This family consists of several insect fibroin P25 proteins. Silk fibroin produced by the silkworm Bombyx mori consists of a heavy chain, a light chain, and a glycoprotein, P25. The heavy and light chains are linked by a disulfide bond, and P25 associates with disulfide-linked heavy and light chains by non-covalent interactions. P25 is plays an important role in maintaining integrity of the complex [1].

1: Silk fibroin of Bombyx mori is secreted, assembling a high molecular mass elementary unit consisting of H-chain, L-chain, and P25, with a 6:6:1 molar ratio. Inoue S, Tanaka K, Arisaka F, Kimura S, Ohtomo K, Mizuno S; J Biol Chem 2000;275:40517-40528. PMID:10986287