Elastin
 
Elastin is the major macromolecule found in the ECM of elastic tissues such as the lung, skin, and blood vessels. The molecular properties of elastin are responsible for the elasticity. Elastin is a hydrophobic protein of about 800 amino acids that assumes a random coil configuration. Networks of elastin near the cell surface are covalently cross-linked via lysine residues, and elasticity is thought to be achieved by stretching the random coils. The protein has an abundance of proline and glycine, similar to collagen, but unlike collagen, elastin has only a small amount of hydroxyproline and lacks both hydroxylysine and carbohydrate side chains. The elastic layer is strengthened by an additional layer of microfibrils made of the protein fibrillin.
Cells may interact with elastin via an unusual 67-kDa elastin receptor. The receptor protein, unlike matrix receptors in the integrin class, is a secreted rather than transmembrane protein. It appears to mediate cell adhesion to elastin via interactions with cell surface proteins. The same receptor also binds laminin, and harbors a lectin like activity for binding sugar moieties. This protein is also found inside the cell where it may act as a chaperone to facilitate tropoelastin secretion and assembly of elastic fibers.
The functional importance of elastin is demonstrated poignantly by the recent discovery that deletions in the human elastin gene give rise to a disease known as Williams syndrome. People affected with this disease are mentally retarded (although occasionally mathematically or musically gifted), have an elfin like facial appearance and are prone to blood vessel narrowing and aneurysms.
Another disease, supravalvular aotric stenosis (SVAS), results from haploinsufficiency of elastin, and is also characterized by lumenal narrowing in blood vessels.