Adenovirus DNA binding protein interacts with the SNF2-related CBP activator protein (SrCap) and inhibits SrCap-mediated transcription

The SNF2-related CBP activator protein, SrCap (pronounced "sir cap"), share s homology with the SNF2/SWI2 protein family. SrCap was cloned through its ability to bind CBP. SrCap can function as a CBP coactivator and can activa te transcription in a reporter assay when expressed as a Gal-SrCap fusion p rotein. A monoclonal antibody raised against the carboxyl terminus of SrCap coimmunoprecipitates CBP/p300, supporting the model that SrCap is a CBP bi nding protein and that these proteins can be found together in a cellular p rotein complex. In addition, several cellular proteins are coimmunoprecipit ated by the SrCap-specific antibody. Since adenovirus E1A proteins interact with CBP/p300 proteins, we examined what proteins could be copurified in a SrCap-specific coimmunoprecipitation assay from lysates of adenovirus-infe cted cells. While EIA proteins were not detected in this complex, to our su rprise, we observed the presence of an infected-cell-specific band of 72 kD a, which we suspected might be the adenovirus DNA binding protein, DBP. The adenovirus DBP is a multifunctional protein involved in several aspects of the adenovirus life cycle, including an ability to modulate transcription. The identity of DBP was confirmed by DBP-specific Western blot analysis an d by reimmunoprecipitating DBP from denatured SrCap-specific protein comple xes. Using in vitro-translated DBP and SrCap proteins, we demonstrated that these proteins interact. To determine whether this interaction could affec t SrCap-mediated transcription, we tested whether increasing amounts of DBP could modulate the Gal-SrCap transcription activity. We observed that DBP inhibited Gal-SrCap transcription activity in a dose-dependent manner. Thes e data suggest a novel mechanism of adenovirus host cell control by which D BP binds to and inactivates SrCap, a member of the SNF2 chromatin-remodelin g protein family.