The eye lens is a unique organ that hosts some of the most long-lived proteins in the body. To obtain lens transparency, differentiated fiber cells lose their inner machinery, resulting in little to no protein turnover post-embryonic development. What results are layered cells that host protein concentrations of more than 400 mg/mL in humans and 1,000 mg/mL in some fish species. The lens proteome is dominated by two protein superfamilies: α-crystallin (holdase chaperone proteins) and βγ-crystallin (refractive and structural proteins). These eye lens proteins participate in a wide variety of protein-protein interactions that are essential to the continuing development and health of aging lenses. It was suggested that lens transparency is facilitated by the presence of short-range order arrangements that minimize light scattering despite high protein concentration. While currently it is accepted that localized organizations are encouraged by depletion effects, clarity of what intermolecular interactions facilitate hydrogel formation is lacking. Better understanding of lens protein behavior during gelation provides the foundation necessary to elucidate residues, motifs, and post-translational modification that strongly influence protein stability in the presence of crowding effects. Here, we show that at high concentrations γS-crystallin forms a transparent hydrogel via transient intermolecular interactions. Using a combination of solid- and solution-state NMR spectroscopy techniques we identified the intermolecular contacts that facilitate hydrogel formation, as well as show how hydrogel formation influences protein conformation and dynamic motion. Furthermore, rheological testing reports a sharp sol-gel phase transition indicating that γS-crystallin hydrogel formation is strongly concentration-dependent.
Suk et al. (Sun,) studied this question.