Specificity of alphaA-crystallin binding to destabilized mutants of betaB1-crystallin.

To elucidate the structural and energetic basis of attractive protein interactions in the aging lens, we investigated the binding of destabilized mutants of betaB1-crystallin to the lens chaperones, alpha-crystallins. We show that the mutations enhance the binding affinity to alphaA- but not alphaB-crystallin at physiological temperatures. Complex formation disrupts the dimer interface of betaB1-crystallin consistent with the binding of a monomer. Binding isotherms obtained at increasing concentrations of betaB1-crystallin deviate from a classic binding equilibrium and display cooperative-like behavior. In the context of betaB1-crystallin unfolding equilibrium, these characteristics are reflective of the concentration-dependent change in the population of a dimeric intermediate that has low affinity to alphaA-crystallin. In the lens, where alpha-crystallin binding sites are not regenerated, this may represent an added mechanism to maintain lens transparency.