| Abstract
| - Protein l-isoaspartyl methyltransferases (PIMT; EC 2.1.1.77) catalyze the S-adenosylmethionine-dependent methylation of l-isoaspartyl residues that arise spontaneously in proteins with age, therebyinitiating a repair process that restores the normal backbone configuration to the damaged polypeptide. InDrosophila melanogaster, overexpression of PIMT in transgenic flies extends the normal life span,suggesting that protein damage can be a limiting factor in longevity. To understand structural features ofthe Drosophila PIMT (dPIMT) important for catalysis, the crystal structure of dPIMT was determined ata resolution of 2.2 Å, and site-directed mutagenesis was used to identify the role of Ser-60 in catalysis.The core structure of dPIMT is similar to the modified nucleotide-binding fold observed in PIMTs fromextreme thermophiles and humans. A striking difference of the dPIMT structure is the rotation of theC-terminal residues by 90° relative to the homologous structures. Effectively, this displacement generatesa more open conformation that allows greater solvent access to S-adenosylhomocysteine, which is almostcompletely buried in other PIMT structures. The enzyme may alternate between the open conformationfound for dPIMT and the more closed conformations described for other PIMTs during its catalytic cycle,thereby allowing the exchange of substrates and products. Catalysis by dPIMT requires the side chain ofthe conserved, active site residue Ser-60, since substitution of this residue with Thr, Gln, or Ala reducesor abolishes the methylation of both protein and isoaspartyl peptide substrates.
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