Lenticular chaperones suppress the aggregation of the cataract-causing mutant T5P γC-crystallin

The T5P mutation in human γC-crystallin produces a lens cataract. Here, we have investigated the effects of the T5P mutation upon the aggregation of γC-crystallin in vitro and in transfected cells. By sedimentation assay and sucrose gradient centrifugation, the mutation significantly increased the aggregation of the protein and reduced dramatically its solubility in vitro. Similar effects were seen when T5P γC-crystallin was transfected into tissue culture cells, resulting in the formation of cytoplasmic aggregates of T5P γC-crystallin. Interestingly, the major lenticular protein chaperones, αA- and αB-crystallin, increased the solubility of the T5P γC-crystallin both in vitro and in transfected cells. More importantly, the size of the T5P γC-crystallin aggregates were also significantly reduced in the presence of the lenticular chaperones. These data therefore suggest a dual role for these chaperones in maintaining transparency in the lens. The first is that these protein chaperones increase the proportion of the soluble T5P γC-crystallin and the second is that they also reduce light scatter by reducing the aggregate size of T5P γC-crystallin. Both activities could modify the cataract phenotype and help explain the observed variability reported for identical γ-crystallin mutations, which identify cataract as a polygenic disease.