Silk Road Revealed: Mechanism of silk fibre formation in Bombyx mori

Abstract

The transition of silk fibroin from liquid to solid is fundamental to silk-fibre production and key to the superior materials properties of native silks. Here we discover that the fibroin heavy chain from the silkworm moth Bombyx mori folds into a novel {beta}-solenoid structure, where the N-terminal domain (NTD) promotes higher-order oligomerization driven by pH reduction. These findings elucidate the complex rheological behaviour of silk and the liquid crystalline textures within the silk gland. We also find that NTD undergoes hydrolysis during standard regeneration, explaining differences between native and regenerated silk feedstocks. Overall, this study establishes a fibroin heavy chain fold, which could be relevant for other similar proteins, and explains mechanistically its liquid-to-solid transition, driven by pH reduction and stress. One sentence summaryThis study redefines the molecular structure of fibroin heavy chain and its role in the transition from solution to fibre.