Self-Assembly in Spider Silk Production

Self-assembly, a key feature of biological materials, is defined as a process in which supramolecular hierarchical organization is achieved spontaneously out of interactions between smaller structural elements. Virtues of self-assembly include build-in error-checking, high efficiency, and convenient control. In vivo spider silk spinning is reportedly a non-covalent protein self-assembly process. In the gland, silk protein molecules are water soluble and globular. As the silk protein proceeds through the duct, a low-viscosity nematic liquid crystal phase gradually appears. A semi-crystalline structure with a complex third phase is formed when the silk proteins pass through the spinneret,. Compared with B. Mori silk, the amino acid sequences of spider silk are more irregular enhancing the tendency of spider silk to form liquid crystals and to have less overall crystallinity. There is evidence of a methionine redox trigger controlling the ß-sheet content of the final assembly. They observe the phenomenon that with the methionine trigger silk will assembly into an oxidized state which contains less ß-sheet crystals. Finally, some in vivo spinning conditions may have important effects on intermolecular interaction of silk protein and thus control the final assembly product. For instance, acidifying will significantly change the electrostatic interactions. Water withdrawal will weaken hydrophobic interaction, rearrange a large number of intermolecular hydrogen bonds into intramolecular ones, and induce the formation of the concentration-dependent liquid crystalline phase. Shear stress also plays a critical role in the transformation into liquid crystals and insoluble ß-sheet structure.