Structure of the spinning apparatus of a wild silkworm Samia cynthia ricini and molecular dynamics calculation on the structural change of the silk fibroin

Silkworms have been developed over thousands years to optimize folding and crystallization of fibroin under highly controlled conditions which have resulted in their efficient fiber formation. In this paper, we reconstructed the three-dimensional architecture of the spinneret of a wild silkworm Samia cynthia ricini from approximately 1000 optical micrographs of the semi-thin cross sections. The chitin plates and muscles were observed in the silk press part together with large change in the diameter of the spinneret lumen at the press part by large shear stress. This is similar to the case of the spinneret of Bombyx mori silkworm, indicating that the structural change in the silk fibroin of S.c. ricini silkworm occurs exclusively at the silk press part due to large shear stresses. Molecular dynamics (MD) calculations were then performed to study the structural change that occurs in the crystalline region of S.c. ricini silk fibroin under shear stress. Namely, using the peptide AGGAGG(A)12GGAGAG as a model of the crystalline part of the silk fibroin under different shear stresses in the presence of water molecules and followed by molecular mechanics (MM) calculation after removal of water molecules. The simulation indicates that the Ala residues in the model peptides adopt a predominantly β-sheet structure under shear stresses of above 1.0 GPa.