The avian eye lens protein delta-crystallin shows a novel packing arrangement of tetramers in a supramolecular helix.

BACKGROUND

Little is known of the intermolecular organization of crystallins in the protein-packed eye lens. The tetrameric structure of the 200,000 Da avian delta-crystallin, which is closely related to the enzyme argininosuccinate lyase and is characteristic of the accommodating, soft lens of birds, has recently been solved at atomic resolution at acidic pH. To help understand how delta-crystallin remains soluble at the very high concentrations found in the avian lens we have now crystallized turkey delta-crystallin at around neutral pH and examined its intermolecular interactions.

RESULTS

Turkey delta-crystallin has been crystallized around neutral pH. The X-ray structure has been solved at 4.5 A resolution in space group C2 with three and a half tetramers in the asymmetric unit. The symmetrical 222 tetramers have a novel packing arrangement consisting of continuous helices, with 7(3)2 non-crystallographic symmetry, in an approximately hexagonal close-packed array. The internal 222 symmetry of the tetramers allows different polymeric chains to be constructed, based on the tetramer-tetramer association observed in the crystalline helix. It is possible to build a model of a tubule of diameter 212 A that is very similar to observed tubules of bovine argininosuccinate lyase.

CONCLUSIONS

Elements of helical organization may occur in the concentrated solution of the avian eye lens where delta-crystallin is the prominent protein. The symmetry of the tetramer provides a choice in the direction of growth of a helix at each link so that highly hydrated irregular polymers may be formed rather than large compact regular structures that would not be compatible with a transparent lens.