Functional interaction between the HIV transactivator Tat and the transcriptional coactivator PC4 in T cells

The human immunodeficiency virus (HIV) transactivator Tat is a potent activ ator of transcription from the HIV long terminal repeat and is essential fo r efficient viral gene expression and replication. Tat has been shown to in teract with components of the basal transcription machinery and transcripti onal activators. Here we identify the cellular coactivator PC4 as a Tat-int eracting protein using the yeast two-hybrid system and confirmed this inter action both in vitro and in vivo by coimmunoprecipitation. We found that th is interaction has a functional outcome in that PC4 overexpression enhanced activation of the HIV long terminal repeat in transient transfection studi es in a Tat-dependent manner. The domains of PC4 and Tat required for the i nteraction were mapped. In vitro binding studies showed that the basic tran sactivation-responsive binding domain of Tat is required for the interactio n with PC4. The minimum region of PC4 required for Tat binding was amino ac ids 22-91, whereas mutation of the lysine-rich domain between amino acids 2 2 and 43 prevented interaction with Tat. Tat-PC4 interactions may be contro lled by phosphorylation, because phosphorylation of PC4 by casein kinase II inhibited interactions with Tat both in vivo and in vitro. We propose that PC4 may be involved in linking Tat to the basal transcription machinery.