Orb-weaving spiders produce up to 7 different types of silks with a wide range of impressive mechanical properties. The toughest of these is aciniform silk, used for prey-wrapping and egg-case lining, due to a unique combination of high strength (~700 MPa) and high extensibility (~60-80%). Aciniform spidroin 1 (AcSp1), the protein comprising wrapping silk, is composed of a string of ~14 identical repeats of about 200 amino acids, flanked by non-repetitive C- and N-terminal regions. In solution, recombinant AcSp1 repeats have been shown to adopt a ”beads-on-a-string” model, including a helix-rich globular “bead” domain and a disordered linker / “string”. Presumably, the structure of recombinant AcSp1 in solution is similar to how the protein is stored in the gland prior to fiber formation, however, the molecular protein structure of the final fiber is largely unstudied. Here we present Solid-State NMR (SSNMR) results on a strategic variety of 13C-enriched Argiope argentata prey-wrapping silks including 13C-Ala/Ser, 13C-Val/Ile/Leu, and 13C-Val. We utilize 2D 13C–13C Dipolar Assisted Rotational Resonance (DARR) through space correlation experiments to yield structure-specific chemical shifts of Ala, Ser, Gly, Gln, Val, Ile, and Leu residues. Peak-fitting of NMR data yielded quantitative analysis of Ser, Ala, and Val residues adopting α-helical, β-sheet, and random-coil secondary structures. The data show that native, freshly spun aciniform silk fibers likely adopt a hybrid coiled-coil / β-sheet structure; the silk is dominated by α-helical (~50% total) and random-coil (~35% total) secondary structures, with only minor β-sheet domains (~15% total). We hypothesize that the high propensity of α-helices contributes to fiber toughness through extensibility, and we anticipate that the α-helix to β-sheet transitions may occur during elongation, contributing to high ultimate strength. We have ongoing efforts to further understand wrapping silk protein structure, structural changes, and their relation to biological role and fiber performance.

Spider prey-wrapping silk is an α-helical coiled-coil / β-sheet hybrid nanofiber

2019 Fall National Meeting

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