Delta crystallin <p>The crystallins are water-soluble structural proteins that occur in high concentration in the cytoplasm of eye lens fibre cells. Four major groups of crystallin have been distinguished on the basis of size, charge and immunological properties: alpha-, beta- and gamma-crystallins occur in all vertebrate classes (though gamma-crystallins are low or absent in avian lenses); and delta-crystallin is found exclusively in reptiles and birds [<cite idref="PUB00005345"/>, <cite idref="PUB00003917"/>].</p><p>Delta-crystallin evolved in a common ancestor of reptiles and birds, by the overexpression of arginosuccinate lyase in the lens. At this time, a gene duplication took place, since when the lens gene has accumulatedmutations in the coding sequence, rendering it enzymatically inactive.The proteins belong to a superfamily of distantly-related metabolicenzymes, all of which are active as homotetramers: they include fumarase,aspartase, adenylosuccinase and 3-carboxy-cis,cis-muconate lactonisingenzyme.</p><p>The structure of delta-crystallin comprises mainly alpha-helical domains [<cite idref="PUB00003917"/>]. One domain is a bundle of 5 long helices, which forms a 20-helixbundle at the core of the tetramer. The structure reveals a putativeactive-site cleft, located on the boundary between 3 subunits of thetetramer.</p>