in the silk glands. Additionally the contents of selected amino acids (L-Ala, L-Pro, L-His, L-Phe, DL-Ile, and DL-Asn) in the hunting webs produced by spiders of this species were assessed. The ability of S. grossa females to accumulate cadmium was higher than that for copper. Long-term exposure of spiders to copper did not change the level of detoxification parameters, and the level of DNA damage in the cells of ampullate silk glands was also low. Cadmium had a stronger prooxidative and genotoxic effect than copper in the cells of the analyzed silk glands. However, regardless of the type of metal used, no significant changes in the level of amino acids in silk were found. The obtained results confirmed the effectiveness of metal neutralization mechanisms in the body of the studied spider species, which results in the protection of the function of ampullate silk glands.

Spidroins and Silk Fibers of Aquatic Spiders

Spiders are commonly found in terrestrial environments and many rely heavily on their silks for fitness related tasks such as reproduction and dispersal. Although rare, a few species occupy aquatic or semi-aquatic habitats and for them, silk-related specializations are also essential to survive in aquatic environments. Most spider silks studied to date are from cob-web and orb-web weaving species, leaving the silks from many other terrestrial spiders as well as water-associated spiders largely undescribed. Here, we characterize silks from three Dictynoidea species: the aquatic spiders Argyroneta aquatica and Desis marina as well as the terrestrial Badumna longinqua. From silk gland RNA-Seq libraries, we report a total of 47 different homologs of the spidroin (spider fibroin) gene family. Some of these 47 spidroins correspond to known spidroin types (aciniform, ampullate, cribellar, pyriform, and tubuliform), while other spidroins represent novel branches of the spidroin gene family. We also report a hydrophobic amino acid motif (GV) that, to date, is found only in the spidroins of aquatic and semi-aquatic spiders. Comparison of spider silk sequences to the silks from other water-associated arthropods, shows that there is a diversity of strategies to function in aquatic environments.

Universal nanothin silk coatings via controlled spidroin self-assembly

Robust, biocompatible, and facile coatings are promising for improving the in vivo performance of medical implants and devices. Here, we demonstrate the formation of nanothin silk coatings by leveraging the biomimetic self-assembly of eADF4(C16), an amphiphilic recombinant protein based on the Araneus diadematus dragline spidroin ADF4. These coatings result from concurrent adsorption and supramolecular assembly of eADF4(C16) induced by KH2PO4, thereby providing a mild one-pot coating strategy in which the coating rate can be controlled by protein and KH2PO4 concentration. The thickness of the coatings ranges from 2-30 nm depending on the time immersed in the aqueous coating solution. Coatings can be formed on hydrophobic and hydrophilic substrates regardless of surface chemistry and without requiring specialized surface activation. Moreover, coatings appear to be stable through vigorous rinsing and prolonged agitation in water. Grazing incidence wide angle X-ray scattering, single-molecule force spectroscopy, and Congo red staining techniques confirm the formation of β-sheet nanocrystals within the eADF4(C16) coating, which contributes to the cohesive and adhesive stability of the material. Coatings are exceptionally smooth in the dry state and are hydrophilic regardless of substrate hydrophobicity. Under aqueous conditions, nanothin silk coatings exhibit the properties of a hydrogel material.

The liquid structure of elastin

The protein elastin imparts extensibility, elastic recoil, and resilience to tissues including arterial walls, skin, lung alveoli, and the uterus. Elastin and elastin-like peptides are hydrophobic, disordered, and undergo liquid-liquid phase separation upon self-assembly. Despite extensive study, the structure of elastin remains controversial. We use molecular dynamics simulations on a massive scale to elucidate the structural ensemble of aggregated elastin-like peptides. Consistent with the entropic nature of elastic recoil, the aggregated state is stabilized by the hydrophobic effect. However, self-assembly does not entail formation of a hydrophobic core. The polypeptide backbone forms transient, sparse hydrogen-bonded turns and remains significantly hydrated even as self-assembly triples the extent of non-polar side chain contacts. Individual chains in the assembly approach a maximally-disordered, melt-like state which may be called the liquid state of proteins. These findings resolve long-standing controversies regarding elastin structure and function and afford insight into the phase separation of disordered proteins.

