Influence of Cholesterol in Alpha-Crystallin Binding to Model of Human Eye Lens Lipid Membrane

A prominent eye lens protein, alpha-crystallin, binds to the membrane increasingly with age and cataract formation. The role of cholesterol (Chol) within the lens membrane is unclear. This study investigated the influence of Chol on alpha-crystallin binding to a model of the human eye lens membrane. The Chol/human model membrane (Chol/HMM) was prepared using four major phospholipids, namely, phosphatidylcholine (PC), sphingomyelin (SM), phosphatidylserine (PS), and phosphatidylethanolamine (PE), and Chol concentration of 0 and 26 mol%. Alpha-crystallin binding to membranes was measured using the electron paramagnetic resonance (EPR) spin-labeling method. The binding affinity (Ka) and maximum membrane surface occupied by alpha-crystallin decreased with 26 mol% Chol, suggesting that Chol inhibits alpha-crystallin binding to the membrane. Furthermore, with increased alpha-crystallin concentration, the mobility parameter of the membranes decreased, implying that membranes become less mobile near the headgroup regions due to the protein binding. Additionally, 26 mol% Chol content increased the maximum splitting, indicating that the membranes became more ordered near the headgroup regions with increased Chol content. These results suggest that Chol decreases the hydrophobicity near the membrane’s headgroup regions, decreasing the hydrophobic binding of alpha-crystallin to the hydrophobic core of the membrane