Active repression of methylated genes by the chromosomal protein MBD1

MBD1 belongs to a family of mammalian proteins that share a methyl-CpG bind ing domain. Previous work has shown that MBD1 binds to methylated sites in vivo and in vitro and can repress transcription from methylated templates i n transcription extracts and in cultured cells. In the present study we est ablished by several experimental criteria that, contrary to a previous repo rt, MBD1 is not a component of the MeCP1 repressor complex. We identified a powerful transcriptional repression domain (TRD) at the C terminus of MBD1 that can actively repress transcription at a distance. Methylation-depende nt repression in vivo depends on the presence of both the TRD and the methy l-CpG binding domain. The mechanism is likely to involve deacetylation, sin ce the deacetylase inhibitor trichostatin A can overcome MBD1-mediated repr ession. Accordingly, we found that endogenous MBD1 is particularly concentr ated at sites of centromeric heterochromatin, where acetylated histone H4 i s deficient. Unlike MBD2 and MeCP2, MBD1 is not depleted by antibodies to t he histone deacetylase HDAC1. Thus, the deacetylase-dependent pathway by wh ich MBD1 actively silences methylated genes is likely to be different from that utilized by the methylation-dependent repressors MeCP1 and MeCP2.