Amino acid composition of a new rubber-like protein, resilin

The chemical nature of an insoluble, rubber-like substance recently found in insect cuticle (e.g. prealar arm and wing-hinge ligement of locusts, elastic tendon of dragonflies) has been investigated. It consists entirely of a cross-linked protein of unique composition and has been termed resilin (rez'i lin). Of the total residues, it contains 66% of non-polar groups, 31% glycyl, 165 from dicarboxylic acids (mainly amidized), and 14% from hydroxy amino acids but no sulphur-containing residues, no hydroxyproxyl and only traces of tryptophan. In a few places, resilin is parent as pure masses of protein but, typically, it occurs as continuous layers (2–5 μ thick) separting thin (0·2 μ) chitinous lamellae from which it can extracted by hot dilute acid. The residue consists of chitin with only about 2% protein. Fresh rubber-like cuticle contains less than 0.3% water-soluble protein. The amino acid patterns of resilin is discussed by comparison with those of collagen, silk fibroin, and elastin from which it differs distinctly.