NMR Study of the Structures of Repeated Sequences, GAGXGA (X = S, Y, V), in Bombyx mori Liquid Silk

The silk fibroin stored in the silk gland of the Bombyx mori silkworm, called liquid silk, is spun out and converted into the silk fiber with extremely high strength and high toughness. Therefore it is important to determine the silk structure before spinning called Silk I at an atomic level to clarify the fiber formation mechanism. We proposed the repeated type II beta-turn structure as Silk I in the solid state with the model peptide (AG)(15) and several solid state NMR techniques previously. In this paper, the solution structure of native liquid silk was determined with solution NMR, especially for tandem repeated sequences with (GAGXGA)(n) (X = S, Y, V) and GAASGA motifs in the B. mori silk fibroin. The assignment of the C-13, N-15, and H-1 solution NMR spectra for the repetitive sequence motifs was achieved, and the chemical shifts were obtained. The program, TALOS-N, to predict the backbone torsion angles from the chemical shifts of proteins was applied to these motifs with C-13 alpha, C-13 beta, (CO)-C-13, H-1 alpha, (HN)-H-1, and N-15 chemical shifts. The twenty-five best matches of torsion angles (phi, phi) were well populated and mainly fell into the regions for typical type II beta-turn structures in the (phi, phi) map for the GAGXGA (X = S, Y, V) motifs. In contrast, (phi, phi) plots for motif GAASGA were scattered, indicating that the motif is in a disordered structure. Furthermore, inter-residue HN-H alpha NOE cross peaks between i-th and (i+2)th residues in GAGXGA (X = S, Y, V) motifs were observed, supporting the repeated type II beta-turn structure. Thus, we could show the presence of the repeated type II beta-turn structure in liquid silk.