Elucidating the Coordination Features Associated to Copper and Zinc Induced Aggregation of Human γ-D Crystallin

Abstract

Cataract is the leading cause of blindness in the world. The monomeric all -sheet Human γD (HγD) crystallin is an abundant crystallin in the core of the lens and its non-amyloid aggregation is associated with cataracts. A variety of experimental and etiological studies implicate metals as a potential etiological agent for cataract. Copper and zinc concentrations in the cataractous lenses are increased significantly, as compared to normal lenses. Micromolar concentrations of Cu(II) and Zn(II) ions specifically induce the aggregation of HγD crystallin. The mechanism for metal-induced aggregation likely involves interaction of the metal ions with the protein, leading to partial unfolding and formation of high molecular weight light scattering aggregates. In this work, we study the coordination features associated to the metal-induced aggregation of HγD crystallin and probe it by site-directed mutagenesis. Several His and Cys residues have been identified as putative metal anchoring sites. The effects of mutations at these residues in the copper- and zinc- induced aggregation of HγD crystallin have been evaluated. A specific interaction of Zn(II) ions with the N-terminal domain that is key for zinc-induced aggregation has been identified. On the other hand, circular dichroism and electron paramagnetic resonance data show that Cu(II) ions can bind at more than one site in the protein. These results will help elucidate the mechanism by which copper and zinc ions induce the aggregation of HγD crystallin, as it relates to cataracts disease. This research has been supported by: MIT-Seed Funds, Conacyt (#221134), NIH EY015834, and Fulbright-Garcia Robles fellowship to L.Q.