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

Abstract

Solid-State NMR results on 13C-Ala/Ser and 13C-Val enriched Argiope argentata prey-wrapping silk show that native, freshly spun aciniform silk nanofibers are dominated by α-helical (∼50% total) and random-coil (∼35% total) secondary structures, with minor β-sheet nanocrystalline domains (∼15% total). This is the most in-depth study to date characterizing the protein structural conformation of the toughest natural biopolymer: aciniform prey-wrapping silks.

Recombinant Silk Fiber Properties Correlate to Prefibrillar Self‐Assembly

“…Alignment relative to the fiber axis in TFA/TFE PS fibers was more pronounced, but a significantly increased strength was not noted for these fibers relative to HFIP fibers. Coupling with the high α‐helical content in extensible native aciniform silk noted through solid‐state NMR spectroscopy, the exceptional difference in extensibility may arise from a greater α‐helical content for TFA/TFE fibers than HFIP fibers (Table S2, Supporting Information) providing spring‐like behavior to facilitate extension as is proposed in α‐helical coiled‐coil‐containing intermediate filaments . This likely couples with contributions from less ordered portions of the fiber, such as 3 1 ‐helical and turn content proposed to be important for dragline silk extensibility …”

“…The previously noted potential importance of prestructuring of β‐strands/sheets in the dope is hard to reconcile with these spinning dope conditions, assuming that this general behavior of fluorinated solvents holds true. Favoring of β‐strand formation is also hard to rationalize with respect to the high degree of α‐helical content retained in aciniform silk fibers according to Raman and NMR spectroscopy. In order to understand the mechanism by which mechanical differences in W 3 fibers are imparted at the dope stage, we next compare the two types of fibers for indications of the potential for protein prestructuring and self‐assembly in the dope solution.…”

“…This morphological difference may also be attributable to differences between the recombinant aciniform protein W 3 , comprising only 3 iterated repeats versus the native protein comprising at least 14 iterated repeats plus a C‐terminal nonrepetitive domain and likely a N‐terminal nonrepetitive domain. Additionally, the smoother surface morphology may more trivially arise from the fact that natural spun aciniform fibers at ≈0.35–0.5 µm are much narrower than either AS recombinant silk fiber at ≈20 µm. This difference in diameter may arise from the inner diameter of the spigot versus the syringe needle spinneret.…”

“…This transition is believed to be key in providing strength to silk fibers. However, the retention of significant—or, almost all, as solid‐state NMR would imply—α‐helical content in the aciniform silk fiber is distinct. Given the unique toughness of aciniform silk, this may be an essential structural feature contributing to its unique extensibility.…”

Can praying mantises escape from spider webs?

“…, Addison et al. ). The “wrapping” attacks with aciniform silk are an important behavior aimed at immobilizing dangerous prey (Fabre , Eisner ), suggesting that praying mantises are not the only selective pressure on the evolution of wrapping attacks.…”