Zemach Assaf, Grafi Gideon
Department of Plant Sciences, The Weizmann Institute of Science, Rehovot 76100, Israel.
Plant J. 2003 Jun;34(5):565-72. doi: 10.1046/j.1365-313x.2003.01756.x.
Cytosine methylation at symmetrical CpG and CpNpG sequences plays a key role in the epigenetic control of plant growth and development; yet, the way by which the methylation signal is interpreted into a functional state has not been elucidated. In animals, the methylation signal is recognized by methyl-CpG-binding domain (MBD) proteins that specifically bind methylated CpG dinucleotides. In Arabidopsis thaliana, 12 putative MBD proteins were identified and classified into seven subclasses. Here, we characterized six MBD proteins representing four subclasses (II, III, IV, and VI) of the Arabidopsis MBD family. We found that AtMBD7 (subclass VI), a unique protein containing a double MBD motif, as well as AtMBD5 and AtMBD6 (subclass IV), bind specifically symmetrically methylated CpG sites. The MBD motif derived from AtMBD6, but not from AtMBD2, was sufficient for binding methylated CpG dinucleotides. AtMBD6 precipitated histone deacetylase (HDAC) activity from the leaf nuclear extract. The examined AtMBD proteins neither bound methylated CpNpG sequences nor did they display DNA demethylase activity. Our results suggest that AtMBD5, AtMBD6, and AtMBD7 are likely to function in Arabidopsis plants as mediators of the CpG methylation, linking DNA methylation-induced gene silencing with histone deacetylation.
对称的CpG和CpNpG序列上的胞嘧啶甲基化在植物生长发育的表观遗传调控中起关键作用；然而，甲基化信号被转化为功能状态的方式尚未阐明。在动物中，甲基化信号由特异性结合甲基化CpG二核苷酸的甲基-CpG结合域（MBD）蛋白识别。在拟南芥中，鉴定出12种假定的MBD蛋白并将其分为7个亚类。在这里，我们对代表拟南芥MBD家族4个亚类（II、III、IV和VI）的6种MBD蛋白进行了表征。我们发现，AtMBD7（VI亚类），一种含有双MBD基序的独特蛋白，以及AtMBD5和AtMBD6（IV亚类），特异性结合对称甲基化的CpG位点。源自AtMBD6而非AtMBD2的MBD基序足以结合甲基化的CpG二核苷酸。AtMBD6从叶核提取物中沉淀出组蛋白脱乙酰酶（HDAC）活性。所检测的AtMBD蛋白既不结合甲基化的CpNpG序列，也不显示DNA去甲基化酶活性。我们的结果表明，AtMBD5、AtMBD6和AtMBD7可能在拟南芥植物中作为CpG甲基化的介质发挥作用，将DNA甲基化诱导的基因沉默与组蛋白去乙酰化联系起来。
