Interactions of beta-crystallin with a model of human eye lens-lipid membranes.

Abstract

β-Crystallin helps maintain the eye lens's structure and refractivity. With age and cataract formation, α-, β-, and γ-crystallin may undergo various mutations and post-translational modifications, and consequently form higher molecular weight complexes, some of which bind to lens membranes. However, the binding of β-crystallin alone to the lens membrane is still unclear. This study aims to investigate the binding of β-crystallin to the model of the human lens-lipid (MHLL) membrane and the physical properties of the MHLL membrane after β-crystallin binding. Small unilamellar vesicles of MHLL membranes, resembling the human lens-lipid composition, were prepared using the rapid solvent exchange method and probe-tip sonication. The 1 mol% cholesterol analogue spin label (CSL) integrated into the membrane, which resides near the lens-lipid headgroup region, probes β-crystallin binding to the membrane using the electron paramagnetic resonance (EPR) spin-labeling method. Our results showed that β-crystallin binds to the MHLL membrane with a binding affinity (Ka) of 0.082 ± 0.006 μM−1 and saturates around 5% of the membrane surface occupied. Furthermore, β-crystallin binding to the MHLL membrane decreased the mobility parameter, making the headgroups of the membrane less mobile. However, β-crystallin binding to the MHLL membrane showed no significant change in maximum splitting, meaning it does not affect the order near the headgroup regions of the MHLL membrane. Moreover, the hydrophobicity of the MHLL membrane also did not change significantly with the binding of β-crystallin, meaning hydrophobicity near the headgroup regions does not significantly change with β-crystallin binding. Our results showed that β-crystallin binding with MHLL membranes changes the physical property of membranes, implying that β-crystallin binding to the lens membrane may change the integrity of the membrane and has the potential to promote cataract formation.