David Thorn (Harvard University), Eugene Serebryany (Harvard University), Gabriel Birrane (Beth Israel Deaconess Medical Center and Harvard Medical School), Ali Kaya (Advanced Photon Source), Eugene Shakhnovich (Harvard University)
Maintaining the transparency and proper functioning of the eye lens throughout adulthood is achieved by the high kinetic and thermodynamic stabilities of the long-lived crystallin proteins, which must retain their soluble, folded conformations to avoid diseases of the lens such as cataract. Vertebrate βγ-crystallins adopt two homologous domains connected via a linking peptide which is considered a key determinant of their tertiary and quaternary arrangements. In vitro, γ-crystallins are monomeric whereas β-crystallins form mainly dimers that are thought to associate further via domain-swapping to form larger oligomers. By mutating the interdomain linker in γD-crystallin, we have engineered a β-crystallin mimic whereby it forms dimers in solution. The crystal structure of the γ-crystallin mutant shows a domain-swapped dimer, distinct from the compact ‘face en face’ dimeric structure of β-crystallin in solution. While preserving its native domain structure, the domain-swapped dimer exhibits reduced thermodynamic stability, suggesting that this arrangement is unfavorable in an aging lens where the risk of protein misfolding and aggregation is pronounced due to cumulative post-translational modifications. However, as higher-order oligomers of β-crystallin are domain-swapped, the γ-crystallin mutant may be representative of a short-lived, conformational intermediate in the oligomerization of β-crystallins.
National Eye Institute, National Institutes of Health, grant R01EY030444
Figure 1. Wildtype, monomeric and mutant, domain-swapped dimeric γ-crystallin represented as surrogate conformational intermediates [red] in the oligomerization of β-crystallin [black]