Analysis of repetitive amino acid motifs reveals the essential features of spider dragline silk proteins

The extraordinary mechanical properties of spider dragline silk are dependent on the highly repetitive sequences of the component proteins, major ampullate spidroin 1 and 2 (MaSp2 and MaSp2). MaSp sequences are dominated by repetitive modules composed of short amino acid motifs; however, the patterns of motif conservation through evolution and their relevance to silk characteristics are not well understood. We performed a systematic analysis of MaSp sequences encompassing infraorder Araneomorphae based on the conservation of explicitly defined motifs, with the aim of elucidating the essential elements of MaSp1 and MaSp2. The results show that the GGY motif is nearly ubiquitous in the two types of MaSp, while MaSp2 is invariably associated with GP and di-glutamine (QQ) motifs. Further analysis revealed an extended MaSp2 consensus sequence in family Araneidae, with implications for the classification of the archetypal spidroins ADF3 and ADF4. Additionally, the analysis of RNA-seq data showed the expression of a set of distinct MaSp-like variants in genus Tetragnatha. Finally, an apparent association was uncovered between web architecture and the abundance of GP, QQ, and GGY motifs in MaSp2, which suggests a co-expansion of these motifs in response to the evolution of spiders' prey capture strategy.

Physicochemical Property Variation in Spider Silk: Ecology, Evolution, and Synthetic Production

The unique combination of great stiffness, strength, and extensibility makes spider major ampullate (MA) silk desirable for various biomimetic and synthetic applications. Intensive research on the genetics, biochemistry, and biomechanics of this material has facilitated a thorough understanding of its properties at various levels. Nevertheless, methods such as cloning, recombination, and electrospinning have not successfully produced materials with properties as impressive as those of spider silk. It is nevertheless becoming clear that silk properties are a consequence of whole-organism interactions with the environment in addition to genetic expression, gland biochemistry, and spinning processes. Here we assimilate the research done and assess the techniques used to determine distinct forms of spider silk chemical and physical property variability. We suggest that more research should focus on testing hypotheses that explain spider silk property variations in ecological and evolutionary contexts.

Major Ampullate Spider Silk with Indistinguishable Spidroin Dope Conformations Leads to Different Fiber Molecular Structures

To plentifully benefit from its properties (mechanical, optical, biological) and its potential to manufacture green materials, the structure of spider silk has to be known accurately. To this aim, the major ampullate (MA) silk of Araneus diadematus (AD) and Nephila clavipes (NC) has been compared quantitatively in the liquid and fiber states using Raman spectromicroscopy. The data show that the spidroin conformations of the two dopes are indistinguishable despite their specific amino acid composition. This result suggests that GlyGlyX and GlyProGlyXX amino acid motifs (X = Leu, Glu, Tyr, Ser, etc.) are conformationally equivalent due to the chain flexibility in the aqueous environment. Species-related sequence specificity is expressed more extensively in the fiber: the β-sheet content is lower and width of the orientation distribution of the carbonyl groups is broader for AD (29% and 58°, respectively) as compared to NC (37% and 51°, respectively). β-Sheet content values are close to the proportion of polyalanine segments, suggesting that β-sheet formation is mainly dictated by the spidroin sequence. The extent of molecular alignment seems to be related to the presence of proline (Pro) that may decrease conformational flexibility and inhibit chain extension and alignment upon drawing. It appears that besides the presence of Pro, secondary structure and molecular orientation contribute to the different mechanical properties of MA threads.

