THE STRUCTURE OF AVIAN EYE LENS DELTA-CRYSTALLIN REVEALS A NEW FOLD FOR A SUPERFAMILY OF OLIGOMERIC ENZYMES

The crystal structure of turkey delta-crystallin, a principal soluble components of the avian lens, has been determined to a resolution of 2 .5 Angstrom. It is a tetramer, of 200,000 M(1), with 222 symmetry. The subunit has a new fold composed of three mainly a-helical domains. On e domain is a bundle of five long helices which forms a 2O-helix bundl e at the cove of the tetramer. delta-crystallin shaves approximately 9 0% sequence identity with the enzyme argininosuccinate lyase (EC 4.3.2 .1), indicating that it is an example of a 'hijacked' enzyme. It is al so distantly related to the class II fumarases, aspartases, adenylosuc cinases and 3-carboxy-cis,cis-mueonate lactonising enzyme. The structu re reveals a putative active-site cleft which is located on the bounda ry between three subunits of the tetramer. This is the first three-dim ensional structure of a representative of this superfamily of enzymes.