Written Paper

The silk of lepidoptera  [2002]

Fedic, R. (University of South Bohemia (Czech R.)) Zurovec, M. Sehnal, F.

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The larvae of most Lepidoptera secrete fibrous proteins from their labial glands, collectively known as silk. Silk is used to construct shelters for the larvae and cocoons in which the larvae pupate. The water insoluble core of the silk filament is produced in the posterior gland section. Its crucial component, the heavy chain fibroin (H- fibroin), is characterized by hierarchical arrangement of species specific amino acid repeats. The H-fibroin genes of different Lepidoptera are homologous but their repetitive region has diversified. Amino acid composition, complexity of basic repetitive units, and arrangement of these units into iterated higher order domains determine physical properties of the silk fiber. In most Lepidoptera, the fiber includes two other peptides, light chain fibroin (L- fibroin) and P25 that occurs as two proteins differing by the degree of glycosylation. For the silkworm it has been shown that six H-fibroin, six L-fibroin, and one P25 molecules are assembled into an elementary silk unit. The fiber of tussah silk is different because it lacks the L-fibroin and P25 components; it seems to be composed of H-fibroin dimers. The fiber core in all species is enveloped by several sericins that are derived from two genes expressed in the middle section of the gland. Differential splicing and probably also different extent of glycosylation provide for the diversity of sericins that serve as a glue for fiber adherence. The silk further contains
several proteins of smaller size that are not essential for silk function but may provide protection against predators, molds, and microbes. Greatly diversified proteins called seroins, to emphasize their production both in the middle, sericin producing and the posterior, fibroin producing gland sections, were identified as products of one or two genes, depending on the species. Two other genes specifically expressed in the silk glands encode a Kunitz and a Kazal type protease inhibitors that are active on certain fungal and bacterial proteases. The inhibitors are produced before cocoon spinning and constitute very stable cocoon components. Structures of several other small silk proteins await elucidation. Silk produced by a handful of species has been woven into textiles for several millennia, and used as additive into cosmetics and occasionally also remedies. Modern technologies offer the potential to produce for practical use specific silk components such as the protease inhibitors. Creation of recombinant silkworms secreting silk fibers of new quality has become feasible. Industrial spinning of a silk dope prepared under in vitro conditions is a challenge for current research.

From the journal

Journal of Insect Biotechnology and Sericology (Japan)

ISSN : 1346-8073