Evidence of Decoupling Protein Structure from Spidroin Expression in Spider Dragline Silks

The exceptional strength and extensibility of spider dragline silk have been thought to be facilitated by two spidroins, major ampullate spidroin 1 (MaSp1) and major ampullate spidroin 2 (MaSp2), under the assumption that protein secondary structures are coupled with the expressed spidroins. We tested this assumption for the dragline silk of three co-existing Australian spiders, Argiope keyserlingi, Latrodectus hasselti and Nephila plumipes. We found that silk amino acid compositions did not differ among spiders collected in May. We extended these analyses temporally and found the amino acid compositions of A. keyserlingi silks to differ when collected in May compared to November, while those of L. hasselti did not. To ascertain whether their secondary structures were decoupled from spidroin expression, we performed solid-state nuclear magnetic resonance spectroscopy (NMR) analysis on the silks of all spiders collected in May. We found the distribution of alanine toward β-sheet and 3,10helix/random coil conformations differed between species, as did their relative crystallinities, with A. keyserlingi having the greatest 3,10helix/random coil composition and N. plumipes the greatest crystallinity. The protein secondary structures correlated with the mechanical properties for each of the silks better than the amino acid compositions. Our findings suggested that a differential distribution of alanine during spinning could decouple secondary structures from spidroin expression ensuring that silks of desirable mechanical properties are consistently produced. Alternative explanations include the possibility that other spidroins were incorporated into some silks.

Protein secondary structure of Green Lynx spider dragline silk investigated by solid-state NMR and X-ray diffraction

In this study, the secondary structure of the major ampullate silk from Peucetia viridans (Green Lynx) spiders is characterized by X-ray diffraction and solid-state NMR spectroscopy. From X-ray diffraction measurement, β-sheet nanocrystallites were observed and found to be highly oriented along the fiber axis, with an orientational order, fc≈0.98. The size of the nanocrystallites was determined to be on average 2.5nm×3.3nm×3.8nm. Besides a prominent nanocrystalline region, a partially oriented amorphous region was also observed with an fa≈0.89. Two-dimensional (13)C-(13)C through-space and through-bond solid-state NMR experiments were employed to elucidate structure details of P. viridans silk proteins. It reveals that β-sheet nanocrystallites constitutes 40.0±1.2% of the protein and are dominated by alanine-rich repetitive motifs. Furthermore, based upon the NMR data, 18±1% of alanine, 60±2% glycine and 54±2% serine are incorporated into helical conformations.

A fast recoiling silk-like elastomer facilitates nanosecond nematocyst discharge

BACKGROUND

The discharge of the Cnidarian stinging organelle, the nematocyst, is one of the fastest processes in biology and involves volume changes of the highly pressurised (150 bar) capsule of up to 50%. Hitherto, the molecular basis for the unusual biomechanical properties of nematocysts has been elusive, as their structure was mainly defined as a stress-resistant collagenous matrix.

RESULTS

Here, we characterise Cnidoin, a novel elastic protein identified as a structural component of Hydra nematocysts. Cnidoin is expressed in nematocytes of all types and immunostainings revealed incorporation into capsule walls and tubules concomitant with minicollagens. Similar to spider silk proteins, to which it is related at sequence level, Cnidoin possesses high elasticity and fast coiling propensity as predicted by molecular dynamics simulations and quantified by force spectroscopy. Recombinant Cnidoin showed a high tendency for spontaneous aggregation to bundles of fibrillar structures.

CONCLUSIONS

Cnidoin represents the molecular factor involved in kinetic energy storage and release during the ultra-fast nematocyst discharge. Furthermore, it implies an early evolutionary origin of protein elastomers in basal metazoans.

Inverse temperature transition of elastin like motifs in major ampullate dragline silk: MD simulations of short peptides and NMR studies of water dynamics

