Damage resistance of gamma crystallins: Weathering heat, UV, and ionizing radiation

The γ-crystallins are structural and refractive proteins of the vertebrate lens. They belong to the βγ-crystallin superfamily, which is characterized by an exceptionally stable double-Greek key fold. In the context of the lens, these proteins must remain stable and soluble for the lifetime of the organism. Recent work in my group focuses on understanding how γ-crystallins maintain functionality in the face of damage caused by aging and oxidation. I will present results on the impact of heat, UV light, and ionizing radiation on human γS-crystallin and cataract-associated variants. We find that the γ-crystallins are not particularly resistant to damage; on the contrary, they appear to be selected for the ability to remain soluble even when the protein surface is significantly modified and transient unfolding intermediates are populated. These have been investigated using a variety of biophysical techniques, including circular dichroism, mass spectrometry, fluorescence, and NMR spectroscopy. In particular, mass spectrometry measurements on irradiated samples show evidence of both deamidation and oxidative damage, while high-resolution NMR data allow us to locate protein regions and specific residues that are involved in conformational changes, altered dynamics, and protein-protein contacts. We will discuss proposed aggregation mechanisms mediated by different types of protein modification, as well as the relevance of these findings to human cataract disease.