Containment of extended length polymorphisms in silk proteins

Characterization of two full-length tubuliform silk gene sequences from Neoscona theisi reveals intragenic concerted evolution and multiple copies in genome

Orb-web weaving spiders use a variety of silk types for particular tasks, and each silk type is composed of at least two spider silk proteins (spidroins). In the early stage of divergence, however, the molecular evolutionary processes act on spidroin variants are still unclear because of a lack of knowledge for full-length paralogous and orthologous gene sequences among closely related species. Here, we present two complete gene sequences encoding the tubuliform spidroin TuSp1 variants (TuSp1-v2 and TuSp1-v3) from orb-weaving spider Neoscona theisi. Both N. theisi TuSp1-v2 and TuSp1-v3 genes contain a single enormous exon (14,139 bp for TuSp1-v2 and 13,152 bp for TuSp1-v3) and dozens of tandemly arrayed repeats (25 repeats for TuSp1-v2 and 23 repeats for TuSp1-v3) with extreme intragenic homogenization. The pattern of expression for these two spidroins revealed that the level of TuSp1-v3 mRNA is ~3-fold higher than that of TuSp1-v2 in tubuliform gland. Phylogenetic analyses of spidroins not only show the occurrence of a gene duplication event for TuSp1-v2 and TuSp1-v3 in the common ancestor of the Neoscona and Araneus lineage but reinforce the role of concerted evolution for the extreme homogenization of TuSp1 repeats.

Two novel tubuliform silk gene sequences from Araneus ventricosus provide evidence for multiple loci in genome

Spiders produce a diversity of silk fibers from multiple morphologically distinct silk glands for specific tasks, and each silk type primarily composed of one or more particular silk proteins encoded by silk gene family members believed to generated by duplication and divergence of ancient silk genes. Egg case silks spun from tubuliform glands are used to construct the tough outer structure of egg cases, are important for their reproduction. Here we present two novel complete TuSp1 sequences from orb weaving spider Araneus ventricosus. Alignment of the two spidroin iterated repeats showed both extreme intragenic homogenization. The pairwise Ka/Ks analysis revealed the terminal and repetitive regions for three TuSp1 loci including the reported TuSp1 gene are all under purifying selection. Phylogenetic analysis showed the two new TuSp1 variants could derive from recent duplication events.

Silk physico-chemical variability and mechanical robustness facilitates intercontinental invasibility of a spider

There are substantive problems associated with invasive species, including threats to endemic organisms and biodiversity. Understanding the mechanisms driving invasions is thus critical. Variable extended phenotypes may enable animals to invade into novel environments. We explored here the proposition that silk variability is a facilitator of invasive success for the highly invasive Australian house spider, Badumna longinqua. We compared the physico-chemical and mechanical properties and underlying gene expressions of its major ampullate (MA) silk between a native Sydney population and an invasive counterpart from Montevideo, Uruguay. We found that while differential gene expressions might explain the differences in silk amino acid compositions and protein nanostructures, we did not find any significant differences in silk mechanical properties across the populations. Our results accordingly suggest that B. longinqua’s silk remains functionally robust despite underlying physico-chemical and genetic variability as the spider expands its range across continents. They also imply that a combination of silk physico-chemical plasticity combined with mechanical robustness might contribute more broadly to spider invasibilities.

Metabolic engineering for recombinant major ampullate spidroin 2 (MaSp2) synthesis in Escherichia coli

In this research, metabolic engineering was employed to synthesize the artificial major ampullate spidroin 2 (MaSp2) in the engineered Escherichia coli. An iterative seamless splicing strategy was used to assemble the MaSp2 gene, which could reach 10000 base pairs, and more than 100 kDa protein was expected. However, only 55 kDa recombinant MaSp2 was obtained. Because MaSp2 is rich in alanine and glycine residues, Glycyl/alanyl-tRNA pool and extra amino acids adding were adopted in order to supplement alanine and glycine in the protein translation process. With the supplementary alanine and glycine (0.05 wt%) in the medium, MaSp2 constructed in pET28a(+) and Gly/Ala-tRNA constructed in pET22b(+) were co-expressed in Escherichia coli BL21 (DE3). As results, the artificial MaSp2 with 110 kDa molecular weight was obtained in the present work. This work demonstrates a successful example of applying metabolic engineering approaches and provided a potential way with the enhanced Glycyl/alanyl-tRNA pool to achieve the expression of high molecular weight protein with the repeated motifs in the engineered Escherichia coli.