Using deuterium 2D T1 − T2 Inverse Laplace Transform (ILT) NMR, we have investigated the distribution, population, and dynamics of waters of hydration in major ampullate N. clavipes and A. aurantia silk as a function of temperature. In both samples studied, correlation times much larger than that of free water are measured, and in some cases, appear to increase with increasing temperature over the range of 5 to 60 °C (corresponding to reduced tumbling). In addition, the experimental data point to a reduction in the population of water localized in the silk with increasing temperature in the range of 20 to 50 °C. Molecular dynamics simulations were performed to probe the thermal characteristics of a variety of repeating motifs found in the two silk samples. The repeating motifs GLGSQ, GAAAAAAG, GPGGY, GPGQQ, GPSG, and GPSGPGS found in N. clavipes, GLGSQ, GYGSG, GPGSG, and GPGSQ found in A. aurantia silk were found to exhibit a thermal property observed in short elastin peptides known as the "inverse temperature transition". This is a well known characteristic exhibited by short peptides consisting of (VPGXG)n motifs (where X is any amino acid other than proline) found in elastin--a protein responsible for the elasticity of vertebrate tissues. In qualitative agreement with experimental measurements of water in the silks, all the peptides studied in simulation show evidence of an increase in sidechain contacts and peptide hydrogen bonds, concomitant with a decrease in radius of gyration and localized water as the temperature is raised from approximately 5 to 60 °C.

Structure-function-property-design interplay in biopolymers: spider silk

Spider silks have been a focus of research for almost two decades due to their outstanding mechanical and biophysical properties. Recent advances in genetic engineering have led to the synthesis of recombinant spider silks, thus helping to unravel a fundamental understanding of structure-function-property relationships. The relationships between molecular composition, secondary structures and mechanical properties found in different types of spider silks are described, along with a discussion of artificial spinning of these proteins and their bioapplications, including the role of silks in biomineralization and fabrication of biomaterials with controlled properties.

Nephila clavipes Flagelliform silk-like GGX motifs contribute to extensibility and spacer motifs contribute to strength in synthetic spider silk fibers

Flagelliform spider silk is the most extensible silk fiber produced by orb weaver spiders, though not as strong as the dragline silk of the spider. The motifs found in the core of the Nephila clavipes flagelliform Flag protein are GGX, spacer, and GPGGX. Flag does not contain the polyalanine motif known to provide the strength of dragline silk. To investigate the source of flagelliform fiber strength, four recombinant proteins were produced containing variations of the three core motifs of the Nephila clavipes flagelliform Flag protein that produces this type of fiber. The as-spun fibers were processed in 80% aqueous isopropanol using a standardized process for all four fiber types, which produced improved mechanical properties. Mechanical testing of the recombinant proteins determined that the GGX motif contributes extensibility and the spacer motif contributes strength to the recombinant fibers. Recombinant protein fibers containing the spacer motif were stronger than the proteins constructed without the spacer that contained only the GGX motif or the combination of the GGX and GPGGX motifs. The mechanical and structural X-ray diffraction analysis of the recombinant fibers provide data that suggests a functional role of the spacer motif that produces tensile strength, though the spacer motif is not clearly defined structurally. These results indicate that the spacer is likely a primary contributor of strength, with the GGX motif supplying mobility to the protein network of native N. clavipes flagelliform silk fibers.

Wet webs work better: Humidity, supercontraction and the performance of spider orb webs

Like many biomaterials, spider silk responds to water through softening and swelling. Major ampullate silk, the main structural element of most prey capture webs, also shrinks dramatically if unrestrained or develops high tension if restrained, a phenomenon called "supercontraction". While supercontraction has been investigated for over 30 years, its consequences for web performance remain controversial. Here, we measure prey capture performance of dry and wet (supercontracted) orb webs of Argiope and Nephila using small wood blocks as prey. Prey capture performance significantly increased at high humidity for Argiope while the improvement was less dramatic for Nephila. This difference is likely due to Argiope silk supercontracting more than Nephila silk. Web deflection, measured as the extension of the web upon prey impact, also increased at high humidity in Argiope, suggesting that silk softening upon supercontraction explains improved performance of wet webs. These results strongly argue that supercontraction is not detrimental to web performance.

