Modifications of Long-Lived Proteins that Affect Protein Solubility

Although long-lived proteins are found throughout the body, the lens contains perhaps the most easily purified and readily available source of aged proteins. This is due to the decrease in protein biosynthetic capacity in mature lens fibers following organelle loss and low protein turnover. This chapter describes the major modifications occurring in aged crystallins: peptide bond cleavage, cross-linking, and the related modifications such as deamidation/isomerization/racemization that have been associated with protein insolubilization, as well as in vitro experiments aimed at determining how these modifications impact crystallin structure. Methods used to identify post-translational modifications (PTMs), including cross-links and sites of deamidation/isomerization/racemization in aged proteins, are also described. One of the first studied modifications associated with crystallin aggregation and insolubilization was disulfide bond formation. Finally, future directions for research on the biological significance of PTMs in long-lived proteins are discussed.