Interaction of Sp1 transcription factor with HIV-1 Tat protein: looking for cellular partners.

The Tat protein of human immunodeficiency virus type 1 (HIV-1) trans-activates HIV-1 transcription by functionally interacting with a number of cellular proteins, among which the Sp1 transcription factor. We recently demonstrated that Tat does not directly interact with Sp1 either in vitro or in vivo, and we suggested that other protein(s) could indirectly mediate Tat-Sp1 interaction. In keeping, here we showed that addition of HeLa cell nuclear extracts to purified Tat and Sp1 proteins allows the formation of a Tat/Sp1 complex in in vitro binding assays. In an attempt to identify the partner(s) that bridge Tat and Sp1, we developed a yeast multi-protein system, in which cellular proteins recently shown to play a relevant role in Tat function, namely TATA box-binding protein, cyclin T1, CDK9, and cyclin T1/CDK9 complex, were coexpressed, individually or in pair-wise combination, with Tat and Sp1 hybrids. We demonstrated that none of these candidate partners bridges Tat and Sp1. However, our yeast multi-protein system, which allows simple and rapid detection of interactions among up to four proteins, will be most helpful to further dissect the interaction of Tat and Sp1 with other candidate partners that participate in the assembly of transcriptionally active complexes at the HIV-1 LTR.