Early events in the evolution of spider silk genes

Silk spinning is essential to spider ecology and has had a key role in the expansive diversification of spiders. Silk is composed primarily of proteins called spidroins, which are encoded by a multi-gene family. Spidroins have been studied extensively in the derived clade, Orbiculariae (orb-weavers), from the suborder Araneomorphae ('true spiders'). Orbicularians produce a suite of different silks, and underlying this repertoire is a history of duplication and spidroin gene divergence. A second class of silk proteins, Egg Case Proteins (ECPs), is known only from the orbicularian species, Lactrodectus hesperus (Western black widow). In L. hesperus, ECPs bond with tubuliform spidroins to form egg case silk fibers. Because most of the phylogenetic diversity of spiders has not been sampled for their silk genes, there is limited understanding of spidroin gene family history and the prevalence of ECPs. Silk genes have not been reported from the suborder Mesothelae (segmented spiders), which diverged from all other spiders >380 million years ago, and sampling from Mygalomorphae (tarantulas, trapdoor spiders) and basal araneomorph lineages is sparse. In comparison to orbicularians, mesotheles and mygalomorphs have a simpler silk biology and thus are hypothesized to have less diversity of silk genes. Here, we present cDNAs synthesized from the silk glands of six mygalomorph species, a mesothele, and a non-orbicularian araneomorph, and uncover a surprisingly rich silk gene diversity. In particular, we find ECP homologs in the mesothele, suggesting that ECPs were present in the common ancestor of extant spiders, and originally were not specialized to complex with tubuliform spidroins. Furthermore, gene-tree/species-tree reconciliation analysis reveals that numerous spidroin gene duplications occurred after the split between Mesothelae and Opisthothelae (Mygalomorphae plus Araneomorphae). We use the spidroin gene tree to reconstruct the evolution of amino acid compositions of spidroins that perform different ecological functions.

Post-secretion processing influences spider silk performance

Phenotypic variation facilitates adaptations to novel environments. Silk is an example of a highly variable biomaterial. The two-spidroin (MaSp) model suggests that spider major ampullate (MA) silk is composed of two proteins-MaSp1 predominately contains alanine and glycine and forms strength enhancing β-sheet crystals, while MaSp2 contains proline and forms elastic spirals. Nonetheless, mechanical properties can vary in spider silks without congruent amino acid compositional changes. We predicted that post-secretion processing causes variation in the mechanical performance of wild MA silk independent of protein composition or spinning speed across 10 species of spider. We used supercontraction to remove post-secretion effects and compared the mechanics of silk in this 'ground state' with wild native silks. Native silk mechanics varied less among species compared with 'ground state' silks. Variability in the mechanics of 'ground state' silks was associated with proline composition. However, variability in native silks did not. We attribute interspecific similarities in the mechanical properties of native silks, regardless of amino acid compositions, to glandular processes altering molecular alignment of the proteins prior to extrusion. Such post-secretion processing may enable MA silk to maintain functionality across environments, facilitating its function as a component of an insect-catching web.

Structural disorder and protein elasticity

An emerging class of disordered proteins underlies the elasticity of many biological tissues. Elastomeric proteins are essential to the function of biological machinery as diverse as the human arterial wall, the capture spiral of spider webs and the jumping mechanism of fleas. In this chapter, we review what is known about the molecular basis and the functional role of structural disorder in protein elasticity. In general, the elastic recoil of proteins is due to a combination of internal energy and entropy. In rubber-like elastomeric proteins, the dominant driving force is the increased entropy of the relaxed state relative to the stretched state. Aggregates of these proteins are intrinsically disordered or fuzzy, with high polypeptide chain entropy. We focus our discussion on the sequence, structure and function of five rubber-like elastomeric proteins, elastin, resilin, spider silk, abductin and ColP. Although we group these disordered elastomers together into one class of proteins, they exhibit a broad range of sequence motifs, mechanical properties and biological functions. Understanding how sequence modulates both disorder and elasticity will help advance the rational design of elastic biomaterials such as artificial skin and vascular grafts.

Variation in protein intake induces variation in spider silk expression

BACKGROUND

It is energetically expensive to synthesize certain amino acids. The proteins (spidroins) of spider major ampullate (MA) silk, MaSp1 and MaSp2, differ in amino acid composition. Glutamine and proline are prevalent in MaSp2 and are expensive to synthesize. Since most orb web spiders express high proline silk they might preferentially attain the amino acids needed for silk from food and shift toward expressing more MaSp1 in their MA silk when starved.

