The effect of urea exposure on isoaspartyl content and protein L-isoaspartate methyltransferase activity in Drosophila melanogaster

Urea is a protein unfolding agent that can accumulate to locally high concentrations in tissues of many organisms. We used Drosophila melanogaster to test the hypothesis that urea loading would promote formation of isoaspartate (beta-carboxyl-linked aspartate), a common form of protein damage that occurs most readily in unstructured polypeptides and flexible regions of folded proteins. Ten populations of flies were tested; five control populations of urea-sensitive flies and five previously selected urea-tolerant populations. We measured the effects of urea consumption on levels of both isoaspartate and protein L-isoaspartate methyltransferase (PIMT), an enzyme believed to function in the repair or removal of isoaspartyl proteins. For both sets of populations, urea feeding for 6 days increased isoaspartyl levels by approximately 60%, supporting the idea that disruption of protein secondary and tertiary structures can accelerate the formation of isoaspartate in vivo. Urea feeding tended to increase PIMT activity in both control and urea-tolerant populations. There were no significant differences in PIMT activities or isoaspartyl levels between the control and urea-tolerant flies raised on normal or urea food. The latter findings indicate that urea tolerance evolved in the selected populations without any significant change in PIMT expression or activity.