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Abstract
DNA and protein sequence of major ampullate silk proteins from a cob-weaving spider, the brown widow, Latrodectus geometricus are reported here for the first time. Sequences identified from the cDNA library revealed the presence of three genes: MaSp1, MaSp2, and MaSp-like. The MaSp1 protein is composed of GGX and (A)n motifs. The MaSp2 protein has GPGXX and (A)n motifs present. The MaSp-like protein has an unusual combination of GGX and GPGXX motifs and was not previously found in other silks. Comparison of amino acid compositions between spooled silk and the identified proteins suggests that all three of these proteins are silk fiber components.
The first report on dragline silk mechanical properties from brown widow L. geometricus are presented. Mechanical testing was performed on single strands of silk fibers from adult female L. geometricus spiders. The breaking stress and strain were measured for 20 replicate samples and averaged. Values of 0.83 ± 0.19 GPa for stress and 0.14 ± 0.06 for strain show that brown widow dragline is weaker than similar fibers from the orb-weaving spiders.
The sequence of the N-terminal regions of major ampullate silk proteins from three spider species were identified and analyzed: Argiope trifasciata, Latrodectus geometricus, and Nephila inaurata madagascariensis . The N-terminal domains have not been previously characterized for these proteins since only partial cDNA sequences near the C-terminus have been obtained for major ampullate proteins. The amino acid sequences were predicted from genomic DNA clones. Northern blotting experiments verified that the predicted 5' end of the transcripts are present in fibroin mRNA. The presence of more than one Met codon in the N-terminal region suggests the possibility of translation of a pair of protein isoforms, designated as a long and a short isoform. The size of the short isoform is consistent with the published, cDNA based, N-terminal sequence found in flagelliform silk. Two DNA sequence motifs identified upstream of the putative transcription start site are potential silk fibroin promoter elements.
