Jørgensen Helle F, Ben-Porath Ittai, Bird Adrian P
The Wellcome Trust Centre for Cell Biology, University of Edinburgh, Edinburgh, United Kingdom.
Mol Cell Biol. 2004 Apr;24(8):3387-95. doi: 10.1128/MCB.24.8.3387-3395.2004.
MBD1 is a vertebrate methyl-CpG binding domain protein (MBD) that can bring about repression of methylated promoter DNA sequences. Like other MBD proteins, MBD1 localizes to nuclear foci that in mice are rich in methyl-CpG. In methyl-CpG-deficient mouse cells, however, Mbd1 remains localized to heterochromatic foci whereas other MBD proteins become dispersed in the nucleus. We find that Mbd1a, a major mouse isoform, contains a CXXC domain (CXXC-3) that binds specifically to nonmethylated CpG, suggesting an explanation for methylation-independent localization. Transfection studies demonstrate that the CXXC-3 domain indeed targets nonmethylated CpG sites in vivo. Repression of nonmethylated reporter genes depends on the CXXC-3 domain, whereas repression of methylated reporters requires the MBD. Our findings indicate that MBD1 can interpret the CpG dinucleotide as a repressive signal in vivo regardless of its methylation status.
MBD1是一种脊椎动物甲基化CpG结合域蛋白（MBD），它能够导致甲基化启动子DNA序列的抑制。与其他MBD蛋白一样，MBD1定位于核内聚集区，在小鼠中这些区域富含甲基化CpG。然而，在甲基化CpG缺陷的小鼠细胞中，Mbd1仍定位于异染色质聚集区，而其他MBD蛋白则分散于细胞核中。我们发现，小鼠主要的异构体Mbd1a含有一个CXXC结构域（CXXC-3），该结构域能特异性结合非甲基化的CpG，这为其不依赖甲基化的定位提供了解释。转染研究表明，CXXC-3结构域在体内确实靶向非甲基化的CpG位点。非甲基化报告基因的抑制依赖于CXXC-3结构域，而甲基化报告基因的抑制则需要MBD。我们的研究结果表明，MBD1在体内能够将CpG二核苷酸解读为一种抑制信号，而不论其甲基化状态如何。
