Spiders store spidroins in their silk glands as high concentration aqueous solutions, spinning these dopes into fibres with outstanding mechanical properties. Aciniform (or wrapping) silk is the toughest spider silk and is devoid of the short amino acid sequence motifs characteristic of the other spidroins. Using solution-state NMR spectroscopy, we demonstrate that the 200 amino acid Argiope trifasciata AcSp1 repeat unit contrasts with previously characterized spidroins, adopting a globular 5-helix bundle flanked by intrinsically disordered N- and C-terminal tails. Split-intein-mediated segmental NMR-active isotope-enrichment allowed unambiguous demonstration of modular and malleable “beads-on-a-string” concatemeric behaviour. Concatemers form fibres upon manual drawing with silk-like morphology and mechanical properties, alongside secondary structuring and orientation consistent with native AcSp1 fibres. AcSp1 structural stability varies locally, with the fifth helix denaturing most readily. The structural transition of aciniform spidroin from a mostly α-helical dope to a mixed α-helix/β-sheet-containing fibre can be directly related to spidroin architecture and stability.

To understand the spinning process of dragline silk by spiders, the protein conformation before spinning has to be determined. Raman confocal spectromicroscopy has been used to study the conformation of the proteins in situ in the intact abdominal major ampullate gland of Nephila clavipes and Araneus diadematus spiders. The spectra obtained are typical of natively unfolded proteins and are very similar to that of a mixture of recombinant silk proteins. Vibrational circular dichroism reveals that the conformation is composed of random and polyproline II (PPII) segments with some alpha-helices. The alpha-helices seem to be located in the C-terminal part whereas the repetitive sequence is unfolded. The PPII structure can significantly contribute to the efficiency of the spinning process in nature.