Aquatic caddisworm silk is solidified by environmental metal ions during the natural fiber-spinning process

Aquatic caddisfly larvae (caddisworms) wet-spin fibers to construct composite cases of silk and stone. The silk emerges from labial ducts as a nanofibrous fluid gel, flowing over the stone substrate and making intimate interfacial adhesive contacts before being drawn into tough fibers that rapidly solidify underwater to span gaps in the construction. Divalent metal ions are responsible for the unique mechanical properties of naturally spun silk fibers; however, when and where divalent metal ions are incorporated into the metallofibers and other aspects of the fiber solidification mechanism are poorly understood. To investigate, the elemental composition and secondary structure of silk precursors stored in the silk gland lumen were compared with naturally spun fibers by inductively coupled plasma optical emission spectroscopy and attenuated total reflection Fourier transform infrared spectroscopy. Naturally spun fibers contained near equimolar ratios of Ca²⁺ to P. In contrast, silk precursors stored in the silk gland lumen contained only traces of Ca²⁺ and other multivalent metal ions. Ca²⁺ was also undetectable in anterior lumenal silk using the histochemical Ca²⁺ indicator, alizarin S red. Addition of Ca²⁺ to isolated lumenal silk resulted in Ca²⁺ complexation by H-fibroin phosphoserines (pSs) and a shift in secondary structure from random coils to β-structures, creating infrared spectra in the phosphate and amide I regions nearly equivalent to those found in naturally spun Ca²⁺-containing silk fibers. Light and electron microscopy within distinct regions of the silk gland suggested that posterior gland silk colloidal complexes transition into a nanofibrous morphology as they pass into the chitin-lined anterior lumen. Altogether, the results suggest that environmental Ca²⁺ absorbed from natural water triggers silk fiber solidification postdraw by complexing H-fibroin pSs, creating Ca²⁺-stabilized crystalline β-nanodomains that cross-link and toughen the freshly drawn silk fibers.-Ashton, N. N., Stewart, R. J. Aquatic caddisworm silk is solidified by environmental metal ions during the natural fiber-spinning process.