InterPro: IPR003031 Delta crystallin

The crystallins are water-soluble structural proteins that occur in high concentration in the cytoplasm of eye lens fiber 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 [1, 2].

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 accumulated mutations in the coding sequence, rendering it enzymatically inactive. The proteins belong to a superfamily of distantly-related metabolic enzymes, all of which are active as homotetramers: they include fumarase, aspartase, adenylosuccinase and 3-carboxy-cis,cis-muconate lactonising enzyme.

The structure of delta-crystallin comprises mainly alpha-helical domains [2]. One domain is a bundle of 5 long helices, which forms a 20-helix bundle at the core of the tetramer. The structure reveals a putative active-site cleft, located on the boundary between 3 subunits of the tetramer.