MeCP2 interacts with chromosomal microRNAs in brain

Although methyl CpG binding domain protein-2 (MeCP2) is commonly understood to function as a silencing factor at methylated DNA sequences, recent studies also show that MeCP2 can bind unmethylated sequences and coordinate gene activation. MeCP2 displays broad binding patterns throughout the genome, with high expression levels similar to histone H1 in neurons. Despite its significant presence in the brain, only subtle gene expression changes occur in the absence of MeCP2. This may reflect a more complex regulatory mechanism of MeCP2 to complement chromatin binding. Using an RNA immunoprecipitation of native chromatin technique, we identify MeCP2 interacting microRNAs in mouse primary cortical neurons. In addition, comparison with mRNA sequencing data from Mecp2-null mice suggests that differentially expressed genes may indeed be targeted by MeCP2-interacting microRNAs. These findings highlight the MeCP2 interaction with microRNAs that may modulate its binding with chromatin and regulate gene expression.     

DNA binding restricts the intrinsic conformational flexibility of methyl CpG binding protein 2 (MeCP2)

Mass spectrometry-based hydrogen/deuterium exchange (H/DX) has been used to define the polypeptide backbone dynamics of full-length methyl CpG binding protein 2 (MeCP2) when free in solution and when bound to unmethylated and methylated DNA. Essentially the entire MeCP2 polypeptide chain underwent H/DX at rates faster than could be measured (i.e. complete exchange in ≤10 s), with the exception of the methyl DNA binding domain (MBD). Even the H/DX of the MBD was rapid compared with that of a typical globular protein. Thus, there is no single tertiary structure of MeCP2. Rather, the full-length protein rapidly samples many different conformations when free in solution. When MeCP2 binds to unmethylated DNA, H/DX is slowed several orders of magnitude throughout the MBD. Binding of MeCP2 to methylated DNA led to additional minor H/DX protection, and only locally within the N-terminal portion of the MBD. H/DX also was used to examine the structural dynamics of the isolated MBD carrying three frequent mutations associated with Rett syndrome. The effects of the mutations ranged from very little (R106W) to a substantial increase in conformational sampling (F155S). Our H/DX results have yielded fine resolution mapping of the structure of full-length MeCP2 in the absence and presence of DNA, provided a biochemical basis for understanding MeCP2 function in normal cells, and predicted potential approaches for the treatment of a subset of RTT cases caused by point mutations that destabilize the MBD.     

Structure-specific binding of MeCP2 to four-way junction DNA through its methyl CpG-binding domain

MeCP2, whose methylated DNA-binding domain (MBD) binds preferentially to DNA containing 5Me-CpG relative to linear unmethylated DNA, also binds preferentially, and with similar affinity, to unmethylated four-way DNA junctions through the MBD. The Arg133Cys (R133C) mutation in the MBD, a Rett syndrome mutation that abolishes binding to methylated DNA, leads to only a slight reduction in the affinity of the MBD for four-way junctions, suggesting distinct but partially overlapping modes of binding to junction and methylated DNA. Binding to unmethylated DNA junctions is likely to involve a subset of the interactions that occur with methylated DNA. High-affinity, methylation-independent binding to four-way junctions is consistent with additional roles for MeCP2 in chromatin, beyond recognition of 5Me-CpG.