What do rubber bands and fleas have in common?

The familiar rubber band exemplifies the central point about elasticity – when a force is applied (via the fingers), it stores the energy and when relaxed, much (in an ideal world all) of the energy is restored or transferred, the rubber band returning to its original shape without distortion (all this more technically involves such matters as stress, strain, Hooke’s law and Young’s modulus). A simple example of the transfer of energy is the slingshot (as well as more sophisticated devices such as the crossbow and the ever-handy trebuchet, much favoured in medieval Europe), where the elastic thong propels the stone or other projectile.

Nature has repeatedly reinvented elastic proteins, with the most striking instances of convergent evolution being the elastins, resilins and abductins. It is important to stress that despite the similarities of these elastic proteins, they differ in their physical properties as well as in their molecular structure. The domains, usually with a characteristic suite of amino acids, typically differ in their composition. Normally, there are at least two distinct domains, one with elastomeric repeat motifs and a non-elastic one where cross-links to adjacent proteins can be formed (although in some cases, e.g. in abductin and resilin, the cross-links are found in the elastomeric repeats). Certain types of silk are also elastic, and these demonstrate extensive molecular convergence.

Elastins

Elastins are typical of vertebrates and important in various organs, including the skin, lungs and penis. In the aorta, elastin serves to smooth the pulses of pressure as the blood is expelled from the heart. This vertebrate vessel shows remarkable convergence with the cephalopod aorta, which contains elastic fibres that are slightly different in their amino acid composition and seem to be a little less flexible. Oddly enough, the vertebrate elastin appears not to be replaced during the animal’s life and must therefore be particularly durable. This, of course, increases the risk of failure by aneurysm, and is a reminder of the potential importance of convergent evolution to health and medicine. It also has an interesting parallel with the crystallin proteins of the eyes, which too have to function without replacement for the life of the animal.

Resilins

The resilins resemble the elastins in many respects and are best known in the insects and their employment in flight. However, resilin is typically found in insects such as locusts and dragonflies that have rather low wing beat frequencies, while the many insects that beat their wings at higher frequencies employ different mechanisms of elasticity. This might also explain why there is no convergent equivalent in hummingbirds, even though they show many striking convergences with the sphinx moths. In addition, resilin is important in other aspects of insect biology, notably the jumping fleas and the even more extraordinary froghoppers, which can accelerate at up to 4000 m per second and jump with a force equivalent to more than 400 times their body weight (compared with about 135 times in the flea). In both cases, energy storage and release is mediated by resilin, although the froghopper employs a different catapult mechanism.

Abductins

Abductin is the elastic protein found in the inner part of the articulatory ligament of bivalve molluscs. It is best known from scallops, where it is quite capable of engendering swimming. The abductin-containing ligament provides the antagonist of the abductor muscle and together they lead to a vigorous flapping of the valve.

Other elastic proteins

The list of elastic proteins extends considerably beyond these three groups. Although the protein collagen, a major structural protein in animals, is adept at storing energy, it does not generally stretch very much. An interesting variant, however, is found in the threads (known as byssus) that anchor bivalve molluscs such as mussels to the sea floor. Here the collagen has extensile properties that mimic the vertebrate elastin, but it is extremely tough and has great tensile strength – an ideal anchor.

Another important elastic protein is titin (or connectin) that confers elasticity to the striated myofibrils. Perhaps the most extraordinary example, however, is found in the gluten of wheat seeds, where there seems to be an analogue to the elasticity of animal proteins. It is suggested that the elasticity is a by-product of a structure (the endosperm) that needs to efficiently and effectively store the protein. And there is another, rather odd by-product, because the gluten gives elasticity to the bread dough and is an essential part of the kneading process. Next time you eat a sandwich think of convergence!