The Assembly and Disassembly of the Complex Containing PTEFb Tat and TAR

The assembly and disassembly of the ternary complex containing P-TEFb, Tat, and TAR is a highly regulated process in vivo. In fact, P-TEFb is intrinsically incapable of forming a stable complex with Tat and TAR due to two built-in autoinhibitory mechanisms in P-TEFb. The first arises from the lack of phosphorylation on several key serine and threonine residues near the C-terminus of CDK9, which renders P-TEFb to adopt a conformation unfavorable for TAR recognition. The second is caused by the intramolecular interaction between the N- and C-terminal regions of CycTl, which sterically blocks the P-TEFb:TAR interaction (Fong and Zhou, 2000; Garber et al., 2000). While the autophosphorylation of CDK9 (at least in vitro, could be mediated by other kinases in vivo) can overcome the first inhibition by inducing conformational changes in P-TEFb and thereby exposing a region in CycTl for possible TAR binding, the second is relieved by the binding of the C-terminal region of CycTl to Tat-SFl and perhaps other cellular factors.

Whereas the phosphorylation of CDK9 stabilizes the P-TEFb:Tat:TAR ternary complex (Fong and Zhou, 2000; Garber et al., 2000), the acetylation of Tat by p300 on Lys50, which is located in the TAR RNA-binding domain of Tat, has been shown to dissociate Tat from the TAR RNA. Once liberated, the acetylated Tat can recruit the P-CAF factor to the elongating Pol II (Fig. 14.4), possibly to facilitate chromatin remodeling (Bres et al., 2002; Bres el al., 2002; Kiernan et al., 1999). Recently, it has been shown that Tat can be deacetylated by SIRT1, which recycles Tat to its unacetylated form and acts as a transcriptional co-activator during Tat trans-activation (Pagans et al., 2005). Taken together, these results have revealed novel control mechanisms for the assembly/disassembly of a multi-component transcription elongation complex at the HIV promoter through reversible modifications of the components within this complex.

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