Cloning of a mammalian transcriptional activator that binds unmethylated CpG motifs and shares a CXXC domain with DNA methyltransferase, human trithorax, and methyl-CpG binding domain protein 1

Ligand screening was utilized to isolate a human cDNA that encodes a novel CpG binding protein, human CpG binding protein (hCGBP), This factor contain s three cysteine-rich domains, two of which exhibit homology to the plant h omeodomain finger domain, A third cysteine-rich domain conforms to the CXXC motif identified in DNA methyltransferase, human trithorax, and methyl-CpG binding domain protein 1, A fragment of hCGBP that contains the CXXC domai n binds to an oligonucleotide probe containing a single CpG site, and this complex is disrupted by distinct oligonucleotide competitors that also cont ain a CpG motif(s). However, hCGBP fails to bind oligonucleotides in which the CpG motif is either mutated or methylated, and it does not bind to sing le-stranded DNA or RNA probes, Furthermore, the introduction of a CpG dinuc leotide into an unrelated oligonucleotide sequence is sufficient to produce a binding site for hCGBP, Native hCGBP is detected as an 88-kDa protein by Western analysis and is ubiquitously expressed. The DNA-binding activity o f native hCGBP is apparent in electrophoretic mobility shift assays, and hC GBP trans-activates promoters that contain CPG motifs but not promoters in which the CpG is ablated. These data indicate that hCGBP is a transcription al activator that recognizes unmethylated CpG dinucleotides, suggesting a r ole in modulating the expression of genes located within CpG islands.