Regulation of transcription and chromatin by methyl-CpG binding protein MBD1

DNA methylation is important for epigenetic regulation of genome, and it is interpreted by specific protein factors that contain a highly conserved methyl-CpG binding domain (MBD). There are at present five mammalian MBD family proteins: MBD1, MBD2, MBD3, MBD4 and MeCP2. In the family of methyl-CpG binding proteins, MBD1 is characterized by the presence of MBD, two or three cysteine-rich CXXC motifs, and the C-terminal transcriptional repression domain (TRD). In addition, MBD1 has at least five isoforms due to alternative splicing events, resulting in the existence of CXXC1, CXXC2, and CXXC3 in MBD1 isoform v1 (MBD1v1) and MBD1v2, and CXXC1 and CXXC2 in MBD1v3 and MBD1v4. MBD1v1 represses transcription preferentially from both unmethylated and hypomethylated promoters, while MBD1v3 inhibits hypermethylated but not unmethylated promoter activities. The MBD and CXXC3 sequences are responsible for the ability to bind methylated and unmethylated DNAs, respectively. MBD1 is also found to be a chromosomal protein that forms many foci within the nucleus. These findings suggest that MBD1 is a unique transcriptional regulator depending on the density of methyl-CpG pairs.