DP‐1: a cell cycle‐regulated and phosphorylated component of transcription factor DRTF1/E2F which is functionally important for recognition by pRb and the adenovirus E4 orf 6/7 protein.

The cellular transcription factor DRTF1/E2F integrates cell cycle events with the transcription apparatus through its cyclical interactions with important regulators of cellular proliferation. Two sequence‐specific DNA binding proteins, DP‐1 and E2F‐1, are components of DRTF1/E2F which synergistically interact in a DP‐1/E2F‐1 heterodimer. Here, we show that DP‐1 is a very frequent, possibly universal, component of DRTF1/E2F in 3T3 cells since it is present in all forms of the DNA binding activity that occur during cell cycle progression. Furthermore, the DP‐1 polypeptide, which is phosphorylated, undergoes a phosphorylation‐dependent mobility shift during the cell cycle suggesting that its level of phosphorylation is regulated during cell cycle progression. A C‐terminal region in DP‐1 can interact with pRb which, in the context of the DP‐1/E2F‐1 heterodimer, contributes to the efficiency of pRb binding. The DP‐1/E2F‐1 heterodimer specifically interacts with the adenovirus type 5 E4 orf 6/7 protein, to produce a DNA binding activity which binds co‐operatively to, and transcriptionally activates through, two appropriately positioned E2F sites in a manner which resembles the regulation of DRTF1/E2F by E4 orf 6/7 during adenovirus infection. We conclude that DP‐1 is a frequent and cell cycle‐regulated component of DRTF1/E2F, and that in the DP‐1/E2F‐1 heterodimer it is functionally important for recognition by pRb and the E4 orf 6/7 protein.