Targeted Gene Disruption of theCaenorhabditis elegansl-Isoaspartyl Protein Repair Methyltransferase Impairs Survival of Dauer Stage Nematodes

The methylation of abnormall-isoaspartyl residues by proteinl-isoaspartate (d-aspartate)O-methyltransferase (EC 2.1.1.77) can lead to their conversion tol-aspartyl residues. For polypeptides damaged by spontaneous reactions that generatel-isoaspartyl residues, these steps represent a protein repair pathway that can limit the accumulation of potentially detrimental proteins in the aging process. We report here the construction and the characterization of an animal model deficient in this methyltransferase. We utilized Tc1-transposon-mediated mutagenesis in the nematodeCaenorhabditis elegansto construct a homozygous excision mutant lacking exons 2–5 of thepcm-1gene encoding this enzyme. Nematodes carrying this deletion exhibited no detectablel-isoaspartyl methyltransferase activity. These worms demonstrated normal morphology and behavior and adult mutant nematodes exhibited a normal lifespan. However, the survival of dauer-phase mutants was diminished by 3.5-fold relative to wild-type dauers after 50 days in the dauer phase. The fitness of thepcm-1deletion nematodes was reduced by about 16% relative to that of wild-type nematodes as measured by the ability of these mutants to compete reproductively against a wild-type population. We found that the absence of the functional methyltransferase gene leads to a modest accumulation of altered protein substrates in aged dauer worms. However, in the viable fraction of these dauer worms, no differences were seen in the levels of altered substrate proteins in the parent and methyltransferase-deficient worms, suggesting that the enzyme in wild-type cells does not efficiently catalyze the repair of spontaneously damaged proteins.