Calcium-dependent protein kinase CPK28 targets the methionine adenosyltransferases for degradation by the 26S proteasome and affects ethylene biosynthesis and lignin deposition in Arabidopsis

S-adenosylmethionine (AdoMet) is synthesized by methionine adenosyltransferase (MAT), and plays an essential role in ethylene biosynthesis and other methylation reactions. Despite increasing knowledge of MAT regulation at transcriptional levels, how MAT is post-translationally regulated remains unknown in plant cells. Phosphorylation is an important post-translational modification for regulating the activity of enzymes, protein function and signaling transduction. Using molecular and biochemical approaches, we have identified the phosphorylation of MAT proteins by calcium-dependent protein kinase (CPK28). Phenotypically, both MAT2-overexpressing transgenic plants and cpk28 mutants display short hypocotyls and ectopic lignifications. Their shortened hypocotyl phenotypes are caused by ethylene overproduction and rescued by ethylene biosynthesis inhibitor aminoethoxyvinylglycine treatment. Genetic evidence reveals that MAT2 mutation restores the phenotype of ectopic lignification in CPK28-deficient plants. We find that total MAT proteins and AdoMet are increased in cpk28 mutants, but decreased in CPK28-overexpressing seedlings. We also find that MATs in OE::CPK28 are degraded through the 26S proteasome pathway. Our work suggests that CPK28 targets MATs (MAT1, MAT2 and MAT3) for degradation by the 26S proteasome pathway, and thus affects ethylene biosynthesis and lignin deposition in Arabidopsis.

S - 腺苷甲硫氨酸（AdoMet）由甲硫氨酸腺苷转移酶（MAT）合成，在乙烯生物合成及其他甲基化反应中起重要作用。尽管在转录水平上对MAT调控的了解日益增多，但在植物细胞中MAT如何在翻译后被调控仍然未知。磷酸化是一种重要的翻译后修饰，用于调节酶的活性、蛋白质功能和信号转导。通过分子和生化方法，我们已经确定了钙依赖蛋白激酶（CPK28）对MAT蛋白的磷酸化作用。在表型上，MAT2过表达的转基因植物和cpk28突变体都表现出下胚轴短和异位木质化。它们下胚轴缩短的表型是由乙烯过量产生引起的，并可通过乙烯生物合成抑制剂氨基乙氧基乙烯基甘氨酸处理来挽救。遗传证据表明，MAT2突变可恢复CPK28缺陷植物的异位木质化表型。我们发现cpk28突变体中总MAT蛋白和AdoMet增加，而在CPK28过表达的幼苗中减少。我们还发现OE::CPK28中的MATs通过26S蛋白酶体途径降解。我们的工作表明，CPK28靶向MATs（MAT1、MAT2和MAT3）通过26S蛋白酶体途径进行降解，从而影响拟南芥中的乙烯生物合成和木质素沉积。