The crystal structure of turkey delta-crystallin, one of the principal soluble components of the avian lens, has been determined to a resolution of 2.5 Angstroms. It is a tetramer, molecular weight 200 kDa, with 222 symmetry. The subunit has a new fold composed of 3 mainly alpha-helical domains. One of the domains is a bundle of five long helices which forms a 20 helix bundle at the core of the tetramer.
Delta-crystallin is closely related to the enzyme argininosuccinate lyase (EC 184.108.40.206), with which it shares approximately 90% sequence identity, indicating that it is an example of a hijacked enzyme. It is also distantly related to the class II fumarases, aspartases, adenylosuccinases and 3-carboxy-cis,cis-muconate lactonising enzyme. Analysis of the structure reveals a putative active site cleft which is located on the boundary between 3 subunits of the tetramer.
This is the first 3D structure of a representative of this superfamily of enzymes which are central to metabolism.