In lens, similar to 90% of ocular proteins are abg-crystallins with concentrations similar to 400 mg/ml, which need to remain soluble for the whole life-span and their aggregation leads to cataract. The G18V mutation of human gamma S-crystallin causes hereditary childhood-onset cortical cataract. Mysteriously, despite being a metabolically-quiescent organ, lens maintains ATP concentrations of 3-7 mM. Very recently, we found that ATP has no significant binding to gamma S-crystallin as well as no alternation of its conformation. Nevertheless, ATP antagonizes the crowding-induced destabilization of gamma S-crystallin even at 1:1, most likely by interacting with the hydration shell. Here by DSF and NMR, we characterized the effect of ATP on binding, conformation, stability of G18V gamma S-crystallin and its interactions with alpha-crystallin. The results reveal: 1) G18V significantly accelerates the crowding-induced destabilization with Tm of 67 degrees C reduced to 50.5 degrees C at 1 mM. 2) Most unexpectedly, G18V almost completely eliminates the antagonizing effect of ATP against the crowding-induced destabilization. 3) ATP shows no significant effect on the interactions of alpha-crystallin with both WT and G18V gamma S-crystallin. Results together decode for the first time that G18V causes cataract not only by accelerating the crowding-induced destabilization, but also by eliminating the antagonizing effect of ATP against the crowding-induced destabilization. (C) 2020 Elsevier Inc. All rights reserved.