Structural features of isomerizable aspartyl residues in human α-crystallins

PURPOSE The aspartyl (Asp) residues 58 and 151 in αA-crystallin, and Asp36 and Asp62 in αB-crystallin in human lenses are known to be highly isomerized with aging. We investigate structural environments of these isomerizable aspartyl residues in α-crystallins of human lenses. METHODS To perform limited proteolysis experiments of purified human αA- and αB-crystallins, endoproteinase Asp-N (EC 3.4.24.33), which selectively cleaves the peptide bonds at the amino side of aspartyl and cysteic acid residues, was employed. By proteolysis approach coupled with the time-of-flight mass spectrometry (TOF-MS) method, we determined the cleavage points along protein sequences. RESULTS Proteolysis by endoproteinase Asp-N occurred preferentially at the site of isomerizable aspartyl residues in αA- and αB-crystallins. CONCLUSIONS It is found that isomerizable aspartyl residues in α-crystallins in human lenses were located not only in the solvent accessible area but also at regions displaying inherent conformational flexibility.