Inhibition of Tat transactivation by the RNA polymerase II CTD-phosphatase FCP1

OBJECTIVES: To asses the role of the RNAPII carboxy-terminal domain (CTD) phosphatase FCP1 on HIV-1 Tat-mediated transactivation. DESIGN: Construction of expression vectors encoding FCP1 phosphatase and analysis of their functions on Tat activity. METHODS: Basal and Tat-mediated transactivation of HIV-1 long terminal repeat (LTR)-driven transcription was compared, by transient transfections, in the presence of FCP1 phosphatase. Protein interactions were analysed by in vitro binding assays. RESULTS: FCP1 specifically and effectively represses Tat transactivation but not HIV-1 LTR-basal transcription. Protein interaction assays demonstrated that FCP1 specifically and directly binds Tat in vitro. CONCLUSION: The specific and efficient inhibitory function of FCP1 highlights the important role of this CTD-phosphatase in Tat-mediated transactivation, and it suggests that FCP1 might represent a specific target for modulation of Tat activity in infected cells.