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Insights into the substrate discrimination mechanisms of methyl-CpG-binding domain 4.

ABSTRACT: G:T mismatches, the major mispairs generated during DNA metabolism, are repaired in part by mismatch-specific DNA glycosylases such as methyl-CpG-binding domain 4 (MBD4) and thymine DNA glycosylase (TDG). Mismatch-specific DNA glycosylases must discriminate the mismatches against million-fold excess correct base pairs. MBD4 efficiently removes thymine opposite guanine but not opposite adenine. Previous studies have revealed that the substrate thymine is flipped out and enters the catalytic site of the enzyme, while the estranged guanine is stabilized by Arg468 of MBD4. To gain further insights into the mismatch discrimination mechanism of MBD4, we assessed the glycosylase activity of MBD4 toward various base pairs. In addition, we determined a crystal structure of MBD4 bound to T:O6-methylguanine-containing DNA, which suggests the O6 and N2 of purine and the O4 of pyrimidine are required to be a substrate for MBD4. To understand the role of the Arg468 finger in catalysis, we evaluated the glycosylase activity of MBD4 mutants, which revealed the guanidinium moiety of Arg468 may play an important role in catalysis. D560N/R468K MBD4 bound to T:G mismatched DNA shows that the side chain amine moiety of the Lys stabilizes the flipped-out thymine by a water-mediated phosphate pinching, while the backbone carbonyl oxygen of the Lys engages in hydrogen bonds with N2 of the estranged guanine. Comparison of various DNA glycosylase structures implies the guanidinium and amine moieties of Arg and Lys, respectively, may involve in discriminating between substrate mismatches and nonsubstrate base pairs.

SUBMITTER: Ouzon-Shubeita H

PROVIDER: S-EPMC8173489 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Methyl CpG binding domain 3 (Mbd3) protein belongs to the MBD family of proteins, responsible for reading the DNA methylation pattern. MBD family proteins bind the methyl-CpG domain and are also involved in heterochromatin formation. Mbd3 protein does not have the ability to selectively recognize methyl-CpG islands, however, its characteristic feature is the ability to bind to 5-hydroxymethylcytosine and unmethylated DNA. Little is known about the role of Mbd3 in epilepsy and epileptogenesis. Our previous study (Bednarczyk et al., 2016) showed an increase in levels of NuRD complex proteins, including Mbd3 protein, in the brain of epileptic animals in a rat model of temporal epilepsy induced by electrical stimulation of the amygdala. Amygdala stimulation induced the binding of NuRD complex containing MBD3 to larger number of regions of DNA. In the present study, we investigated whether the Mbd3 protein is involved in the determination of the seizure threshold. An increase in Mbd3 protein levels was demonstrated in the entorhinal cortex/amygdala in the rat’s brain 4 hours after pentylenetetrazole (PTZ)-induced seizures. No alterations in MBD3 protein levels were detected in hippocampus and somatosensory cortex. No alterations in MBD3 mRNA expression were observed at any time point in any studied brain area. Reduction of Mbd3 level using AAV vector coding shRNA against Mbd3 injected to the amygdala prolonged the latency time to the onset of an acute seizure in PTZ challenge test indicating increase in seizure threshold. This was accompanied by increased anxiety in the open field test. In the contrast, an overexpression of Mbd3 using AAV decreased anxiety and increased their excitability in the open field test. Moreover Mbd3 overexpression in the amygdala accelerated epileptogenesis in the PTZ-kindling model. In order to identify the role of Mbd3 protein in the regulation of gene expression, mRNA profiling with RNA-seq was performed in a model of magnesium deficiency-induced epileptiform discharges in vitro. Mbd3 overexpression in vitro induced changes in gene expression in a time- and state-specific manner. Our data indicate the pro-epileptic properties of the Mbd3 protein in vivo and in vitro.

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Insights into the substrate discrimination mechanisms of methyl-CpG-binding domain 4.

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