Different biological activities of the hetero- and homodimers formed by the 47- and 43-kilodalton proteins of transcription factor ATF/E4TF3.

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The transcription factor ATF/E4TF3 stimulates transcription from the adenovirus early region 4 (E4) promoter by binding to specific promoter elements. Among the multiple forms of ATF/E4TF3, two forms with molecular masses of 47 and 43 kDa, which are most active in transcription in vitro from the E4 promoter, have been purified to homogeneity from HeLa cells by sequence-specific DNA affinity chromatography and biochemically characterized. Each purified protein formed a homodimer. These two homodimers were easily altered into a heterodimer when mixed together in the absence, but not in the presence, of the specific DNA sequence. All of these dimers were able to activate transcription in vitro from the E4 promoter by binding to the specific DNA sequence. Their activities to bind to DNA or stimulate transcription were different. The ability of the 47-kDa homodimer to stimulate transcription in vitro from the E4 promoter was approximately nine and three times higher than the abilities of the 43 kDa homodimer and the heterodimer, respectively, at the same level of DNA-binding activity. However, the affinity of the 47-kDa homodimer for DNA was lower than that of the 43-kDa homodimer, and the heterodimer had intermediate affinity. These results are the first to show differential binding and transcriptional activation activities of the different dimers of ATF/E4TF3, using purified cellular proteins rather than cloned gene products.

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