Caddisfly silk

Caddisfly silk is a heterodimer of heavy and light fibroin proteins with approximately 450 and 250 amino acid residues, respectively.[1] Similar to Lepidoptera, Trichoptera H and L fibroin also exhibit conserved cysteine residues needed for disulfide linkage of the two proteins.[2] Unlike the fibroin dimer of Lepidoptera, Trichoptera dimers are not escorted by a P25 protein, likely due to the increased hydrophilicity of trichopteran fibroin unnecessary.[3] Trichoptera H-fibroin contains repeated β-sheet motifs similar to those found in Lepidoptera fibroin, characterized by the highly conserved proline-glycine turn[4] found within a repeating sequence in the protein and by the highly crystalline structure found within trichoptera silk fibers. However, where the fibroin of silkworms contains β sheets rich in glycine and alanine, the β sheet motif of trichoptera fibroin contains a significantly different amino acid makeup. The β-sheet motif of caddisfly H-fibroin was found to have a (SX)4 repeating pattern, meaning serine alternating with usually isoleucine or valine.[5] Throughout the H-fibroin protein, greater than 60% of all serines were phosphorylated. Though negative charges are generally considered a destabilizer of β sheets, their consistent appearance within the β motif suggests a potentially novel structure.[6] This negatively charged group suggests that ionic interactions, rather than hydrogen bonding, may explain the unusual strength of caddisfly silk. Phosphorylated serines have been found in other underwater bioadhesives, including in molluscs and sea cucumbers[7]

This diagram depicts the ionic interactions between negatively charged amino acids and cations in freshwater caddisfly larvae silk

The heavily phosphorylated β-sheet of caddisfly larvae contains strong negative charges which have been found to interact with di- and trivalent cations found naturally within the larvae’s aquatic environment. These cations, including calcium, magnesium, and iron, are vital to maintaining the rigid structure of the silk’s β-sheet.[8] The ionic interactions between negatively charged serines and these cations produce a crystal with a unit cell of 5.9 Angstroms x 23.3 Angstroms x 17.3 Angstroms, as determined by X-Ray Diffraction.[9] The necessity of these cations was shown using EDTA to chelate and remove them from the protein structure, producing a mobile, noncrystalline protein.[10] Reintroduction of monovalent ions failed to restore the crystalline structure, however reintroduction of calcium or other multivalent ions successfully restored the rigidity to the H-fibroin protein[11]