Fig. 4: The mechanism underlying the silk protein structural transitions.
a Freeze-fractured cryo-electron microscopy image of the anterior part of the silkworm silk gland revealing an event of linear ordering of the fibroin nanocompartments (denoted by a black rectangle). b AFM image of silk fibroin protein self-assembled into nanoscale fibrils. c Enlarged image of (b) revealing the presence of spherical assemblies in nanofibrillar structures. d Volumetric calculation of unordered silk fibroin nanocompartments not subjected to elongational flow stress (average volume of 1.2*107 nm3) vs. nanocompartments that have aligned due to shear produced by elongational flow stress (average of 60,160 nm3) vs. nanocompartments subjected to shear stress (average volume of 16,750 nm3). The standard error and the mean are indicated in the graph. e Molecular dynamics simulations of silk fibroin conformational transitions from a relaxed state (representative of an unfolded state) (i) to intermolecular interactions (ii), which gradually induce the formation of β-sheets (iii and iv) due to the pulling of the tetrameric silk fibroin hydrophobic domain’s GAGAGS sequence. (v) MD simulation of a 12-meric hydrophobic silk fibroin domain, formed by the pulling of β-sheets. f SAXS analysis of silk fibroin monomers, nanocompartments and silk fibroin nanofibrils. Azimuthally-interacted background-subtracted solution X-ray scattering absolute intensity, I, as a function of the magnitude of the scattering vector, q, from monomer, nanocompartment and nanofibril- containing samples (open blue symbols). Solid red curves correspond to a linear combination of g silk monomers (left illustration) with either elongated (nanofibrils) and spherical (right illustration) structures. Source data are provided as a Source Data file.
