Methionine sulfoxide reductase A (MsrA) restores α-crystallin chaperone activity lost upon methionine oxidation

Abstract

Background Lens cataract is associated with protein oxidation and aggregation. Two proteins that cause cataract when deleted from the lens are methionine sulfoxide reductase A (MsrA) that repairs protein methionine sulfoxide (PMSO) oxidized proteins and α-crystallin which is a two-subunit (αA and αB) chaperone. Here, we tested whether PMSO formation damages α-crystallin chaperone function and whether MsrA could repair PMSO-α-crystallin. Methods Total α-crystallin was oxidized to PMSO and evaluated by CNBr-cleavage and mass spectrometry. Chaperone activity was measured by light scattering using lysozyme as target. PMSO-α-crystallin was treated with MsrA, and repair was assessed by CNBr cleavage, mass spectrometry and recovery of chaperone function. The levels of α-crystallin-PMSO in the lenses of MsrA-knockout relative to wild-type mice were determined. Results PMSO oxidation of total α-crystallin (met 138 of αA and met 68 of αB) resulted in loss of α-crystallin chaperone activity. MsrA treatment of PMSO-α-crystallin repaired its chaperone activity through reduction of PMSO. Deletion of MsrA in mice resulted in increased levels of PMSO-α-crystallin. Conclusions Methionine oxidation damages α-crystallin chaperone function and MsrA can repair PMSO-α-crystallin restoring its chaperone function. MsrA is required for maintaining the reduced state of α-crystallin methionines in the lens. Significance Methionine oxidation of α-crystallin in combination with loss of MsrA repair causes loss of α-crystallin chaperone function. Since increased PMSO levels and loss of α-crystallin function are hallmarks of cataract, these results provide insight into the mechanisms of cataract development and likely those of other age-related diseases.