Cloning and Characterization of a Novel Mouse AP-2 Transcription Factor, Ap-2δ, with Unique DNA Binding and Transactivation Properties

AP-2 transcription factors are sequence-specific DNA-binding proteins expressed in neural crest and other tissues during mammalian development. Three mammalian genes, AP-2α, AP-2β, and AP-2γ, have been reported previously. A partial predicted AP-2 gene was identified in tandem with AP-2β on human chromosome 6p12-p21.1. The orthologous mouse gene, which we named Ap-2δ, was identified from a fetal mouse head cDNA library. Northern analysis revealed two transcripts in embryonic and newborn mouse brain, with markedly higher steady-state levels in the former. The predicted Ap-2δ protein comprised 452 amino acids and was highly similar to other AP-2 proteins across the DNA-binding and dimerization domains. Ap-2δ formed homodimers and heterodimers in vitro, bound an optimized AP-2 consensus DNA sequence, and transactivated gene expression in eukaryotic cells. Ap-2δ dimers bound poorly to an AP-2 binding sequence from the human metallothionein IIa promoter in vitro, revealing a sequence specificity not previously observed among other AP-2 proteins. The PY motif and critical residues in the transactivation domain, which are highly conserved in the AP-2 family and believed necessary for transactivation, were divergent in Ap-2δ. The unique protein sequence and functional features of Ap-2δ suggest mechanisms, besides tissue-specific AP-2 gene expression, for specific control of target gene activation.