HIV-1 Tat protein interacts with mammalian capping enzyme and stimulates capping of TAR RNA

HIV gene expression is subject to a transcriptional checkpoint, whereby neg ative transcription elongation factors induce an elongation block that is o vercome by HIV Tat protein in conjunction with P-TEFb. P-TEFb is a cyclin d ependent kinase that catalyzes Tat-dependent phosphorylation of Ser-5 of th e Pol II C-terminal domain (CTD). Ser-5 phosphorylation confers on the CTD the ability to recruit the mammalian mRNA capping enzyme (Mcel) and stimula te its guanylyltransferase activity. Here we show that Tat spearheads a sec ond and novel pathway of capping enzyme recruitment and activation via a di rect physical interaction between the C-terminal domain of Tat and Mcel. Ta t stimulates the guanylyl transferase and triphosphatase activities of Mcel and thereby enhances the otherwise low efficiency of cap formation on a TA R stem-loop RNA. Our findings suggest that multiple mechanisms exist for co upling transcription elongation and mRNA processing.