Methyl-CpG-binding domain protein 4 (MBD4) is a mismatch-specific DNA N-glycosylase involved in DNA repair. It has thymine glycosylase activity and is specific for G:T mismatches within methylated and unmethylated CpG sites [, ].
Methylation at CpG dinucleotide, the most common DNA modification ineukaryotes, has been correlated with gene silencing associated with variousphenomena such as genomic imprinting, transposon and chromosome X inactivation, differentiation, and cancer. Effects of DNA methylation are mediated through proteins which bind to symmetrically methylated CpGs. Such proteins contain a specific domain of ~70 residues, the methyl-CpG-binding domain (MBD), which is linked to additional domains associated with chromatin, such as the bromodomain, the AT hook motif,the SET domain, or the PHD finger. MBD-containing proteins appear to act as structural proteins, which recruit a variety of histone deacetylase (HDAC) complexes and chromatin remodelling factors, leading to chromatin compaction and, consequently, to transcriptional repression. The MBD of MeCP2, MBD1, MBD2, MBD4 and BAZ2 mediates binding to DNA, in case of MeCP2, MBD1 and MBD2 preferentially to methylated CpG. In case of human MBD3 and SETDB1 the MBD has been shown to mediate protein-protein interactions [, ].The MBD folds into an alpha/beta sandwich structure comprising a layer oftwisted beta sheet, backed by another layer formed by the alpha1 helix and ahairpin loop at the C terminus. These layers are both amphipathic, with the alpha1 helix and the beta sheet lying parallel and the hydrophobic faces tightly packed against each other. The beta sheet is composed of two long inner strands (beta2 and beta3) sandwiched by two shorter outer strands (beta1 and beta4) [].
