Human γ crystallins form fibrillar amyloid aggregates through a partially unfolded intermediate

Abstract

The transparency of the human eye lens depends on the stability and solubility of the lens crystallins. Age‐related cataract is a result of aggregation of damaged or misfolded forms of crystallins. The most abundant monomeric crystallins, namely human γC, γD, and γS, have been found in the insoluble fraction of age‐related cataracts. Mutations in the genes encoding these proteins are associated with congenital cataracts. Here we examined the aggregation of wild type and a number of congenital mutants of γ crystallins under acidic conditions (which might be encountered in lysosomes of differentiating lens fiber cells). When incubated without denaturant at pH3 and 37°C, γ crystallins assembled into fibril structures imaged with TEM. These fibrils also bound Thioflavin T, measured by fluorescence at 485nm. Congenital mutants, especially those which destabilize the protein (L5S and V75D of γD), aggregated more readily than their wild type counterpart. The tryptophan fluorescence of these proteins exhibited a red‐shift under aggregation‐prone conditions, suggesting the presence of a partially unfolded intermediate. The aggregation of γ crystallins strongly depended upon the pH of the solution, correlating with the fluorescence red‐shift. Such correlation suggests that this partially unfolded intermediate is on the aggregation pathway. This research was supported by NIH (GM17980) and NEI (EY015834) to J.K.