METHODOLOGY/PRINCIPAL FINDINGS

We fed three spiders; Argiope aetherea, Cyrtophora moluccensis and Leucauge blanda, high protein, low protein or no protein solutions. A. aetherea and L. blanda MA silks are high in proline, while C. moluccesnsis MA silks are low in proline. After 10 days of feeding we determined the amino acid compositions and mechanical properties of each species' MA silk and compared them between species and treatments with pre-treatment samples, accounting for ancestry. We found that the proline and glutamine of A. aetherea and L. blanda silks were affected by protein intake; significantly decreasing under the low and no protein intake treatments. Glutmaine composition in C. moluccensis silk was likewise affected by protein intake. However, the composition of proline in their MA silk was not significantly affected by protein intake.

CONCLUSIONS

Our results suggest that protein limitation induces a shift toward different silk proteins with lower glutamine and/or proline content. Contradictions to the MaSp model lie in the findings that C. moluccensis MA silks did not experience a significant reduction in proline and A. aetherea did not experience a significant reduction in serine on low/no protein. The mechanical properties of the silks could not be explained by a MaSp1 expressional shift. Factors other than MaSp expression, such as the expression of spidroin-like orthologues, may impact on silk amino acid composition and spinning and glandular processes may impact mechanics.

Humidity affects the extensibility of an orb-weaving spider's viscous thread droplets

The prey-capture threads found in most spider orb webs rely on viscous droplets for their stickiness. Each droplet is formed of a central mass of viscoelastic glycoprotein glue surrounded by an aqueous covering, both of which incorporate hydrophilic components. We found that the extensibility of droplets on Larinioides cornutus threads increased as humidity increased. However, the deflection of the droplets' supporting axial lines did not change, indicating that atmospheric water uptake increases glycoprotein plasticity, but not glycoprotein adhesion. The extensibility of droplets, along with that of the thread's supporting axial fibers, is responsible for summing the adhesion of multiple thread droplets. Therefore, daily changes in humidity have the potential to significantly alter the performance of viscous threads and orb webs.

The critical role of water in spider silk and its consequence for protein mechanics

Due to its remarkable mechanical and biological properties, there is considerable interest in understanding, and replicating, spider silk's stress-processing mechanisms and structure-function relationships. Here, we investigate the role of water in the nanoscale mechanics of the different regions in the spider silk fibre, and their relative contributions to stress processing. We propose that the inner core region, rich in spidroin II, retains water due to its inherent disorder, thereby providing a mechanism to dissipate energy as it breaks a sacrificial amide-water bond and gains order under strain, forming a stronger amide-amide bond. The spidroin I-rich outer core is more ordered under ambient conditions and is inherently stiffer and stronger, yet does not on its own provide high toughness. The markedly different interactions of the two proteins with water, and their distribution across the fibre, produce a stiffness differential and provide a balance between stiffness, strength and toughness under ambient conditions. Under wet conditions, this balance is destroyed as the stiff outer core material reverts to the behaviour of the inner core.

Plasticity in major ampullate silk production in relation to spider phylogeny and ecology

Spider major ampullate silk is a high-performance biomaterial that has received much attention. However, most studies ignore plasticity in silk properties. A better understanding of silk plasticity could clarify the relative importance of chemical composition versus processing of silk dope for silk properties. It could also provide insight into how control of silk properties relates to spider ecology and silk uses. We compared silk plasticity (defined as variation in the properties of silk spun by a spider under different conditions) between three spider clades in relation to their anatomy and silk biochemistry. We found that silk plasticity exists in RTA clade and orbicularian spiders, two clades that differ in their silk biochemistry. Orbiculariae seem less dependent on external spinning conditions. They probably use a valve in their spinning duct to control friction forces and speed during spinning. Our results suggest that plasticity results from different processing of the silk dope in the spinning duct. Orbicularian spiders seem to display better control of silk properties, perhaps in relation to their more complex spinning duct valve.

