Specificity of αA-crystallin binding to destabilized mutants of βB1-crystallin

To elucidate the structural and energetic basis of attractive protein interactions in the aging lens, we investigated the binding of destabilized mutants of beta B31-crystallin to the lens chaperones, a-crystallins. We show that the mutations enhance the binding affinity to alpha A- but not alpha B-crystallin at physiological temperatures. Complex formation disrupts the dimer interface of DBI-crystallin consistent with the binding of a monomer. Binding isotherms obtained at increasing concentrations of OBI-crystallin deviate from a classic binding equilibrium and display cooperative-like behavior. In the context of PBI-crystallin unfolding equilibrium, these characteristics are reflective of the concentration-dependent change in the population of a dimeric intermediate that has low affinity to alpha A-crystallin. In the lens, where a-crystallin binding sites are not regenerated, this may represent an added mechanism to maintain lens transparency. (C) 2007 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.