Human T-lymphotropic virus type 1 Tax protein complexes with P-TEFb and competes for Brd4 and 7SK snRNP/HEXIM1 binding.

Positive transcription elongation factor b (P-TEFb) plays an important role in stimulating RNA polymerase II elongation for viral and cellular gene expression. P-TEFb is found in cells in either an active, low-molecular-weight (LMW) form or an inactive, high-molecular-weight (HMW) form. We report here that human T-lymphotropic virus type 1 (HTLV-1) Tax interacts with the cyclin T1 subunit of P-TEFb, forming a distinct Tax/P-TEFb LMW complex. We demonstrate that Tax can play a role in regulating the amount of HMW complex present in the cell by decreasing the binding of 7SK snRNP/HEXIM1 to P-TEFb. This is seen both in vitro using purified Tax protein and in vivo in cells transduced with Tax expression constructs. Further, we find that a peptide of cyclin T1 spanning the Tax binding domain inhibits the ability of Tax to disrupt HMW P-TEFb complexes. These results suggest that the direct interaction of Tax with cyclin T1 can dissociate P-TEFb from the P-TEFb/7SK snRNP/HEXIM1 complex for activation of the viral long terminal repeat (LTR). We also show that Tax competes with Brd4 for P-TEFb binding. Chromatin immunoprecipitation (ChIP) assays demonstrated that Brd4 and P-TEFb are associated with the basal HTLV-1 LTR, while Tax and P-TEFb are associated with the activated template. Furthermore, the knockdown of Brd4 by small interfering RNA (siRNA) activates the HTLV-1 LTR promoter, which results in an increase in viral expression and production. Our studies have identified Tax as a regulator of P-TEFb that is capable of affecting the balance between its association with the large inactive complex and the small active complex.