Deamidation of N76 in human γS-crystallin promotes dimer formation.

BACKGROUND: Cataract formation is often attributed to the build-up of post-translational modifications in the crystallin proteins of the eye lens. One such modification, the deamidation of N76 in human γS-crystallin to D76, is highly correlated with age-related cataract (Hooi et al. Invest. Ophthalmol. Vis. Sci. 53 (2012) 3554-3561). In the current work, this modification has been extensively characterised in vitro. METHODS: Biophysical characterisation was performed on wild type and N76D γS-crystallins using turbidity measurements to monitor aggregation, intrinsic fluorescence and circular dichroism spectroscopy to determine the folded state and NMR spectroscopy for identifying local changes in structure. Protein mass was determined using SEC-MALLS and analytical ultracentrifugation methods. RESULTS: Relative to the wild type protein, deamidation at N76 in γS-crystallin causes an increase in the thermal stability and resistance to thermally induced aggregation alongside a decrease in stability to denaturants, a propensity to aggregate rapidly once destabilised and a tendency to form a dimer. We ascribe the apparent increase in thermal stability upon deamidation to the formation of dimer which prevents the unfolding of the inherently less stable monomer. CONCLUSIONS: Deamidation causes a decrease in stability of γS-crystallin but this is offset by an increased tendency for dimer formation. GENERAL SIGNIFICANCE: Deamidation at N76 in human γS-crystallin likely has a combinatorial effect with other post-translational crystallin modifications to induce age-related cataract. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.