Calcium Binding Properties of γ-Crystallin

The β- and γ-crystallins are closely related lens proteins that are members of the βγ-crystallin superfamily, which also include many non-lens members. Although β-crystallin is known to be a calcium-binding protein, this property has not been reported in γ-crystallin. We have studied the calcium binding properties of γ-crystallin, and we show that it binds 4 mol eq of calcium with a dissociation constant of 90 µm. It also binds the calcium-mimic spectral probes, terbium and Stains-all. Calcium binding does not significantly influence protein secondary and tertiary structures. We present evidence that the Greek key crystallin fold is the site for calcium ion binding in γ-crystallin. Peptides corresponding to Greek key motif of γ-crystallin (42 residues) and their mutants were synthesized and studied for calcium binding. These peptides adopt β-sheet conformation and form aggregates producing β-sandwich. Our results with peptides show that, in Greek key motif, the amino acid adjacent to the conserved aromatic corner in the “a” strand and three amino acids of the “d” strand participate in calcium binding. We suggest that the βγ superfamily represents a novel class of calcium-binding proteins with the Greek key βγ-crystallin fold as potential calcium-binding sites. These results are of significance in understanding the mechanism of calcium homeostasis in the lens.