Structural studies of MeCP2 in complex with methylated DNA
DNA methylation is a common epigenetic mark that affects gene regulation, genomic stability and chromatin structure. In mammals, methylation is mainly found in the CpG dinucleotides. The CpG methylation signals can be recognised by the Methyl-CpG-Binding Protein (MBP) family which includes MeCP2, MBD1, MBD2, MBD3, MBD4 and Kiaso. MeCP2 and MBD1-4 (except mammalian MBD3) recognise methyl-CpG via their MBD domain whereas Kaiso interprets methylation through its Zn finger DNA binding domain. The TRD domains of MeCP2, MBD1 and MBD2 have been reported to recruit transcriptional co-repressors to the methylated DNA. A thymine DNA glycosylase domain is located at the C-terminal region of MBD4. This study concerns the molecular details of the methyl-CpG recognition by the MBD domain of MeCP2. To achieve this, the MeCP2 MBD domain has been expressed, purified and co-crystallised with a 20 bp DNA fragment from the BDNF promoter. The DNA-protein cocrystal diffracted X-rays to a maximum resolution of 2.5Å using synchrotron sources. It belongs to space group C2 with unit cell dimensions: a = 79.71Å, b = 53.60Å, c = 65.73Å, and β = 132.1°. The X-ray structure of the MeCP2 MBD-DNA complex was solved using the SAD method. Structural analyses of the refined X-ray structure reveal that the methyl groups of the DNA make contact with a predominantly hydrophilic surface that includes tightly bound water molecules. From a structure of the MBD domain in MBD1, established by NMR, the binding specificity of the MBD domain had been thought to depend on hydrophobic interactions between the cytosine methyl groups and a hydrophobic patch within the MBD domain. The findings of this study suggest that MeCP2 recognises the hydration pattern of the major groove of methylated DNA rather than cytosine methylation per se. The X-ray structure also identifies a unique role of T158 and R106, the sites of the two most frequent Rett missense mutations. Both residues stabilise the tandem Asx-ST motif at the C-terminal region of MBD domain. Disruption of this tandem motif destabilises the DNA-protein interaction. The BDNF sequence in this study contains an AT run which displays unique properties of AT tract DNA. Previously, mutation of the AT run has been reported to decrease MeCP2 binding specificity. This study however demonstrated that a significant reduction can only be observed when both AT runs close to the methyl-CpG have been mutated. The X-ray structure of the MeCP2 MBD-DNA complex in this study rationalises the effects of the most common Rett mutations and provides a general model for methylated DNA binding that is dependent on structured water molecules.