Role of human γD-crystallin tryptophans in the ultraviolet radiation response

Cataracts are the leading cause of reversible blindness worldwide, primarily associated with the aggregation of proteins such as γ-crystallins, which are essential for maintaining lens transparency. Among these, human γD-crystallin (HγD) contains four conserved tryptophans, hypothesized to act as a protective mechanism against ultraviolet (UV) radiation. This study investigated the effects of low-dose UV-B radiation on HγD and its variants, in which each tryptophan was replaced by phenylalanine. The substitutions did not significantly affect the protein's secondary or tertiary structure but markedly reduced thermal stability, particularly in the W42F mutant. Aggregation kinetics were accelerated in all variants, with pronounced increases observed in the W130F and W156F mutants. Molecular dynamics simulations revealed that these substitutions disrupt hydrophobic interactions in both the N- and C-terminal domains, promoting instability and enhancing aggregation propensity. UV radiation induced chemical modifications, where Trp42 and Trp130 were the most affected, further driving aggregation. Changes in fluorescence spectra after UV exposure indicated the breakdown of the tryptophan indole ring and the formation of degradation products. These results confirm that tryptophans in HγD serve a crucial protective role against UV-induced damage by preserving structural stability and minimizing aggregation.