L-Aspartyl (L-Asp) and L-asparaginyl residues in proteins isomerize or racemize to D,L-isoaspartyl (D,L-iSO-Asp) or D-aspartyl (D-Asp) residues during protein aging. These atypical aspartyl residues can interfere with the biological function of the protein and lead to cellular dysfunction. Protein L-isoaspartyl (D-aspartyl) methyltransferase (PIMT) is a repair enzyme that facilitates conversion Of L-isoAsp and D-Asp to L-Asp. PIMT deficient mice exhibit accumulation Of L-isoAsp in several tissues and die, on average, 12 days after birth from progressive epileptic seizures with grand mal and myoclonus features. However, little is known about the molecular mechanisms by which accumulation of the aberrant residues leads to cellular abnormalities. In this study, we established PIMT-knockdown cells using a short interfering RNA expression system and characterized the resultant molecular abnormalities in intracellular signaling pathways. PIMT-knockdown cells showed significant accumulation of proteins with isomerized residues, compared to control cells. In the PIMT-knockdown cells, Raf-1, MEK, and ERK, members of the MAPK cascade, were hyperphosphorylated after EGF stimulation compared to control cells. These results suggest that PIMT repair of abnormal proteins is necessary to maintain normal MAPK signaling. (c) 2008 Elsevier Inc. All rights reserved.