This domain is found at the N-terminal end of Fibroin heavy chain (Fib-H) from Bombyx mori and similar proteins mainly from arthropods and bacteria. This hydrophilic region plays an important role in the assembly of silk by changing its conformation in response to pH decreases: it transitions from random coil to beta sheets and form micelle-like oligomers at around pH 6.0 [1].

1: N-Terminal domain of Bombyx mori fibroin mediates the assembly of silk in response to pH decrease. He YX, Zhang NN, Li WF, Jia N, Chen BY, Zhou K, Zhang J, Chen Y, Zhou CZ; J Mol Biol. 2012;418:197-207. PMID:22387468