gamma S-crystallin, a crucial structural lens protein, plays an important role in maintaining lens transparency through its solubility and stability. The S39C mutation, a proven pathogenic mutation involved in congenital cataract, resulted in progressive cataract in adolescents. In this study, using biophysical methods, we thoroughly investigated the effects of the S39C mutation on the gamma S-crystallin structure, stability and propensity for aggregations. The data from spectroscopy analyses did not reveal an effect of the S39C mutation on the native structure of monomeric gamma S-crystallin. However, when faced with oxidative conditions, the S39C mutation prevented gamma S-crystallin from forming stable disulfide-linked dimers and remarkably increased hydrophobicity and the propensity to aggregate and precipitate. Under UV irradiation, heat shock, and GdnHCl-induced denaturation, the S39C mutant tended to aggregate and was prone to form more deleterious aggregates than the wild type protein. Therefore, the S39C mutation significantly increased the sensitivity of gamma S-crystallin to environmental stress. However, the addition of alpha A-crystallin and lanosterol did not change the tendency of the mutant to aggregate. According to molecular dynamic (MD) simulations, the S39C mutation had little effect on the secondary or tertiary structures of monomeric gamma S-crystallin but disrupted the disulfide-linked structure of the gamma S-crystallin dimer. The cleavage of this bond might largely reduce the structural stability of gamma S-crystallin. The significant decrease in the structural stability along with the increasing aggregation tendency under environmental stress might be the major causes of progressive juvenile onset cataracts induced by the S39C mutation. (C) 2020 Elsevier Inc. All rights reserved.