Biomimetic self-assembly of recombinant marine snail egg capsule proteins into structural coiled-coil units

The egg capsules of the marine snails from the Melongenidea family feature unique biomechanical properties, including large reversible elasticity combined with a relatively high stiffness and outstanding strain energy absorption, making it an attractive biomimetic model system for restorative and tissue engineering applications. The capsules' building blocks are proteins called egg capsule proteins (ECPs), which we recently sequenced. ECPs are predicted to contain relatively large coiled-coil domains, which are directly responsible for the high elasticity arising from the extension of alpha-helical coiled-coil domains into extended beta-sheet domains. In this work, de novo synthesized ECPs genes were cloned and expressed in a bacterial expression system. Following purification under denaturing conditions by strong ion-exchange chromatography, individual and paired mixtures of ECPs were self-assembled using a controlled dialysis protocol, resulting in the folding of ECPs into soluble coiled-coil units. Circular Dichroism (CD) spectroscopy of the fibrils suggested that ECPs self-assembled into heteromeric coiled-coil units. The enhancement of the alpha-helical coiled-coil content during pair assembly was confirmed by Fourier Transform Infrared (FTIR) spectroscopy. Transmission Electron Microscopy (TEM) imaging of covalently-fixed self-assembled units corroborated the formation of elongated intermediate filaments-like structures. Polymer statistical analysis of Atomic Force Microscopy (AFM) images of unfixed self-assembled fibrils suggested that the observed coiled-coils were made of dimeric subunits. This study establishes the key protein engineering and physicochemical parameters needed to assemble ECPs into building blocks that can be processed into biomaterials that mimic the unique biomechanical properties of marine snail egg capsules.