Abstract
Spider dragline (major ampullate) silk outperforms virtually all other natural and manmade materials in terms of tensile strength and toughness. For this reason, the mass-production of artificial spider silks through transgenic technologies has been a major goal of biomimetics research. Although all known arthropod silk proteins are extremely large (>200 kiloDaltons), recombinant spider silks have been designed from short and incomplete cDNAs, the only available sequences. Here we describe the first full-length spider silk gene sequences and their flanking regions. These genes encode the MaSp1 and MaSp2 proteins that compose the black widow's high-performance dragline silk. Each gene includes a single enormous exon (>9000 base pairs) that translates into a highly repetitive polypeptide. Patterns of variation among sequence repeats at the amino acid and nucleotide levels indicate that the interaction of selection, intergenic recombination, and intragenic recombination governs the evolution of these highly unusual, modular proteins. Phylogenetic footprinting revealed putative regulatory elements in non-coding flanking sequences. Conservation of both upstream and downstream flanking sequences was especially striking between the two paralogous black widow major ampullate silk genes. Because these genes are co-expressed within the same silk gland, there may have been selection for similarity in regulatory regions. Our new data provide complete templates for synthesis of recombinant silk proteins that significantly improve the degree to which artificial silks mimic natural spider dragline fibers.
Highlights
Spider silks have received much economic and biomedical attention because of their outstanding mechanical properties [e.g. 1–3]
Based on these multi-gene comparisons, we identify putative regulatory sequences that may be involved in coexpression of the two major ampullate silk genes
Black widow dragline silk is an exceptionally tough biomaterial, even compared to the high-performance draglines spun by other spiders [43,45]
Summary
Spider silks have received much economic and biomedical attention because of their outstanding mechanical properties [e.g. 1–3]. The dragline silk of araneoids (ecribellate orb-weaving spiders and their relatives) displays both high tensile strength and extensibility, making it tougher than most other natural or synthetic materials [4,5,6]. An individual orb-weaving spider spins up to five different types of silk fibers, each serving critical ecological functions, including prey capture, shelter, predator avoidance, egg protection, and dispersal [7,8]. Each distinct fiber is made from one or two unique types of silk structural proteins (fibroins), almost all of which are encoded by members of a single gene family [9,10,11,12]. The spectacular diversity of spider silk proteins evolved through successive rounds of gene duplication and divergence
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