Overexpression of transcription factor AP-2 has been implicated in the tumorigenicity of the human teratocarcinoma cell lines PA-1 that contain an activated ras oncogene. Here we show evidence that overexpression of AP-2 sequesters transcriptional coactivators which results in self-inhibition. We identified AP-2-interacting proteins and determined whether these proteins were coactivators for AP-2-mediated transcription. One such interacting protein is polyADP-ribose polymerase (PARP). PARP suppresses AP-2 self-inhibition and enhances AP-2 activity in PA-1 cells indicating that it is a coactivator for AP-2-transcription. PARP significantly restores AP-2 transcriptional activity in ras oncogene-transformed cells suggesting that it might suppress transformation in these cells. Another AP-2-interacting protein, RAP74, a subunit of transcription factor TFIIF, does not affect AP-2-mediated transcriptional activation alone or in the presence of RAP30, the other subunit of TFIIF. RAP74 also fails to relieve AP-2-mediated transcriptional self-interference and cross-interference. These studies suggest that the interaction between AP-2 and RAP74 may have functions other than activation of AP-2-mediated transcription.

Despite sequence variation, all AP-2 isotypes are capable of activating transcription, which indicates a functional conservation. We used this property to gain a unique insight into the structure and function of the activation motifs of AP-2 family transcription factors. We have precisely localized the activation motif of human AP-2␣ to amino acids 52-108. Our experiments indicate that similar sequence of amino acids in all AP-2 isotypes except Drosophila AP-2␣ harbor their activation motifs. Within this sequence, fewer than 36 residues are critical for transcription activation. Our comparison studies and site-directed mutagenic analyses show that these critical amino acids are strategically placed within this sequence. These residues are interspersed with nonessential and influential residues that vary in composition and length, indicating a structural flexibility. The Drosophila AP-2␣ has its partly conserved activation motif in an extended region about twice the length of other AP-2 isotypes. Our results reveal essential elements of the amino acid composition of activators in general and shed new light on the mechanism of transcription activation.