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 βB1-crystallin to the lens chaperones, α-crystallins. We show that the mutations enhance the binding affinity to αA- but not αB-crystallin at physiological temperatures. Complex formation disrupts the dimer interface of βB1-crystallin consistent with the binding of a monomer. Binding isotherms obtained at increasing concentrations of βB1-crystallin deviate from a classic binding equilibrium and display cooperative-like behavior. In the context of βB1-crystallin unfolding equilibrium, these characteristics are reflective of the concentration-dependent change in the population of a dimeric intermediate that has low affinity to αA-crystallin. In the lens, where α-crystallin binding sites are not regenerated, this may represent an added mechanism to maintain lens transparency.