Conformation and dynamics of soluble repetitive domain elucidates the initial beta-sheet formation of spider silk

The beta-sheet is the key structure underlying the excellent mechanical properties of spider silk. However, the comprehensive mechanism underlying beta-sheet formation from soluble silk proteins during the transition into insoluble stable fibers has not been elucidated. Notably, the assembly of repetitive domains that dominate the length of the protein chains and structural features within the spun fibers has not been clarified. Here we determine the conformation and dynamics of the soluble precursor of the repetitive domain of spider silk using solutionstate NMR, far-UV circular dichroism and vibrational circular dichroism. The soluble repetitive domain contains two major populations: similar to 65% random coil and similar to 24% polyproline type II helix (PPII helix). The PPII helix conformation in the glycine-rich region is proposed as a soluble prefibrillar region that subsequently undergoes intramolecular interactions. These findings unravel the mechanism underlying the initial step of beta-sheet formation, which is an extremely rapid process during spider silk assembly.