Increasing βB1-crystallin sensitivity to proteolysis caused by the congenital cataract-microcornea syndrome mutation S129R.

Congenital hereditary cataract, which is mainly caused by the deposition of crystallins in light-scattering particles, is one of the leading causes of newborn blindness in human beings. Recently, an autosomal dominant congenital cataract-microcornea syndrome in a Chinese family has been associated with the S129R mutation in βB1-crystallin. To investigate the underlying molecular mechanism, we examined the effect of the mutation on βB1-crystallin structure and thermal stability. Biophysical experiments indicated that the mutation impaired the oligomerization of βB1-crystallin and shifted the dimer-monomer equilibrium to monomer. Molecular dynamic simulations revealed that the mutation altered the hydrogen-bonding network and hydrophobic interactions in the subunit interface of the dimeric protein, which resulted in the opening of the tightly associated interacting sites to allow the infiltration of the solvent molecules into the interface. Despite the disruption of βB1-crystallin assembly, the thermal stability of βB1-crystallin was increased by the mutation accompanied by the reduction of thermal aggregation at high temperatures. Further analysis indicated that the mutation significantly increased the sensitivity of βB1-crystallin to trypsin hydrolysis. The digested fragments of the mutant were prone to aggregate and unable to protect βA3-crystallin against aggregation. These results indicated that the thermal stability-beneficial mutation S129R in βB1-crystallin provided an excellent model for discovering molecular mechanisms apart from solubility and stability. Our results also highlighted that the increased sensitivity of mutated crystallins towards proteases might play a crucial role in the pathogenesis of congenital hereditary cataract and associated syndrome.