Insect Silks and Cocoons: Structural and Molecular Aspects

In this chapter, we extensively describe insect-secreting silk proteins. Silk proteins are produced in the labial glands (functioning as silk glands), from which they are then secreted, which is a characteristic feature of the orders Trichoptera, Lepidoptera, and some other Holometabola insects.We first describe lepidopteran silk formation and describe how the two types of fibroin (fibroin heavy chain: H-fibroin and fibroin light chain: L-fibroin) observed in non-saturniid moths, represented by Bombyx mori. Specifically, we present how the two types of fibroins, which are linked by disulfide bonds, and P25 or fibrohexamerin as a chaperone) contribute to silk fiber organization. Saturniidae moths, which produce only one type of fibroin, are also discussed here about their silk formation. Following the description of lepidopteran silk fiber proteins, we present recent progress in the study of sericin proteins of B. mori and other lepidopteran. Sericins wrap silk fibers to seal two silk filaments together like glue. We also present the differences in expression patterns and gene regulation observed in the silk glands of B. mori and Samia ricini. Then, we comprehensively summarize the features of hornet and trichopteran silks different from lepidopterans. According to the current understanding, these species produce no sericin-like proteins. The principal molecular structure of hornet silk is α-helices, frequently in a coiled-coil conformation, a molecular structure distinct from the β-sheet structures that dominate the silks of lepidopterans. Finally, we describe the present status of transgenic technology that is being used to modify fibroins in order to add features that are lacking in the host.