How sequence determines elasticity of disordered proteins

How nature tunes sequences of disordered protein to yield the desired coiling properties is not yet well understood. To shed light on the relationship between protein sequence and elasticity, we here investigate four different natural disordered proteins with elastomeric function, namely: FG repeats in the nucleoporins; resilin in the wing tendon of dragonfly; PPAK in the muscle protein titin; and spider silk. We obtain force-extension curves for these proteins from extensive explicit solvent molecular dynamics simulations, which we compare to purely entropic coiling by modeling the four proteins as entropic chains. Although proline and glycine content are in general indicators for the entropic elasticity as expected, divergence from simple additivity is observed. Namely, coiling propensities correlate with polyproline II content more strongly than with proline content, and given a preponderance of glycines for sufficient backbone flexibility, nonlocal interactions such as electrostatic forces can result in strongly enhanced coiling, which results for the case of resilin in a distinct hump in the force-extension curve. Our results, which are directly testable by force spectroscopy experiments, shed light on how evolution has designed unfolded elastomeric proteins for different functions.

Spider silk as a load bearing biomaterial: tailoring mechanical properties via structural modifications

Spider silk shows great potential as a biomaterial: in addition to biocompatibility and biodegradability, its strength and toughness are greater than native biological fibres (e.g. collagen), with toughness exceeding that of synthetic fibres (e.g. nylon). Although the ultimate tensile strength and toughness at failure are unlikely to be limiting factors, its yield strain of 2% is insufficient, particularly for biomedical application because of the inability to mimic the complex ultrastructure of natural tissues with current tissue engineering approaches. To harness the full potential of spider silk as a biomaterial, it is therefore necessary to increase its yield strain. In this paper, we discuss the means by which the mechanical properties of spider silk, particularly the yield strain, can be optimized through structural modifications.

Proline periodicity modulates the self-assembly properties of elastin-like polypeptides

Elastin is a self-assembling protein of the extracellular matrix that provides tissues with elastic extensibility and recoil. The monomeric precursor, tropoelastin, is highly hydrophobic yet remains substantially disordered and flexible in solution, due in large part to a high combined threshold of proline and glycine residues within hydrophobic sequences. In fact, proline-poor elastin-like sequences are known to form amyloid-like fibrils, rich in β-structure, from solution. On this basis, it is clear that hydrophobic elastin sequences are in general optimized to avoid an amyloid fate. However, a small number of hydrophobic domains near the C terminus of tropoelastin are substantially depleted of proline residues. Here we investigated the specific contribution of proline number and spacing to the structure and self-assembly propensities of elastin-like polypeptides. Increasing the spacing between proline residues significantly decreased the ability of polypeptides to reversibly self-associate. Real-time imaging of the assembly process revealed the presence of smaller colloidal droplets that displayed enhanced propensity to cluster into dense networks. Structural characterization showed that these aggregates were enriched in β-structure but unable to bind thioflavin-T. These data strongly support a model where proline-poor regions of the elastin monomer provide a unique contribution to assembly and suggest a role for localized β-sheet in mediating self-assembly interactions.

Solid-state NMR comparison of various spiders' dragline silk fiber

Major ampullate (dragline) spider silk is a coveted biopolymer due to its combination of strength and extensibility. The dragline silk of different spiders have distinct mechanical properties that can be qualitatively correlated to the protein sequence. This study uses amino acid analysis and carbon-13 solid-state NMR to compare the molecular composition, structure, and dynamics of major ampullate dragline silk of four orb-web spider species ( Nephila clavipes , Araneus gemmoides , Argiope aurantia , and Argiope argentata ) and one cobweb species ( Latrodectus hesperus ). The mobility of the protein backbone and amino acid side chains in water exposed silk fibers is shown to correlate to the proline content. This implies that regions of major ampullate spidroin 2 protein, which is the only dragline silk protein with any significant proline content, become significantly hydrated in dragline spider silk.

