Folding and stability of the isolated Greek key domains of the long‐lived human lens proteins γD‐crystallin and γS‐crystallin

The transparency of the eye lens depends on the high solubility and stability of the lens crystallin proteins. The monomeric γ‐crystallins and oligomeric β‐crystallins have paired homologous double Greek key domains, presumably evolved through gene duplication and fusion. Prior investigation of the refolding of human γD‐crystallin revealed that the C‐terminal domain folds first and nucleates the folding of the N‐terminal domain. This result suggested that the human N‐terminal domain might not be able to fold on its own. We constructed and expressed polypeptide chains corresponding to the isolated N‐ and C‐terminal domains of human γD‐crystallin, as well as the isolated domains of human γS‐crystallin. Both circular dichroism and fluorescence spectroscopy indicated that the isolated domains purified from Escherichia coli were folded into native‐like monomers. After denaturation, the isolated domains refolded efficiently at pH 7 and 37°C into native‐like structures. The in vitro refolding of all four domains revealed two kinetic phases, identifying partially folded intermediates for the Greek key motifs. When subjected to thermal denaturation, the isolated N‐terminal domains were less stable than the full‐length proteins and less stable than the C‐terminal domains, and this was confirmed in equilibrium unfolding/refolding experiments. The decrease in stability of the N‐terminal domain of human γD‐crystallin with respect to the complete protein indicated that the interdomain interface contributes of 4.2 kcal/mol to the overall stability of this very long‐lived protein.