Specific protection of methylated CpGs in mammalian nuclei

DNA methylation and chromatin structure

“…In our Iaboratory, Antequera et al [5] have shown that the accessibility of sites in nuclei for CpG enzymes depends on their methylation status. This result was unexpected because these enzymes normally cut naked DNA irrespective of methylation.…”

“…It is found only at very low levels in F9 and PC13 cell lines, both of which are embryonically derived. Interestingly, Antequera et al [5] found that the level of mSCpG protection was reduced in PC13 cells, establishing a correlation between levels of MeCP and degree of m'CpG protection. These studies also established that binding of MeCP to methylated DNA in vitro reduces the accessibility of methylated MspI sites.…”

“…The method was based on the earlier observation of Antequera et al [5] that complete digestion of nuclei with CpG enzymes gives short oligonucleosomes derived from CpG islands, but leaves bulk chromatin intact. This means that transcriptionally active chromatin can be very highly enriched and its protein composition analysed.…”

Structure, function and regulation of mammalian DNA methyltransferase

“…At such times the de novo activity of DNA MTase might be stimulated by proteolytic cleavage between the N-and C-terminal domains or interaction with a factor which counteracts the inhibitory effects of the N-terminal domain. The masking model cannot be looked on with much favor, as it is precisely the unmethylated CpG sites which are accessible to diffusible factors in nuclei (Antequera et al, 1989), and genomic sequencing has not shown a bias in the sequences Banking methylated and unmethylated CpG sites (Jost et al, 1990). Sequencespecific masking proteins would be expected to leave some evidence of a consensus sequence around unmethylated CpG sites.…”

“…The unmethylated fraction of the genome is exposed to diffusible factors in nuclei (Antequera et al, 1989), perhaps due to the action of proteins which bind to methylated sequences and induce their condensation (Meehan et al, 1989). Methylation may therefore control the availability of regulatory sequences for interaction with the transcriptional apparatus.…”