Fig. 5

Structure and characterization Tat RBD in complex with TAR. a Secondary structure of TAR where the dashed arcs represent the triple-base interactions between U₂₃ from the bulge to the A₂₇-U₃₈ base pair in the stem, giving rise to the roof of the sandwich. b Representative ITC data of Tat RBD (left) and the Tat:CycT1:AFF4 (right) titration into TAR RNA, respectively. Continuous lines represent the fit for a one-site binding model. c ¹H–¹H two- dimensional NOESY showing the NOEs between the R49 and R52 hη guanidinium protons and G₂₈ and G₂₆ H8 protons, respectively, placing these residues in an arginine fork interaction. d NMR structure of TAR bound to Tat RBD (black) showing R49 and R52 sandwiching the U₂₃ cap of the ASM sandwich. e The ASM binding pocket with the A₂₂ base and the U₂₃ cap (magenta), G₂₆ and G₂₈ (gray), and R49 and R52 (black). The dotted lines indicate hydrogen bond interactions from the NH group of arginine 49 and 52 to the O6 and N7 of G₂₈ and G₂₆, respectively. f Positions of K50 and K51 (black) in the major groove. g The R53 and Q54 interaction with U40 and G43, respectively. Close proximity of the R53δ to the C₃₉, U₄₀, and C₄₁ H5 protons evidenced by the data and a dotted line represents the potential hydrogen bonding interaction between a guanidinium proton of R53 to the U₄₀ phosphate. The hydrogen bond interaction between the Q54 hε proton and the G₄₃ O6 represents the C-terminal exit of Tat RBD from TAR