XAB1: Proteins and Enzymes

The DNA methylation system is composed of methyl-CpG-binding proteins, as well as of DNA cytosine methyl transferases. Five methyl-CpG binding proteins were isolated: MeCP2, MBD1, MBD2, MBD3 and MBD4. MBD2 consists of two isoforms, MBD2a and MBD2b, which are generated from a single gene. MBD2a is a component of MeCP1, a large corepressor complex that represses transcription from densely methylated genes. Components of MeCP1 include MBD2, Mi-2, MTA2, MBD3 and HDAC1/2. MIZF and MBDin were isolated as proteins interacting with MBD2. MBDin, in turn, is identical to XPA binding protein 1 (XAB1). At the N erminus, the protein contains the consensus sequence for a GTP binding site. At the C terminus, the protein is characterized by an acidic domain containing a cluster of acidic amino acid residues. The transcriptional repression by MBD2 at methylated promoters is relieved by MBdin. XAB1 is expressed ubiquitously, and may be involved in nuclear localization of XPA.