Properties of Resilin, a Rubberlike Protein

Abstract

Abstract There is much circumstantial evidence in favor of the belief that elastin and some other structural proteins exist in the rubberlike state of matter (see reviews by Wöhlisch, 1939; Astbury, 1940; Meyer, 1950; Kendrew, 1954), but it will be shown that the crucial evidence from thermodynamic experiments is ambiguous. The discovery of a new type of highly elastic structural protein, resilin ( Weis-Fogh, 1960; Bailey and Weis-Fogh, 1961), made it possible to re-investigate the problem by using more suitable samples. Since the thermodynamic experiments reported here gave a clear-cut answer, it was legitimate to interpret in detail the mechanical and optical properties of resilin according to the kinetic theories of rubber elasticity and so to obtain valuable information about the molecular structure of at least one rubberlike protein (Weis-Fogh, in preparation). Resilin is the essential elastic component in certain mechanical springs in the cuticle of arthropods (cf. the Latin resilire, to spring back) and it has a number of properties which make it ideally suited for this type of investigation (Weis-Fogh, 1959; 1960). When swollen with water, it exhibits typical long-range elasticity, it is heat-stable up to 140° C, crosslinked by very stable covalent bonds and insoluble in all solvents that do not break the peptide backbone but it is easily digested by all kinds of proteolytic enzymes. Moreover, it is secreted as thick continuous layers by the epidermis in a pure form unmixed with other proteins, lipids or polysaccharides and spatially separated from chitin, the only other structural component of the rubberlike cuticle (Weis-Fogh, 1960; Bailey and Weis-Fogh, 1961). It is thus possible to obtain native resilin in the form of relatively large, homogeneous samples, 0.1 mm thick and 1 mm long. Such samples are hyaline, devoid of color, and mechanically as well as optically isotropic in all directions and at all degrees of swelling. They are also devoid of structure (light and electron microscopy), and no trace of crystallinity has been found (X-ray diffraction) even in samples that were stretched near to the breaking point and then slowly dried (Elliott, Huxley, and Weis-Fogh, in preparation). The amino acid composition is unique and different from that of elastin, elastoidin, silk fibroin and collagen, and about one third of the side chains carry polar groups (Bailey and Weis-Fogh, 1961). The rubberiness of elastin is generally thought to be intimately connected with the scarcity of polar groups (5%; cf. Lloyd and Garrod, 1946). This paper will deal with experiments that show that the elastic force of resilin is connected with entropy changes caused by deformation, as in true rubbers, rather than with changes in internal energy, as in most solids.