INTERACTION OF HUMAN T-CELL LYMPHOTROPIC VIRUS TYPE-I TAX, ETS1, AND SP1 IN TRANSACTIVATION OF THE PTHRP P2 PROMOTER

We have previously shown that the parathyroid hormone-related protein (PTHrP) promoter contains binding sites for transcription factors Ets1 and Spl and that human T-cell lymphotropic virus type I (HTLV I) Tax cooperates with Ets1 to transactivate the PTHrP P2 promoter. Using the yeast two-hybrid interaction system, we now provide evidence that Tax interacts with Ets1. Moreover, a double mutation (D22A,C23S) in the T ax protein that abrogated the Tax/Ets1 interaction also inhibited the Tax/Ets1 cooperative effect, suggesting that the interaction between T ax and Ets1 is important for transactivation of the PTHrP promoter. In coimmunoprecipitation assays, we find that Tax facilitates the intera ction between Ets1 and Spl, forming a ternary complex. When the Spl si te in the PTHrP promoter was mutated, the Tax/Ets1 cooperative effect was dramatically decreased. This suggests that Spl plays an important role in the Ets1-dependent Tax transactivation of the PTHrP P2 promote r, Finally, we demonstrate that Gal4-Tax is a strong activator of the Gal PTHrP promoter, implying that Tax contributes directly to the tran scriptional activation of the promoter. We propose a model in which th e Tax/Ets1 cooperative effect on the PTHrP P2 promoter is based on the ability of Tax, Ets1, and Sp1 to form a ternary complex on the templa te DNA, Tax facilitates the interaction of Ets1/Sp1 and participates d irectly in the transcription initiation process.