Cys2His2 Zinc Finger Methyl-CpG Binding Proteins: Getting a Handle on Methylated DNA.

DNA methylation is an essential epigenetic modification involved in the maintenance of genomic stability, preservation of cellular identity, and regulation of the transcriptional landscape needed to maintain cellular function. In an increasing number of disease conditions, DNA methylation patterns are inappropriately distributed in a manner that supports the disease phenotype. Methyl-CpG binding proteins (MBPs) are specialized transcription factors that read and translate methylated DNA signals into recruitment of protein assemblies that can alter local chromatin architecture and transcription. MBPs thus play a key intermediary role in gene regulation for both normal and diseased cells. Here, we highlight established and potential structure-function relationships for the best characterized members of the zinc finger (ZF) family of MBPs in propagating DNA methylation signals into downstream cellular responses. Current and future investigations aimed toward expanding our understanding of ZF MBP cellular roles will provide needed mechanistic insight into normal and disease state functions, as well as afford evaluation for the potential of these proteins as epigenetic-based therapeutic targets.

Zinc Finger Readers of Methylated DNA.

DNA methylation is a prevalent epigenetic modification involved in regulating a number of essential cellular processes, including genomic accessibility and transcriptional outcomes. As such, aberrant alterations in global DNA methylation patterns have been associated with a growing number of disease conditions. Nevertheless, the full mechanisms by which DNA methylation information is interpreted and translated into genomic responses is not yet fully understood. Methyl-CpG binding proteins (MBPs) function as important mediators of this essential process by selectively reading DNA methylation signals and translating this information into down-stream cellular outcomes. The Cys2His2 zinc finger scaffold is one of the most abundant DNA binding motifs found within human transcription factors, yet only a few zinc finger containing proteins capable of conferring selectivity for mCpG over CpG sites have been characterized. This review summarizes our current structural understanding for the mechanisms by which the zinc finger MBPs evaluated to date read this essential epigenetic mark. Further, some of the biological implications for mCpG readout elicited by this family of MBPs are discussed.

Structural insights into methylated DNA recognition by the C-terminal zinc fingers of the DNA reader protein ZBTB38.

Methyl-CpG-binding proteins (MBPs) are selective readers of DNA methylation that play an essential role in mediating cellular transcription processes in both normal and diseased cells. This physiological function of MBPs has generated significant interest in understanding the mechanisms by which these proteins read and interpret DNA methylation signals. Zinc finger and BTB domain-containing 38 (ZBTB38) represents one member of the zinc finger (ZF) family of MBPs. We recently demonstrated that the C-terminal ZFs of ZBTB38 exhibit methyl-selective DNA binding within the ((A/G)TmCG(G/A)(mC/T)(G/A)) context both in vitro and within cells. Here we report the crystal structure of the first four C-terminal ZBTB38 ZFs (ZFs 6-9) in complex with the previously identified methylated consensus sequence at 1.75 Å resolution. From the structure, methyl-selective binding is preferentially localized at the 5' mCpG site of the bound DNA, which is facilitated through a series of base-specific interactions from residues within the α-helices of ZF7 and ZF8. ZF6 and ZF9 primarily stabilize ZF7 and ZF8 to facilitate the core base-specific interactions. Further structural and biochemical analyses, including solution NMR spectroscopy and electrophoretic mobility gel shift assays, revealed that the C-terminal ZFs of ZBTB38 utilize an alternative mode of mCpG recognition from the ZF MBPs structurally evaluated to date. Combined, these findings provide insight into the mechanism by which this ZF domain of ZBTB38 selectively recognizes methylated CpG sites and expands our understanding of how ZF-containing proteins can interpret this essential epigenetic mark.

The C-Terminal Zinc Fingers of ZBTB38 are Novel Selective Readers of DNA Methylation.

Methyl-CpG binding proteins play an essential role in translating DNA methylation marks into a downstream transcriptional response, which has implications for both normal cell function as well as disease. Although for many of these proteins, a detailed mechanistic understanding for how this cellular process is mediated remains to be determined. ZBTB38 is an under-characterized member of the zinc finger (ZF) family of methyl-CpG binding proteins. Functional knowledge has been gained for its conserved methylated DNA binding N-terminal ZF region; however, a specific role for the C-terminal set of five ZFs remains to be elucidated. Here we demonstrate for the first time that a subset of the C-terminal ZBTB38 ZFs exhibit high-affinity DNA interactions and that preferential targeting of the consensus DNA site is methyl specific. Utilizing a hybrid approach, a model for the C-terminal ZBTB38 ZFs in complex with its cognate DNA target is proposed, providing insight into a possible novel mode of methylated DNA recognition. Furthermore, it is shown that the C-terminal ZFs of ZBTB38 can directly occupy promoters harboring the newly identified sequence motif in cell in a methyl-dependent manner and, depending on the gene context, contribute to modulating transcriptional response. Combined, these findings provide evidence for a key and novel physiological function for the C-terminal ZF domain of ZBTB38.