Self-tensioning aquatic caddisfly silk: Ca2+-dependent structure, strength, and load cycle hysteresis

Caddisflies are aquatic relatives of silk-spinning terrestrial moths and butterflies. Casemaker larvae spin adhesive silk fibers for underwater construction of protective composite cases. The central region of Hesperophylax sp. H-fibroin contains a repeating pattern of three conserved subrepeats, all of which contain one or more (SX)n motifs with extensively phosphorylated serines. Native silk fibers were highly extensible and displayed a distinct yield point, force plateau, and load cycle hysteresis. FTIR spectroscopy of native silk showed a conformational mix of random coil, β-sheet, and turns. Exchanging multivalent ions with Na(+) EDTA disrupted fiber mechanics, shifted the secondary structure ratios from antiparallel β-sheet toward random coil and turns, and caused the fibers to shorten, swell in diameter, and disrupted fiber birefringence. The EDTA effects were reversed by restoring Ca(2+). Molecular dynamic simulations provided theoretical support for a hypothetical structure in which the (pSX)n motifs may assemble into two- and three-stranded, Ca(2+)-stabilized β-sheets.