Biomacromolecules Research Team, RIKEN Center for Sustainable Resource Science, 529 5F Main Research Building, 2-1 Hirosawa, Wako, Saitama, 351-0198, Japan.
Advanced NMR Application and Platform Team, NMR Research and Collaboration Group, NMR Science and Development Division, RIKEN SPring-8 Center, Yokohama, 1-7-22 Suehiro-cho, Tsurumi-ku, Kanagawa, 230-0045, Japan.
Nat Commun. 2018 May 29;9(1):2121. doi: 10.1038/s41467-018-04570-5.
The β-sheet is the key structure underlying the excellent mechanical properties of spider silk. However, the comprehensive mechanism underlying β-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 solution-state NMR, far-UV circular dichroism and vibrational circular dichroism. The soluble repetitive domain contains two major populations: ~65% random coil and ~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 β-sheet formation, which is an extremely rapid process during spider silk assembly.
β-折叠是蜘蛛丝具有优异机械性能的关键结构。然而，可溶性丝蛋白在向不溶性稳定纤维转变过程中形成β-折叠的综合机制尚未阐明。值得注意的是，主导蛋白质链长度的重复结构域的组装以及纺丝纤维内的结构特征尚未阐明。在这里，我们使用溶液态 NMR、远紫外圆二色性和振动圆二色性来确定蜘蛛丝重复结构域的可溶性前体的构象和动力学。可溶性重复结构域包含两个主要的结构：65%的无规卷曲和24%的聚脯氨酸 II 型螺旋（PPII 螺旋）。富含甘氨酸的区域中的 PPII 螺旋构象被提议为可溶性原纤维形成区，随后发生分子内相互作用。这些发现揭示了β-折叠形成的初始步骤的机制，这是蜘蛛丝组装过程中一个极其迅速的过程。
