Coiled-coil structure-mediated dimerization of template activating factor-I is critical for its chromatin remodeling activity

Template activating factor-I (TAF-I)α and TAF-Iβ have been identified as the host factors that activate DNA replication of the adenovirus genome complexed with viral basic core proteins (Ad core). TAF-I causes a structural change of the Ad core, thereby stimulating not only replication but also transcription from the Ad core DNA in vitro. TAF-I also activates transcription from the reconstituted chromatin consisting of DNA fragments and purified histones through chromatin remodeling. Although the carboxyl-terminal region, which is highly rich in acidic amino acids, is essential for the TAF-I activity, it remains unclear how other parts are involved in its activity. The native TAF-I isolated from HeLa cells exists as either hetero- or homo-oligomer. Here, we have demonstrated by cross-linking assays that most of TAF-I exists as a dimer. Analyses using deletion mutant TAF-I proteins revealed that the amino-terminal region of TAF-I common to both α and β is essential for dimerization. This region is predicted to form a coiled-coil structure. Indeed, mutations disrupting this putative structure abolished the dimerization capability and reduced the TAF-I activity in the Ad core DNA replication assay. Furthermore, we found that TAF-I mutants lacking the acidic tail act in a dominant-negative manner in this assay. These observations strongly suggest that the dimerization of TAF-I is important for its activity.