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Jonathan A. King, Kate D. Moreau; A Congenital Cataract Mutant Of gammaD-Crystallin Aggregates Through A Pathway That Is Not Suppressed By alpha-Crystallin Chaperone. Invest. Ophthalmol. Vis. Sci. 2012;53(14):3668.
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© ARVO (1962-2015); The Authors (2016-present)
To understand the mechanism of cataract formation associated with congenital mutants of the γ-crystallins.
The transparency of the eye lens depends on maintenance of the native state of the Greek key γ- and β crystallins. This is aided by the presence of high concentrations of the passive small heat shock chaperones αA and αB-crystallins. Cataract involves the polymerization of covalently damaged or partially unfolded conformations of the lens crystallins into aggregates large enough to scatter visible light. Congenital cataracts are associated with a number of single amino acid substitutions in the monomeric γD-crystallin. Two mutants of the buried core of γ-crystallins, V75D and I90F, lead to congenital cataract in mice. We have studied the aggregation properties and chaperone interactions of human γD-crystallin (HγD) carrying the equivalent amino acid substitutions.
Both V75D and I90F substitutions destabilize the mutant proteins. Upon initiation of refolding by dilution from denaturant, aggregation competed with productive refolding. This aggregation was suppressed by αB-crystallin chaperone. Complexes between the chaperone and its refolding substrate proteins were stable and the substrate populated a partially folded intermediate in the chaperone/substrate complex as determined by tryptophan fluorescence. The results are consistent with prior experiments indicating that αB-crystallin chaperone recognizes a partially folded Greek key motif.The wild type native state does not have any propensity to aggregate even after weeks of incubation and was not recognized by the chaperone. However upon continued incubation of V75D crystallin in buffer a conformer derived from the native state formed high molecular weight aggregates. This slow aggregation pathway was not suppressed by HαB-crystallin.
The failure of the αB-crystallin chaperone to suppress the aggregation of the conformer derived from the V75D mutant native state can account for the congenital cataract pathology. Apparently the destabilized crystallin can adopt a conformation that results in aggregation, but is not efficiently recognized by the chaperone. Failure of chaperone recognition is likely to be the cause of pathology for other mutant proteins that depend on chaperone/chaperonin interactions.
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