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

Whether and how intramolecular crosslinks in polymeric materials contribute to mechanical properties is debated in both experimental and theoretical arenas. The tethering threads of Octopus bimaculoides egg cases provide a rare window to investigate this question in a biomaterial. The only detectable component of the load-bearing fibers in octopus threads is a 135 kDa protein, octovafibrin, comprising 29 tandem repeats of epidermal growth factor (EGF) each of which contains 3 intramolecular disulfide linkages. The N- and C-terminal C-type lectins mediate linear end-to-end octovafibrin self-assembly. Mechanical testing of threads shows that the regularly spaced disulfide linkages result in improved stiffness, toughness, and energy dissipation. In response to applied loads, molecular dynamics and X-ray scattering show that EGF-like domains deform by recruiting two hidden length β-sheet structures nested between the disulfides. The results of this study further the understanding of intramolecular crosslinking in polymers and provide a foundation for the mechanical contributions of EGF domains to the extracellular matrix.