NMR evidence for a base triple in the HIV-2 TAR C-G.C+ mutant-argininamide complex.

Formation of a specific complex between the HIV Tat protein and the small RNA element TAR is critical for activation of viral transcription. A model complex for this interaction composed of HIV-2 TAR and the amide derivative of arginine has been developed to study how Tat and TAR interact specifically. We have previously determined a high resolution NMR structure of the HIV-2 TAR-argininamide complex. The argininamide guanidium group hydrogen bonds to the major groove face of G26 and is stacked between U23 and A22, forming an arginine sandwich. This structure also provided evidence for formation of a U38-A27.U38 base triple, as U23 is positioned in the major groove within hydrogen bonding distance to A27. However, the expected U23 imino proton was not observed, preventing unambiguous identification of the base triple. Previous work on an isomorphic C38-G27.C23+ base triple mutant of the three base bulge HIV-1 TAR-argininamide complex demonstrated that the base triple is required for specific argininamide binding. Here we investigate the same C38-G27.C23+ base triple mutant in the context of two base bulge HIV-2 TAR. The improved NMR spectral properties of HIV-2 TAR allowed observation of the C23 amino and imino protons for the first time, providing direct evidence that a hydrogen bonding interaction is occurring. The NOEs observed correspond to those observed in the high resolution structure of the HIV-2 TAR-argininamide complex, confirming that a base triple is an important feature of the TAR-argininamide interaction.