Distinct Patterns of Expression But Similar Biochemical Properties of Protein l-Isoaspartyl Methyltransferase in Higher Plants

Protein l-isoaspartyl methyltransferase is a widely distributed repair enzyme that initiates the conversion of abnormal l-isoaspartyl residues to their normal l-aspartyl forms. Here we show that this activity is expressed in developing corn (Zea mays) and carrot (Daucus carota var. Danvers Half Long) plants in patterns distinct from those previously seen in winter wheat (Triticum aestivum cv Augusta) and thale cress (Arabidopsis thaliana), whereas the pattern of expression observed in rice (Oryza sativa) is similar to that of winter wheat. Although high levels of activity are found in the seeds of all of these plants, relatively high levels of activity in vegetative tissues are only found in corn and carrot. The activity in leaves was found to decrease with aging, an unexpected finding given the postulated role of this enzyme in repairing age-damaged proteins. In contrast with the situation in wheat and Arabidopsis, we found that osmotic or salt stress could increase the methyltransferase activity in newly germinated seeds (but not in seeds or seedlings), whereas abscisic acid had no effect. We found that the corn, rice, and carrot enzymes have comparable affinity for methyl-accepting substrates and similar optimal temperatures for activity of 45°C to 55°C as the wheat and Arabidopsis enzymes. These experiments suggest that this enzyme may have specific roles in different plant tissues despite a common catalytic function.