Calcium-binding to lens betaB2- and betaA3-crystallins suggests that all beta-crystallins are calcium-binding proteins.
Crystallins are the major proteins of a mammalian eye lens. The topologically similar eye lens proteins, beta- and gamma-crystallins, are the prototype and founding members of the betagamma-crystallin superfamily. Betagamma-crystallins have until recently been regarded as structural proteins. However, the calcium-binding properties of a few members and the potential role of betagamma-crystallins in fertility are being investigated. Because the calcium-binding elements of other member proteins, such as spherulin 3a, are not present in betaB2-crystallin and other betagamma-crystallins from fish and mammalian genomes, it was argued that lens betagamma-crystallins should not bind calcium. In order to probe whether beta-crystallins can bind calcium, we selected one basic (betaB2) and one acidic (betaA3) beta-crystallin for calcium-binding studies. Using calcium-binding assays such as 45Ca overlay, terbium binding, Stains-All and isothermal titration calorimetry, we established that both betaB2- and betaA3-crystallin bind calcium with moderate affinity. There was no significant change in their conformation upon binding calcium as monitored by fluorescence and circular dichroism spectroscopy. However, 15N-1H heteronuclear single quantum correlation NMR spectroscopy revealed that amide environment of several residues underwent changes indicating calcium ligation. With the corroboration of calcium-binding to betaB2- and betaA3-crystallins, we suggest that all beta-crystallins bind calcium. Our results have important implications for understanding the calcium-related cataractogenesis and maintenance of ionic homeostasis in the lens.
Jobby
Maroor K
MK
Centre for Cellular and Molecular Biology (CCMB), Hyderabad, India.
Sharma
Yogendra
Y
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002132:Calcium Radioisotopes; D002135:Calcium-Binding Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D002151:Calorimetry / Q000379:methods; D002417:Cattle; D002942:Circular Dichroism; D004591:Electrophoresis, Polyacrylamide Gel; D007908:Lens, Crystalline / Q000378:metabolism*; D008969:Molecular Sequence Data; D019906:Nuclear Magnetic Resonance, Biomolecular; D011485:Protein Binding; D011487:Protein Conformation / Q000187:drug effects; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D013050:Spectrometry, Fluorescence; D017186:Titrimetry / Q000379:methods; D038201:alpha-Crystallins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D038206:beta-Crystallin B Chain / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D038204:beta-Crystallins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
D002132:Calcium Radioisotopes; D002135:Calcium-Binding Proteins; D011994:Recombinant Proteins; D038201:alpha-Crystallins; D038206:beta-Crystallin B Chain; D038204:beta-Crystallins; C113634:beta-crystallin B2; D002118:Calcium