Construction of Silk Fiber Core in Lepidoptera

Abstract

The formation and properties of lepidopteran silk fibers depend on amino acid repeats in the principal protein, heavy chain fibroin (H-fibroin). In H-fibroins of the "bombycoid" type, concatenations of alanine or of the GAGAGS crystalline motifs (1st tier repeats) and adjacent sequences breaking periodicity make 2nd tier repeats. Two to six such repeats comprise a 3rd tier assembly, and 12 assemblies, linked by an amorphous sequence, constitute the repetitive H-fibroin region. Heterogeneity in the repeat length and intercalation of amorphous regions prevent excessive crystallization. In the "pyraloid" H-fibroins, iterations of simple motifs are absent and assemblies of several complex motifs constitute highly regular repeats that are organized in about 12 highest order reiterations without specific spacers. Repeat homogeneity appears crucial for the alignment and interaction of the disjunct motifs that must be registered precisely to form crystallites; repeat heterogeneity is associated with decreased fiber strength. Both H-fibroin types are typically hydrophobic, and their secretion requires disulfide linkage to light chain fibroin and participation of another protein, P25. These auxiliary proteins are absent in saturniid moths with amphiphilic H-fibroin repeats. The selection at nucleic acid and protein levels and the availability of nutrients play roles in H-fibroin evolution.

Silk tape nanostructure and silk gland anatomy of trichoptera

“…P25 is postulated to be involved in the assembly of silk precursor micelles of silkworm silk. 12,15,16 Comparison of H-fibroin amino acid compositions reveals the striking distinctions between terrestrial moth and aquatic caddisfly silks. First, moth silks contain 20-30 mol% alanine residues, whereas caddisfly silks contain less than 5 mol% alanine.…”

“…10,11,15,17 The alanine-rich regions of moth silks are widely considered to form rigid b-sheet structures dispersed within extensible, disordered glycine-rich matrices, a molecular design shared with spider silks. 13,16 Tensile strength and extensibility of dry silks are correlated with the proportions of ordered and disordered regions. 18 The comparative absence of alanine in caddisfly silks suggests either a fundamentally different molecular fiber design or that residues, other than alanine, contribute to rigid semicrystalline motifs functionally equivalent to the b-sheets of moth and spider silks.…”

“…21 As a result of the high charge density, caddisfly H-fibroins are ampholytic in character, with alternating blocks of oppositely charged residues in the central H-fibroin region, whereas moth H-fibroins are amphiphilic consisting of alternating blocks of uncharged hydrophilic and hydrophobic residues. 16 Alternating oppositely charged residues also occur in the silk of aquatic black fly larvae (Chironomus tentans); the silk contains an estimated 8 mol% phosphoserine and a combined 25 mol% basic lysine and arginine residues. 17,22 Ampholytic silk precursors likely evolved independently in the two aquatic insect orders.…”

Applicability of biotechnologically produced insect silks

“…Upon acidification, the side chains are protonated and enable the formation of homodimers. The importance of this mechanism is further underlined by the finding that the residues involved in the inhibition of premature dimer formation are conserved in Lepidoptera and Trichoptera [31,32]. In general, silk processing in silk glands of spiders shows similarities to the process in silkworms.…”

Proteomic Studies of Isoforms of the P25 Component ofBombyx moriFibroin

“…1) SG is divided into the following three sub-organs: posterior silk gland (PSG), middle silk gland (MSG), and anterior silk gland (ASG). In the larva of the silkworm, Bombyx mori, fibroin is synthesized in the PSG, secreted into the lumen of the PSG, and associated with a sericin in the lumen of the MSG.…”