Mudgett M B, Clarke S
Department of Chemistry and Biochemistry, University of California, Los Angeles 90024-1569.
J Biol Chem. 1994 Oct 14;269(41):25605-12.
The L-isoaspartyl protein methyltransferase (EC 2.1.1.77) has been proposed to be involved in the repair of spontaneously damaged proteins by facilitating the conversion of abnormal L-isoaspartyl residues to normal L-aspartyl residues. Based on the abundance of this enzyme in the seeds of a variety of plants and its unique substrate specificity, it has been hypothesized that it functions to prevent the accumulation of abnormal aspartyl residues in the proteins of aging seeds that can limit the viability of the embryo or its chances for germination. In this work, we show that the expression of the L-isoaspartyl methyltransferase is under developmental regulation in the winter wheat, Triticum aestivum. Methyltransferase mRNA and active enzyme are first detected in seeds during the late stages (III-IV) of caryopsis development. As mature seeds germinate, methyltransferase mRNA levels decline and are nearly undetectable by 72 h post-imbibition. Enzyme activity remains constant for 24 h post-imbibition and then decreases rapidly following the reduction of its corresponding mRNA. Methyltransferase activity is very low or undetectable in wheat seedlings, including leaf and root tissues. We show, however, that the L-isoaspartyl methyltransferase can be induced in vegetative tissues in response to hormone treatment and environmental stress. Abscisic acid, a phytohormone involved in seed development and desiccation tolerance, induces both methyltransferase mRNA and enzyme activity in 4-day-old wheat seedlings. Dehydration and salt stress also induce its transcription and enzymatic activity in seedlings. The ability of a plant to regulate methyltransferase activity in its seeds and vegetative tissues in response to desiccation, aging, and environmental stress may allow the plant to efficiently repair protein damage associated with these physiological changes.
L-异天冬氨酰蛋白甲基转移酶（EC 2.1.1.77）被认为通过促进异常L-异天冬氨酰残基向正常L-天冬氨酰残基的转化，参与自发损伤蛋白质的修复。基于该酶在多种植物种子中的丰富含量及其独特的底物特异性，有人推测它的功能是防止衰老种子蛋白质中异常天冬氨酰残基的积累，这些异常残基可能会限制胚的活力或其发芽的机会。在这项研究中，我们表明L-异天冬氨酰甲基转移酶的表达在冬小麦（Triticum aestivum）中受发育调控。甲基转移酶mRNA和活性酶首先在颖果发育后期（III-IV期）的种子中被检测到。随着成熟种子萌发，甲基转移酶mRNA水平下降，在吸胀后72小时几乎检测不到。酶活性在吸胀后24小时保持恒定，然后随着相应mRNA的减少而迅速下降。甲基转移酶活性在小麦幼苗包括叶和根组织中非常低或检测不到。然而，我们发现L-异天冬氨酰甲基转移酶可以在营养组织中响应激素处理和环境胁迫而被诱导。脱落酸是一种参与种子发育和耐干燥性的植物激素，可诱导4日龄小麦幼苗中的甲基转移酶mRNA和酶活性。脱水和盐胁迫也会诱导其在幼苗中的转录和酶活性。植物响应干燥、衰老和环境胁迫而调节其种子和营养组织中甲基转移酶活性的能力，可能使植物能够有效地修复与这些生理变化相关的蛋白质损伤。