Natural Occurring Silks and Their Analogues as Materials for Nerve Conduits

Spider silk and its synthetic derivatives have a light weight in combination with good strength and elasticity. Their high cytocompatibility and low immunogenicity make them well suited for biomaterial products such as nerve conduits. Silk proteins slowly degrade enzymatically in vivo, thus allowing for an initial therapeutic effect such as in nerve scaffolding to facilitate endogenous repair processes, and then are removed. Silks are biopolymers naturally produced by many species of arthropods including spiders, caterpillars and mites. The silk fibers are secreted by the labial gland of the larvae of some orders of Holometabola (insects with pupa) or the spinnerets of spiders. The majority of studies using silks for biomedical applications use materials from silkworms or spiders, mostly of the genus Nephila clavipes. Silk is one of the most promising biomaterials with effects not only in nerve regeneration, but in a number of regenerative applications. The development of silks for human biomedical applications is of high scientific and clinical interest. Biomaterials in use for biomedical applications have to meet a number of requirements such as biocompatibility and elicitation of no more than a minor inflammatory response, biodegradability in a reasonable time and specific structural properties. Here we present the current status in the field of silk-based conduit development for nerve repair and discuss current advances with regard to potential clinical transfer of an implantable nerve conduit for enhancement of nerve regeneration.

Relationships between physical properties and sequence in silkworm silks

Silk has attracted widespread attention due to its superlative material properties and promising applications. However, the determinants behind the variations in material properties among different types of silk are not well understood. We analysed the physical properties of silk samples from a variety of silkmoth cocoons, including domesticated Bombyx mori varieties and several species from Saturniidae. Tensile deformation tests, thermal analyses, and investigations on crystalline structure and orientation of the fibres were performed. The results showed that saturniid silks produce more highly-defined structural transitions compared to B. mori, as seen in the yielding and strain hardening events during tensile deformation and in the changes observed during thermal analyses. These observations were analysed in terms of the constituent fibroin sequences, which in B. mori are predicted to produce heterogeneous structures, whereas the strictly modular repeats of the saturniid sequences are hypothesized to produce structures that respond in a concerted manner. Within saturniid fibroins, thermal stability was found to correlate with the abundance of poly-alanine residues, whereas differences in fibre extensibility can be related to varying ratios of GGX motifs versus bulky hydrophobic residues in the amorphous phase.

Ancient properties of spider silks revealed by the complete gene sequence of the prey-wrapping silk protein (AcSp1)

Spider silk fibers have impressive mechanical properties and are primarily composed of highly repetitive structural proteins (termed spidroins) encoded by a single gene family. Most characterized spidroin genes are incompletely known because of their extreme size (typically >9 kb) and repetitiveness, limiting understanding of the evolutionary processes that gave rise to their unusual gene architectures. The only complete spidroin genes characterized thus far form the dragline in the Western black widow, Latrodectus hesperus. Here, we describe the first complete gene sequence encoding the aciniform spidroin AcSp1, the primary component of spider prey-wrapping fibers. L. hesperus AcSp1 contains a single enormous (∼19 kb) exon. The AcSp1 repeat sequence is exceptionally conserved between two widow species (∼94% identity) and between widows and distantly related orb-weavers (∼30% identity), consistent with a history of strong purifying selection on its amino acid sequence. Furthermore, the 16 repeats (each 371-375 amino acids long) found in black widow AcSp1 are, on average, >99% identical at the nucleotide level. A combination of stabilizing selection on amino acid sequence, selection on silent sites, and intragenic recombination likely explains the extreme homogenization of AcSp1 repeats. In addition, phylogenetic analyses of spidroin paralogs support a gene duplication event occurring concomitantly with specialization of the aciniform glands and the tubuliform glands, which synthesize egg-case silk. With repeats that are dramatically different in length and amino acid composition from dragline spidroins, our L. hesperus AcSp1 expands the knowledge base for developing silk-based biomimetic technologies.

Chromosome mapping of dragline silk genes in the genomes of widow spiders (Araneae, Theridiidae)

With its incredible strength and toughness, spider dragline silk is widely lauded for its impressive material properties. Dragline silk is composed of two structural proteins, MaSp1 and MaSp2, which are encoded by members of the spidroin gene family. While previous studies have characterized the genes that encode the constituent proteins of spider silks, nothing is known about the physical location of these genes. We determined karyotypes and sex chromosome organization for the widow spiders, Latrodectus hesperus and L. geometricus (Araneae, Theridiidae). We then used fluorescence in situ hybridization to map the genomic locations of the genes for the silk proteins that compose the remarkable spider dragline. These genes included three loci for the MaSp1 protein and the single locus for the MaSp2 protein. In addition, we mapped a MaSp1 pseudogene. All the MaSp1 gene copies and pseudogene localized to a single chromosomal region while MaSp2 was located on a different chromosome of L. hesperus. Using probes derived from L. hesperus, we comparatively mapped all three MaSp1 loci to a single region of a L. geometricus chromosome. As with L. hesperus, MaSp2 was found on a separate L. geometricus chromosome, thus again unlinked to the MaSp1 loci. These results indicate orthology of the corresponding chromosomal regions in the two widow genomes. Moreover, the occurrence of multiple MaSp1 loci in a conserved gene cluster across species suggests that MaSp1 proliferated by tandem duplication in a common ancestor of L. geometricus and L. hesperus. Unequal crossover events during recombination could have given rise to the gene copies and could also maintain sequence similarity among gene copies over time. Further comparative mapping with taxa of increasing divergence from Latrodectus will pinpoint when the MaSp1 duplication events occurred and the phylogenetic distribution of silk gene linkage patterns.