Transcriptional Repression by Drosophila Methyl-CpG-Binding Proteins

Abstract

In vertebrates, including the mammals, the chromosomal DNAs are modified by C methylation at a limited number of CpG dinucleotides, resulting in m 5 CpG, methylated at position 5 of the C residues. This methylation at CpG has been implicated in the regulation of a number of genetic activities during mammalian cell differentiation and embryo development. These activities include tissue-specific gene transcription, X chromosome inactivation, genomic imprinting, cellular defense against viral agents, and tumorigenesis (references 1, 2, 11, 13, 15, and 34 and references therein). The level of m 5 CpG in the vertebrate cells is presumably balanced by the combined actions of DNA cytosine (C-5) methyltransferases (CpG MTases) (2, 26) and DNA demethylases (3).It is generally accepted that regulation of the different cellular activities by DNA methylation is modulated mainly through the m 5 CpG residues. Indeed, m 5 CpG could be recognized by a specific class of proteins that consist of the methylated-DNA binding domain (MBD) (21). These "MBD proteins," in particular MeCP2 and MBD2/3, could preferentially bind to m 5 CpG-containing DNA, recruit histone deacetylase (HDAC)-containing complexes, and thus cause chromatin condensation in the vicinity of m 5 CpG (12,22,23,33). This is very likely one major, but not the only, scheme involved in the functioning of vertebrate DNA methylation.Unlike the vertebrates, several invertebrate species, including Drosophila melanogaster (27, 31), do not have apparent DNA methylation in their genomes. Nor has CpG MTase been reported to occur in these invertebrate animals. It was thus surprising to find mammalian CpG MTase-related proteins as well as MBD proteins expressed in Drosophila. A Drosophila protein, DmMTR1, is characteristically similar to the vertebrate maintenance CpG MTase, dnmt1, in several aspects (9). In particular, DmMTR1 molecules are located outside the nuclei during interphase of the syncytial Drosophila embryos, as is dnmt1 in the mouse blastocyst (19). However, DmMTR1 molecules appear to be rapidly transported into and then out of the nuclei again, as the embryos undergo the mitotic waves (9). Immunofluorescence data further indicate that DmMTR1 molecules "paint" the whole set of condensed Drosophila chromosomes throughout the mitotic phase, suggesting that it may play an essential role in the cell cycle-regulated condensation of the Drosophila chromosomes. In addition to DmMTR1, another Drosophila polypeptide, DmMT2, exhibiting high sequence homology to mammalian dnmt2, a relatively short homolog of dnmt1 but without detectable methylation activities (25), has been identified through a database search (9, 30). Expression of DmMT2 in Drosophila is developmentally regulated (9).Interestingly, several recent reports have also noted the identification of an MBD protein encoded by the Drosophila genome (9,30,33). Like the mammalian MBD (12,22,23,33), the Drosophila MBD protein, which was termed the "Drosophila MBD-like sequence" (33) or dMBD2/3 (30), interacts with HDAC in...

The Drosophila MBD2/3 protein mediates interactions between the MI-2 chromatin complex and CpT/A-methylated DNA

“…Two previous studies have failed to detect an interaction between MBD2/3 and CpG-methylated probes (Ballestar et al, 2001;Tweedie et al, 1999). A third study described a preferential binding of the short MBD2/3 isoform to a human DNA probe containing a single methylated CpG (Roder et al, 2000). However, the short isoform lacks parts of the methyl-DNA binding domain, which rendered the significance of this finding unclear.…”

“…Confocal analysis of mutant embryos showed local displacement of MI-2 from DNA and indicated that MBD2/3 is associated CpG binding domain of MBD2/3 contains a number of deviations from the consensus MBD that seemed to be incompatible with a standard methyl-CpG binding activity (Ballestar et al, 2001;Tweedie et al, 1999). A very weak association with a CpG-methylated DNA fragment could be demonstrated in other experiments, but this interaction was observed with the short isoform and therefore seemed to be independent of the full-length methyl-CpG binding domain (Roder et al, 2000). Together, these results suggested that MBD2/3 might represent a functional homologue of mammalian MBD3, rather than MBD2 (Ballestar et al, 2001;Tweedie et al, 1999).…”

DNA methylation, chromatin inheritance, and cancer

“…S. cerevisiae, C. elegans) has long raised questions about the importance of DNA methylation for gene control in eukaryotes. Interestingly, while Drosophila was long thought not to have DNA methylation, recent reports have identi®ed genes with homology to the vertebrate DNA methylation machinery (Tweedie et al, 1999;Hung et al, 1999;Roder et al, 2000;Lyko et al, 2000a). In addition, trace amounts of cytosine methylation have been detected in this organism (Gowher et al, 2000;Lyko et al, 2000b).…”

The mysterious presence of a 5-methylcytosine oxidase in theDrosophilagenome

“…The dMBD2/3 protein was shown to have the capacity to bind to CpG-methylated DNA, at least in vitro. 16,17 In the year 2000, Lyko et al reported that Drosophila DNA is methylated, but that Drosophila genome methylation is restricted to early stages of embryo development. 18 Confocal analysis of immunostained Drosophila embryos provided further evidence for the methylation of embryonic DNA.…”