Isoaspartyl damage occurring spontaneously and accumulating over time could reduce protein function sufficiently that a cell could no longer carry out its normal activities. However, isoAsp can be readily recognized as a site of damage, because this amino acid is normally not found in proteins. The L-isoaspartyl protein carboxyl methyltransferase, or PCM for short, is an enzyme capable of specifically recognizing isoAsp in proteins¹. This enzyme is nearly universal in the living world, having been found in bacteria, nematodes, insects, plants, mammals and humans (but, interestingly, not in yeast or Gram-positive bacteria)².

PCM specifically methylates isoAsp residues (shown below) by transferring a methyl (CH₃) group from S-adenosylmethionine (SAM)³. The methylated isoAsp readily re-forms the succinimide intermediate, which can again be hydrolyzed into either normal Asp or isoAsp. The net result (perhaps requiring multiple rounds of methylation and hydrolysis) is repair of isoAsp to normal Asp.

Fountain of youth? Protein function can be restored or improved when PCM is allowed to repair damaged proteins⁴, suggesting the potential importance of PCM for aging cells that may accumulate isoAsp damage over time. Several lines of evidence support the idea that PCM is an important cellular defense against aging. (i) Mice lacking PCM have an average lifespan of only 42 days and die of seizures resembling epilepsy⁵. (ii) Plants activate PCM synthesis in response to stress and during seed formation; long-lived seeds have high levels of PCM⁶. (iii) Increased PCM expression in Drosophila subjected to heat stress improves their longevity⁷. (iv) Bacteria⁸ (more about this later) and nematodes⁹ require PCM to survive during periods of low metabolic activity. Ongoing studies are investigating when, where and how this enzyme functions to maintain protein activity and help cells age successfully.

Next: Aging in E. coli>>

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¹Geiger, T., and S. Clarke. 1987. Deamidation, isomerization, and racemization at asparaginyl and aspartyl residues in peptides: succinimide-linked reactions that contribute to protein degradation. J. Biol. Chem. 262:785-794.

²Kagan, R. M., H. J. McFadden, P. N. McFadden, C. O'Connor, and S. Clarke. 1997. Molecular phylogenetics of a protein repair methyltransferase. Comp. Biochem. Physiol. 117:379-385.

³McFadden, P. N., and S. Clarke. 1987. Conversion of isoaspartyl peptides to normal peptides by coupled enzymatic/nonenzymatic reactions: implications for the cellular repair of damaged proteins. Proc. Natl. Acad. Sci. USA 84:2595-2599.

⁴Reviewed in: Clarke, S. 2003. Aging as war between chemical and biochemical processes: protein methylation and the recognition of age-damaged proteins for repair. Ageing Res. Rev. 2:263-285.

⁵Kim, E., J. D. Lowenson, D. C. MacLaren, S. Clarke, and S. G. Young. 1997. Deficiency of a protein-repair enzyme results in the accumulation of altered proteins, retardation of growth, and fatal seizures in mice. Proc. Natl. Acad. Sci. USA 94:6132-6137.

⁶Mudgett, M. B., and S. Clarke. 1994. Hormonal and environmental responsiveness of a developmentally regulated protein repair L-isoaspartyl methyltransferase in wheat. J. Biol. Chem. 269:25605-25612.

⁷Chavous, D. A., F. R. Jackson, and C. M. O'Connor. 2001. Extension of the Drosophila lifespan by overexpression of a protein repair methyltransferase. Proc. Natl. Acad. Sci. USA 98:14814-14818.

⁸Visick, J. E., H. Cai, and S. Clarke. 1998. The L-isoaspartyl protein repair methyltransferase enhances survival of aging Escherichia coli subjected to secondary environmental stresses. J. Bacteriol. 180:2623-2629.

⁹Kagan, R. M., A. Niewmierzycka, and S. Clarke. 1997. Targeted gene disruption of the Caenorhabditis elegans L-isoaspartyl protein repair methyltransferase impairs survival of dauer stage nematodes. Arch. Biochem. Biophys. 348:320-328.