A protein phosphatase from human T cells augments tat transactivation of the human immunodeficiency virus type 1 long-terminal repeat.

HIV-1 Tat protein regulates viral gene expression by modulating the activity and association of cellular transcription factors with RNA polymerase II (RNAPII). Possible mechanisms include Tat-associated protein kinase(s) and phosphatase(s) that regulate phosphorylation of the C-terminal domain (CTD) of the large subunit of RNAPII. Hypophosphorylated RNAPII (RNAPIIa) is recruited to promoters during formation of a preinitiation complex, whereas hyperphosphorylated RNAPII (RNAPIIo) is associated with the elongation complex. The role of phosphatases in maintaining the equilibrium between the two phosphorylated states of RNAPII, which is required for sustained transcriptional activation from the HIV-1 LTR, is not clear. In this study, we discuss the properties of a Tat-associated CTD phosphatase fractionated from Jurkat T cells. The Tat-associated protein phosphatase (TAPP) is related to the serine/threonine, type 1, protein phosphatase (PP1) family. TAPP dephosphorylates the hyperphosphorylated form of recombinant CTD specifically on serine 2, and augments Tat-mediated transcriptional transactivation of HIV-1 LTR in an in vitro transcription reaction. TAPP is associated with the transcription complex during the early initiation steps, and its release from the HIV-1 promoter coincides with the Tat-specific activation of CDK9. The results suggest a unique role of the Tat-associated phosphatase which regulates viral transcription by target-specific dephosphorylation of RNAPII during the early stages of elongation.