Analysis of the effect of natural sequence variation in Tat and in cyclin T on the formation and RNA binding properties of Tat-cyclin T complexes

The biological activity of the human immunodeficiency virus type 1 (HIV-1) Tat (Tat1) transcriptional activator requires the recruitment of a Tat1-Cyc linT1 (CycT1) complex to the TAR RNA target encoded within the viral long t erminal repeat (LTR). While other primate immunodeficiency viruses, such as HIV-2 and mandrill simian immunodeficiency virus (SIVmnd), also encode Tat proteins that activate transcription via RNA targets, these proteins diffe r significantly, both from each other and from Tat1, in terms of their abil ity to activate transcription directed by LTR promoter elements found in di fferent HIV and SIV isolates. Here, we show that CycT1 also serves as an es sential cofactor for HIV-2 Tat (Tat2) and SIVmnd Tat (Tat-M) function. More over, the CycT1 complex formed by each Tat protein displays a distinct RNA target specificity that accurately predicts the level of activation observe d with a particular LTR. While Tat2 and Tat-M share the ability of Tat1 to bind to CycT1, they differ from Tat1 in that they are also able to bind to the related but distinct CycT2. However, the resultant Tat-CycT2 complexes fail to bind TAR and are therefore abortive. Surprisingly, mutation of a si ngle residue in CycT2 (asparagine 260 to cysteine) rescues the ability of C ycT2 to bind Tat1 and also activates not only TAR binding by all three Tat- CycT2 complexes but also Tat function. Therefore, the RNA target specificit y of different Tat-CycT1 complexes is modulated by natural sequence variati on in both the viral Tat transcriptional activator and in the host cell Cyc T molecule recruited by Tat. Further, the RNA target specificity of the res ultant Tat-CycT1 complex accurately predicts the ability of that complex to activate transcription from a given LTR promoter element.