Epigenetic Regulation of Transcription and Chromatin by Methyl-CPG Binding Proteins

Mitsuyoshi Nakao
School of Medicine; Kumamoto University

DNA methylation is important for mammalian development especially genomic imprinting and cancer, and is interpreted by protein factors that contain a shared methyl-CpG binding domain (MBD). There are five MBD-containing proteins including MBD1, MBD2, MBD3, MBD4, and MeCP2. Among the MBD family of proteins, MBD1 is characterized by sequences similar to a cysteine-rich CXXC domain which was originally found in DNA methyltransferase and trithorax group protein ALL-1. MBD1 has at least five isoforms due to alternative splicing events, resulting in the presence of CXXCl, CXXC2, and CXXC3 in MBD1 isoform vl(MBDlvl) and MBDlv2, and CXXC1 and CXXC2 in MBDlv3 and -v4. In the present study, we have investigated the significance of MBD, CXXC, and the C-terminal transcriptional repression domain (TRD) in MBD1. Further, we have constructed mutant-type MBDs in which the functionally important residues Arg22, Arg30, Asp32, Tyr34, Arg44, Ser45 and Tyr52 are changed to alanine to investigate the correlation between the structure and function of the MBD in MBD1. Based on these findings we have concluded that MBD1 acts as a transcriptional regulator depending on the density of methyl-CpG pairs. Finally, MeCP2, MBD2, and MBD3 are embedded in the historic deacetylase complexes and are involved in packing the genomic DNA into the inactive chromatin, leading to transcriptional repression. However, MBD1 has not been found in known histone deacetylase complexes nor in the MeCP1 complex, suggesting that MBD1 may form a novel repressor complex or chromatin.