A new paradigm in eukaryotic biology: HIV Tat and the control of transcriptional elongation

Abstract

Studies of the transcriptional transactivator (Tat), a key regulatory protein of HIV, have yielded insight into the control of eukaryotic transcription

Figure 1. Activation of HIV Transcription by Tat

Activators (red circles) that bind the HIV LTR promoter (light-blue rectangle) assemble the PIC and recruit RNAPIIa to the start site of transcription. For simplicity, only RNAPIIa in the PIC is presented. The yellow sphere with two open circles, depicting serines at position 5 and 2 within the CTD (S5 and S2, respectively), represents the unphosphorylated CTD of RNAPIIa (white sphere). TFIIH, which performs DNA-helicase and CTD-kinase activities, melts the DNA and phosphorylates S5 (red circle in the CTD; P-S5), resulting in promoter clearance. RNAPIIa transcribes TAR (red hairpin) and is paused by the binding of N-TEF, DSIF, and NELF, which are presented as blue spheres. The RD subunit of NELF binds the bottom stem in TAR. P-TEFb (comprising the red [CDK9] and pink [CycT1] spheres), which binds TAR together with Tat (small red sphere), phosphorylates S2 (red circle in the CTD; P-S2) to form elongating RNAPIIo (large red sphere). It also phosphorylates Spt5 in DSIF and RD in NELF, which become elongation factors, with the latter dissociating from TAR. In addition, P-TEFb, possibly independent of its kinase activity, assembles PIC via recruitment of TBP and RNAPIIa (dotted arrow). The phosphorylated CTD in RNAPIIo now binds the Elongator, which contains splicing machinery and polyadenylation factors. The red sphere at the 5′ end of the HIV transcript (red line) represents its cap. Finally, p300 acetylates Tat (magenta circle) and dissociates it from TAR. Acetylated Tat binds P-CAF and transfers it to RNAPIIo, possibly facilitating chromatin remodeling. Collectively, efficient RNAPII elongation of viral transcription ensues.

Figure 2. Inhibition of P-TEFb by the Coordinate Actions of HEXIM1 and 7SK snRNA

HEXIM1 (blue sphere) binds the 5′ half of 7SK snRNA (red structure with multiple hairpins). Upon this binding, P-TEFb joins this RNA–protein complex and becomes enzymatically inactive, depicted by CDK9 as a black sphere. For simplicity, only the CDK9/CycT1 heterodimer is presented. Multiple stimuli, including stress, ultraviolet light, actinomycin D, DRB, and hypertrophic signals, dissociate HEXIM1 and 7SK snRNA from P-TEFb, possibly by preventing the RNA–protein interaction. In this way, P-TEFb is rendered active, depicted by CDK9 as a red sphere.