Mechanical Behavior of Octopus Egg Tethers Composed of Topologically Constrained, Tandemly Repeated EGF Domains

Whether and how intramolecularcrosslinks in polymeric materialscontribute to mechanical properties is debated in both experimentaland theoretical arenas. The tethering threads of Octopusbimaculoides egg cases provide a rare window to investigatethis question in a biomaterial. The only detectable component of theload-bearing fibers in octopus threads is a 135 kDa protein, octovafibrin, comprising 29 tandem repeats of epidermalgrowth factor (EGF) each of which contains 3 intramolecular disulfidelinkages. The N- and C-terminal C-type lectins mediate linear end-to-endoctovafibrin self-assembly. Mechanical testing of threads shows thatthe regularly spaced disulfide linkages result in improved stiffness,toughness, and energy dissipation. In response to applied loads, moleculardynamics and X-ray scattering show that EGF-like domains deform byrecruiting two hidden length beta-sheet structures nested betweenthe disulfides. The results of this study further the understandingof intramolecular crosslinking in polymers and provide a foundationfor the mechanical contributions of EGF domains to the extracellularmatrix.

Automated summary

This study investigates the contribution of intramolecular crosslinks in polymeric materials to mechanical properties using the tethering threads of Octopus bimaculoides egg cases. The presence of regularly spaced disulfide linkages in the load-bearing fibers improves stiffness, toughness, and energy dissipation. Molecular dynamics and X-ray scattering reveal that the EGF-like domains deform by recruiting hidden length beta-sheet structures between the disulfides. This study enhances our understanding of intramolecular crosslinking in polymers and provides insights into the mechanical contributions of EGF domains in the extracellular matrix.