Posttranslational modification of human alphaA-crystallin: correlation with electrophoretic migration.

alphaA-crystallin is a major protein component of the human lens. It is known to undergo posttranslational modification. This study was done to further elucidate the temporal and spatial nature of these posttranslational modifications and to correlate the modified forms with electrophoretic migration. We dissected normal human lenses into concentric shells of fiber cells, separated the proteins by two-dimensional electrophoresis, and identified modified forms by mass spectrometry. We found that alphaA-crystallin migrated as a major spot and in over 20 additional protein spots. The extent of modification correlated with the age of the fiber cells and the depth within a lens. A correlation was also seen between these parameters and the concentration of modified forms that had full-length sequences but migrated at more acidic positions. These proteins were phosphorylated, acetylated, and/or deamidated. A few proteins migrated to a more basic position than the major form of alphaA-crystallin. The locations of several species that were truncated after C-terminal residues Ser172 and Ser162 were identified. Each of these species had intact N termini. The similarity of the C-terminal cleavage sites found in alphaA- and alphaB-crystallins was noted.