Nanostructure and molecular mechanics of spider dragline silk protein assemblies

Spider silk is a self-assembling biopolymer that outperforms most known materials in terms of its mechanical performance, despite its underlying weak chemical bonding based on H-bonds. While experimental studies have shown that the molecular structure of silk proteins has a direct influence on the stiffness, toughness and failure strength of silk, no molecular-level analysis of the nanostructure and associated mechanical properties of silk assemblies have been reported. Here, we report atomic-level structures of MaSp1 and MaSp2 proteins from the Nephila clavipes spider dragline silk sequence, obtained using replica exchange molecular dynamics, and subject these structures to mechanical loading for a detailed nanomechanical analysis. The structural analysis reveals that poly-alanine regions in silk predominantly form distinct and orderly beta-sheet crystal domains, while disorderly regions are formed by glycine-rich repeats that consist of 3₁-helix type structures and beta-turns. Our structural predictions are validated against experimental data based on dihedral angle pair calculations presented in Ramachandran plots, alpha-carbon atomic distances, as well as secondary structure content. Mechanical shearing simulations on selected structures illustrate that the nanoscale behaviour of silk protein assemblies is controlled by the distinctly different secondary structure content and hydrogen bonding in the crystalline and semi-amorphous regions. Both structural and mechanical characterization results show excellent agreement with available experimental evidence. Our findings set the stage for extensive atomistic investigations of silk, which may contribute towards an improved understanding of the source of the strength and toughness of this biological superfibre.

Prey type, vibrations and handling interactively influence spider silk expression

The chemical and mechanical properties of spider major ampullate (MA) silks vary in response to different prey, mostly via differential expression of two genes – MaSp1 and MaSp2 – although the spinning process exerts additional influence over the mechanical properties of silk. The prey cues that initiate differential gene expression are unknown. Prey nutrients, vibratory stimuli and handling have been suggested to be influential. We performed experiments to decouple the vibratory stimuli and handling associated with high and low kinetic energy prey (crickets vs flies) from their prey nutrients to test the relative influence of each as inducers of silk protein expression in the orb web spider Nephila pilipes. We found that the MA silks from spiders feeding on live crickets had greater percentages of glutamine, serine, alanine and glycine than those from spiders feeding on live flies. Proline composition of the silks was unaffected by feeding treatment. Increases in alanine and glycine in the MA silks of the live-cricket-feeding spiders indicate a probable increase in MaSp1 gene expression. The amino acid compositions of N. pilipes feeding on crickets with fly stimuli and N. pilipes feeding on flies with cricket stimuli did not differ from each other or from pre-treatment responses, so these feeding treatments did not induce differential MaSp expression. Our results indicate that cricket vibratory stimuli and handling interact with nutrients to induce N. pilipes to adjust their gene expression to produce webs with mechanical properties appropriate for the retention of this prey. This shows that spiders can genetically alter their silk chemical compositions and, presumably, mechanical properties upon exposure to different prey types. The lack of any change in proline composition with feeding treatment in N. pilipes suggests that the MaSp model determined for Nephila clavipes is not universally applicable to all Nephila.

High-performance spider webs: integrating biomechanics, ecology and behaviour

Spider silks exhibit remarkable properties, surpassing most natural and synthetic materials in both strength and toughness. Orb-web spider dragline silk is the focus of intense research by material scientists attempting to mimic these naturally produced fibres. However, biomechanical research on spider silks is often removed from the context of web ecology and spider foraging behaviour. Similarly, evolutionary and ecological research on spiders rarely considers the significance of silk properties. Here, we highlight the critical need to integrate biomechanical and ecological perspectives on spider silks to generate a better understanding of (i) how silk biomechanics and web architectures interacted to influence spider web evolution along different structural pathways, and (ii) how silks function in an ecological context, which may identify novel silk applications. An integrative, mechanistic approach to understanding silk and web function, as well as the selective pressures driving their evolution, will help uncover the potential impacts of environmental change and species invasions (of both spiders and prey) on spider success. Integrating these fields will also allow us to take advantage of the remarkable properties of spider silks, expanding the range of possible silk applications from single threads to two- and three-dimensional thread networks.