Abnormal assemblies and subunit exchange of alphaB-crystallin R120 mutants could be associated with destabilization of the dimeric substructure.

Mutation of the Arg120 residue in the human alphaB-crystallin sequence has been shown to be associated with a significant ability to aggregate in cultured cells and have an increased oligomeric size coupled to a partial loss of the chaperone-like activity in vitro. In the present study, static and dynamic light scattering, small-angle X-ray scattering, and size exclusion chromatography were used to follow the temperature and pressure induced structural transitions of human alphaB-crystallin and its R120G, R120D, and R120K mutants. The wild type alphaB-crystallin was known to progressively increase in size with increasing temperature, from 43 to 60 degrees C, before aggregating after 60 degrees C. The capacity to increase in size with temperature or pressure, while remaining soluble, had disappeared with the R120G mutant and was found to be reduced for the R120K and R120D mutants. The R120K mutant, which preserves the particle charge, was the less impaired. The deficit of quaternary structure plasticity was well correlated with the decrease in chaperone-like activity previously observed. However, the mutant ability to exchange subunits, measured with a novel anion exchange chromatography assay, was found to be increased, suggesting subtle relationships between structural dynamics and function. From molecular dynamic simulations, the R120 position appeared critical to conserve proper intra- and intersubunit interactions. In silico mutagenesis followed by simulated annealing of the known small heat shock protein 3D structures suggested a destabilization of the dimeric substructure by the R120 mutations. The whole of the results demonstrated the importance of the R120 residue for structural integrity, both static and dynamic, in relation with function.