Do Damaged Proteins Accumulate inCaenorhabditis elegansl-Isoaspartate Methyltransferase (pcm-1) Deletion Mutants?1

The proteinl-isoaspartate (d-aspartate)O-methyltransferase (E.C. 2.1.1.77) can initiate the conversion of isomerized and racemized aspartyl residues to their normall-aspartyl forms and has therefore been hypothesized to function as a repair enzyme, responsible for helping to limit the accumulation of damaged proteins in aging organisms. In this study, the effect of a disruption in thepcm-1gene encoding thel-isoaspartyl methyltransferase was investigated in the nematodeCaenorhabditis elegans.It was found that damaged proteins recognized by this enzyme accumulated to significant levels during long-term incubation of bothpcm-1+andpcm-1−nematodes in a specialized larval stage called the dauer. Thel-isoaspartyl methyltransferase-deficient mutants accumulated about twice the level of damaged proteins as the control nematodes during dauer aging. The mutants also accumulated higher levels of damage when both strains were incubated at 30°C for up to 3 days. However, when nonviable nematodes were removed in a Percoll separation, similar levels of damage were measured between the two strains following both dauer aging and 30°C incubation. Both strains were able to effectively eliminate damaged proteins recognized by the methyltransferase after recovery from dauer. Characterization of the methyl-accepting polypeptide substrates which accumulate in aged dauers revealed that although substrates of all molecular weights are present, the majority of substrates are peptides not precipitated by acetone. These results suggest that protein degradation, rather than repair, may be the major mechanism by whichC. eleganseliminates damaged proteins containingl-isoaspartyl residues.