Binding of phage-displayed HIV-1 Tat to TAR RNA in the presence of cyclin T1

The transactivator protein (Tat) of the human immunodeficiency virus (HIV) is a key regulatory protein in the viral replication cycle. Together with c ellular cyclin T1 and an RNA element (transactivation response; TAR) locate d at the 5' end of all viral transcripts, it forms a ternary complex that u ltimately enhances the expression of all viral genes. In this ternary compl ex, cyclin T1 interacts directly with Tat and TAR. The presence of cyclin T 1 is essential for high TAR RNA affinity and specificity of Tat. To study p rotein-protein and protein-RNA interaction, we developed a phage display sy stem that displays functional Tat on the surface of bacteriophage M13. The addition of recombinant cyclin T1 to the selections yielded a phage display system that mirrors all binding properties of the cyclin T1-Tat-TAR comple x known from cell assays and biochemical studies. Phage-displayed Tat prote in as well as the cyclin T1 are fully functional. The relative binding capa bilities of wild-type- and mutant Tat-displaying phages show that the prese nce of cyclin T1 significantly reduces the importance of basic residues in the basic sequence region of Tat for its binding to TAR. Copyright (C) 2001 National Science Council, ROC and S. Karger AG, Basel.