Mechanism of aggregation of UV-irradiated βL-crystallin

Thermal denaturation and aggregation of UV-irradiated βL-crystallin from eye lenses of steers have been studied. The data on size-exclusion chromatography and SDS-PAGE indicated that UV irradiation of βL-crystallin at 10 °С resulted in fragmentation of the protein molecule and formation of cross-linked aggregates. Fluorescence data showed that tryptophan fluorescence in the irradiated protein decreased exponentially with the UV dose. Decrease in tryptophan fluorescence is a result of photochemical destruction, but not of conformational changes of protein, because there is no red shift in the fluorescence maximum. The differential scanning calorimetry (DSC) profiles of the samples of UV-irradiated and wild type βL-crystallin were registered. The area under curves, which is proportional to the amount of the native protein, decreased exponentially with increasing the irradiation dose. The shape of the DSC profiles for the samples of UV-irradiated βL-crystallin was identical to that for wild type βL-crystallin. The DSC data allowed estimating the portion of UV-denatured βL-crystallin, which is not registered by DSC, and the portion of the combined fraction consisting of native and UV-damaged molecules retaining the native structure. A conclusion has been made that UV-induced denaturation of βL-crystallin follows the one-hit model. The study of the kinetics of thermal aggregation of UV-irradiated βL-crystallin at 37 °С using dynamic light scattering showed that the initial stage of aggregation was that of formation of the start aggregates with the hydrodynamic radius of 20 nm. Further sticking of the start aggregates proceeded in the regime of reaction-limited cluster-cluster aggregation. Splitting of the aggregate population into two components occurred above a definite point in time.

Research highlights► UV irradiation causes denaturation, fragmentation and aggregation of βL-crystallin. ► UV-induced denaturation of βL-crystallin follows the one-hit model. ► The initial stage of the protein aggregation is the formation of the start aggregates. ► Aggregation proceeds in the regime of reaction-limited cluster-cluster aggregation.