title: Calcium binding properties of gamma-crystallin: calcium ion binds at the Greek key beta gamma-crystallin fold.
abstract: The beta- and gamma-crystallins are closely related lens proteins that are members of the betagamma-crystallin superfamily, which also include many non-lens members. Although beta-crystallin is known to be a calcium-binding protein, this property has not been reported in gamma-crystallin. We have studied the calcium binding properties of gamma-crystallin, and we show that it binds 4 mol eq of calcium with a dissociation constant of 90 microm. 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 gamma-crystallin. Peptides corresponding to Greek key motif of gamma-crystallin (42 residues) and their mutants were synthesized and studied for calcium binding. These peptides adopt beta-sheet conformation and form aggregates producing beta-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 betagamma superfamily represents a novel class of calcium-binding proteins with the Greek key betagamma-crystallin fold as potential calcium-binding sites. These results are of significance in understanding the mechanism of calcium homeostasis in the lens.
authors.affiliation: Centre for Cellular and Molecular Biology, Uppal Road, Hyderabad-500007, India.
authors.forename: B
authors.initials: B
authors.lastname: Rajini
authors.forename: P
authors.initials: P
authors.lastname: Shridas
authors.forename: C S
authors.initials: CS
authors.lastname: Sundari
authors.forename: D
authors.initials: D
authors.lastname: Muralidhar
authors.forename: S
authors.initials: S
authors.lastname: Chandani
authors.forename: F
authors.initials: F
authors.lastname: Thomas
authors.forename: Y
authors.initials: Y
authors.lastname: Sharma
mesh_terms: D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D002417:Cattle; D002851:Chromatography, High Pressure Liquid; D002942:Circular Dichroism; D003459:Crystallins / Q000737:chemistry* / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007477:Ions; D007908:Lens, Crystalline / Q000737:chemistry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010455:Peptides / Q000737:chemistry; D011485:Protein Binding; D011487:Protein Conformation; D017510:Protein Folding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D017386:Sequence Homology, Amino Acid
chemical_list: D003459:Crystallins; D007477:Ions; D010455:Peptides; D002118:Calcium