Arabidopsis protein repair L-isoaspartyl methyltransferases:: predominant activities at lethal temperatures

Protein L-isoaspartyl (D-aspartyl) O-methyltransferases (Enzyme Commission (EC) 2.1.1.77; PIMTor PCMT) are enzymes that initiate the full or partial repair of damaged L-aspartyl and L-asparaginyl residues, respectively. These enzymes are found in most organisms and maintain a high degree of sequence conservation. Arabidopsis thaliana (Arabidopsis L. Heynh.) is unique among eukaryotes in that it contains two genes, rather than one, that encode PIMT isozymes. We describe a novel A. thaliana PIMT isozyme, designated AtPIMT2 alpha omega, encoded by the PIMT2 gene (At5g50240). We characterized the enzymatic activity of the recombinant AtPIMT2 alpha omega in comparison to the other AtPIMT2 isozymes, AtPIMT1, and to the human PCMT1 ortholog, to better understand its role in Arabidopsis. All Arabidopsis PIMT isozymes are active over a relatively wide pH range. For AtPIMT2 alpha omega maximal activity is observed at 50 degrees C (a lethal temperature for Arabidopsis); this activity is almost 10 times greater than the activity at the growth temperature of 25 degrees C. Interestingly, enzyme activity decreases after pre-incubation at temperatures above 30 degrees C. A similar situation is found for the recombinant AtPIMT2 psi and the AtPIMT2 omega isozymes, as well as for the AtPIMT1 and human PCMT1 enzymes. These results suggest that the short-term ability of these methyltransferases to initiate repair under extreme temperature conditions may be a common feature of both the plant and animal species.