Dissimilarity in the Contributions of the N-Terminal Domain Hydrophobic Core to the Structural Stability of Lens β/γ-Crystallins

Vertebrate lens beta/gamma-crystallins share a conserved tertiary structure consisting of four Greek-key motifs divided into two globular domains. Numerous inherited mutations in beta/gamma-crystallins have been linked to cataractogenesis. In this research, the folding mechanism underlying cataracts caused by the I21N mutation in beta B2 was investigated by comparing the effect of mutagenesis on the structural features and stability of four beta/gamma-crystallins, beta B1, beta B2, gamma C, and gamma D. Our results showed that the four beta/gamma-crystallins differ greatly in solubility and stability against various stresses. The I21N mutation greatly impaired beta B2 solubility and native structure as well as its stability against denaturation induced by guanidine hydrochloride, heat treatment, and ultraviolet irradiation. However, the deleterious effects were much weaker for mutations at the corresponding sites in beta B1, gamma C, and gamma D. Molecular dynamics simulations indicated that the introduction of a nonnative hydrogen bond contributed to twisting Greek-key motif I outward, which might direct the misfolding of the I21N mutant of beta B2. Meanwhile, partial hydration of the hydrophobic interior of the domain induced by the mutation destabilized beta B1, gamma C, and gamma D. Our findings highlight the importance of nonnative hydrogen bond formation and hydrophobic core hydration in crystallin misfolding caused by inherited mutations.