Structural analysis of valine residues in silk fibroin by solid-state NMR

The silk fibroin produced by the silkworm Bombyx mori has a highly repetitive sequence, composed mainly of repeats of GAGAGS or GAGAGA, with some of the A/S residues being replaced by Y or V. Silk fiber is approximately 60% composed of crystalline regions, separated by less structured regions, generally described as amorphous. The crystalline regions are widely thought to be composed mainly of GAGAGS and GAGAGA repeats, and the role of the YV-repeats is not well understood. Because of the heterogeneous nature of silk fiber, it has been difficult to relate structural features to sequence. Here, we use solid-state NMR to study the conformation of valine residues in silk fiber, and in liquid silk before spinning, and we show that the valines have an identical secondary structure distribution to the alanines. This implies that the YV-repeats are part of the crystalline regions and function to demarcate their boundaries, and that the amorphous regions are formed from both AS and YV-repeats.