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

Abstract

Ligand screening was utilized to isolate a human cDNA that encodes a novel CpG binding protein, human CpG binding protein (hCGBP). This factor contains three cysteine-rich domains, two of which exhibit homology to the plant homeodomain 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 domain binds to an oligonucleotide probe containing a single CpG site, and this complex is disrupted by distinct oligonucleotide competitors that also contain 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 single-stranded DNA or RNA probes. Furthermore, the introduction of a CpG dinucleotide 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 of native hCGBP is apparent in electrophoretic mobility shift assays, and hCGBP trans-activates promoters that contain CpG motifs but not promoters in which the CpG is ablated. These data indicate that hCGBP is a transcriptional activator that recognizes unmethylated CpG dinucleotides, suggesting a role in modulating the expression of genes located within CpG islands.

ACKNOWLEDGMENTS

We are grateful to Robert Hromas for providing the HL60 λgt11 cDNA library and to Nancy Andrews and Harinder Singh for helpful discussions regarding protein purification and ligand screening, respectively. We also acknowledge the Cell Culture Center and the National Center for Research Resources for providing large-scale tissue culture services at a subsidized rate.

This work was supported by Public Health Service grant CA58997 from the National Cancer Institute, awarded to D.G.S.; an Arthritis Foundation postdoctoral fellowship, awarded to K.S.V., an American Heart Association postdoctoral fellowship, awarded to D.L.C.; and a GAANN fellowship from the Department of Education, awarded to B.M.J.