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 negative transcription elongation factors induce an elongation block that is overcome by HIV Tat protein in conjunction with P-TEFb. P-TEFb is a cyclin-dependent kinase that catalyzes Tat-dependent phosphorylation of Ser-5 of the Pol II C-terminal domain (CTD). Ser-5 phosphorylation confers on the CTD the ability to recruit the mammalian mRNA capping enzyme ( Mce1) and stimulate its guanylyltransferase activity. Here we show that Tat spearheads a second and novel pathway of capping enzyme recruitment and activation via a direct physical interaction between the C-terminal domain of Tat and Mce1. Tat stimulates the guanylyltransferase and triphosphatase activities of Mce1 and thereby enhances the otherwise low efficiency of cap formation on a TAR stem-loop RNA. Our findings suggest that multiple mechanisms exist for coupling transcription elongation and mRNA processing.[1]References
- HIV-1 Tat protein interacts with mammalian capping enzyme and stimulates capping of TAR RNA. Chiu, Y.L., Coronel, E., Ho, C.K., Shuman, S., Rana, T.M. J. Biol. Chem. (2001) [Pubmed]
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