Thiol-Mediated Enhancement of Nε-Acetyllysine Formation in Lens Proteins

Lysine acetylation (AcK) is a prominent post-translational modification in eye lens crystallins. We have observed that AcK formation is preferred in some lysine residues over others in crystallins. In this study, we have investigated the role of thiols in such AcK formation. Upon incubation with acetyl-CoA (AcCoA), αA-Crystallin, which contains two cysteine residues, showed significantly higher levels of AcK than αB-Crystallin, which lacks cysteine residues. Incubation with thiol-rich γS-Crystallin resulted in higher AcK formation in αB-Crystallin from AcCoA. External free thiol (glutathione and N-acetyl cysteine) increased the AcK content in AcCoA-incubated αB-Crystallin. Reductive alkylation of cysteine residues significantly decreased (p < 0.001) the AcCoA-mediated AcK formation in αA-Crystallin. Introduction of cysteine residues within ∼5 Å of lysine residues (K92C, E99C, and V169C) in αB-Crystallin followed by incubation with AcCoA resulted in a 3.5-, 1.3- and 1.3-fold increase in the AcK levels when compared to wild-type αB-Crystallin, respectively. Together, these results suggested that AcK formation in α-Crystallin is promoted by the proximal cysteine residues and protein-free thiols through an S → N acetyl transfer mechanism.