ABSTRACT
Several point mutations in human γD-crystallin (HGD) are now known to be associated with cataract. So far, the in vitro studies of individual mutants of HGD alone have been sufficient in providing plausible molecular mechanisms for the associated cataract in vivo. Nearly all the mutant proteins in solution showed compromised solubility and enhanced light scattering due to altered homologous γ-γ crystallin interactions. In sharp contrast, here we present an intriguing case of a human nuclear cataract-associated mutant of HGD--namely Glu107 to Ala (E107A), which is nearly identical to the wild type in structure, stability, and solubility properties, with one exception: Its pI is higher by nearly one pH unit. This increase dramatically alters its interaction with α-crystallin. There is a striking difference in the liquid-liquid phase separation behavior of E107A-α-crystallin mixtures compared to HGD-α-crystallin mixtures, and the light-scattering intensities are significantly higher for the former. The data show that the two coexisting phases in the E107A-α mixtures differ much more in protein density than those that occur in HGD-α mixtures, as the proportion of α-crystallin approaches that in the lens nucleus. Thus in HGD-α mixtures, the demixing of phases occurs primarily by protein type while in E107A-α mixtures it is increasingly governed by protein density. Analysis of these results suggests that the cataract due to the E107A mutation could result from the instability caused by the altered attractive interactions between dissimilar proteins--i.e., heterologous γ-α crystallin interactions--primarily due to the change in surface electrostatic potential in the mutant protein.
