Monomer-dimer equilibrium of normal and modified beta A3-crystallins: experimental determination and molecular modeling.

Beta- and gamma-crystallins are major protein constituents of the mammalian lens, where their stability and association into higher order complexes are critical for clarity and refraction. Two regions of the betagamma-crystallins have been suggested to modulate protein association, namely, the flexible N-terminal extensions and the intramolecular domain interfaces. The oligomeric state of wild-type recombinant murine betaA3-crystallin (rbetaA3) was compared to that of modified betaA3-crystallins with either an N-terminal deletion of residues 1 to 29 (rbetaA3tr) or with residues 114 to 123 of the interdomain linker replaced with the analogous linker from murine gammaB-crystallin (rbetaA3cp). All three proteins exhibited reversible monomer-dimer formation. The modifications to the N-terminus and domain linker resulted in tighter dimer formation as compared to wild-type protein as indicated by disassociation constants determined by sedimentation equilibrium: 6.62 x 10(-6) M (rbetaA3), 0.86 x 10(-6) M (rbetaA3cp), and 1.83 x 10(-7) M (rbetaA3tr). Homology modeling of betaA3-crystallins and solvation energy calculations also predicted tighter binding of the modified crystallins consistent with the centrifugation results. The findings suggest that under physiological conditions betaA3 crystallin exists in a dynamic equilibrium between monomeric and dimeric protein and that modification, especially to the N-terminal extension, can promote self-association.