Elastin is one of the five rubber-like solids used by animals to provide elasticity in biological systems; the others are resilin, abductin, spider silk, and ColP. Elastin is of particular note because it is the one found in humans, although it is only found in composite tissues of which elastin is only one element. It is most particularly found as a component of skin and arterial walls, as these need to return to their initial form after distortion without active support (from muscles or otherwise), and are undergoing constant flexion in various directions.
Elastin appears in most animals, although nearly always a component of a more complex
tissue. Large
ruminants (e.g. horses) have a large
ligament, the
nuchal ligament, that is nearly pure elastin, which acts like a suspension cable running along the vertebrae to the back of the skull, and supports the head.
Elastin, resilin, and the other elastomeric proteins are all composed of proteins, but their composition is surprisingly unrelated other than that. They do, however, all provide their elasticity largely because pulling on these structures result in pulling against molecular entanglements, rather than on chemical bonds; they are also aided by covalent bonds between long chain molecules, in the same was as plastic polymers are.