Binding by TRBP-dsRBD2 Does Not Induce Bending of Double-Stranded RNA

Protein-nucleic acid interactions are central to a variety of biological processes, many of which involve large-scale conformational changes that lead to bending of the nucleic acid helix. Here, we focus on the nonsequence-specific protein TRBP, whose double-stranded RNA-binding domains (dsRBDs) interact with the A-form geometry of double-stranded RNA (dsRNA). Crystal structures of dsRBD-dsRNA interactions suggest that the dsRNA helix must bend in such a way that its major groove expands to conform to the dsRBD's binding surface. We show through isothermal titration calorimetry experiments that dsRBD2 of TRBP binds dsRNA with a temperature-independent observed binding affinity (K_D ∼500 nM). Furthermore, a near-zero observed heat capacity change (ΔC_p = 70 ± 40 cal·mol^-1·K^-1) suggests that large-scale conformational changes do not occur upon binding. This result is bolstered by molecular-dynamics simulations in which dsRBD-dsRNA interactions generate only modest bending of the RNA along its helical axis. Overall, these results suggest that this particular dsRBD-dsRNA interaction produces little to no change in the A-form geometry of dsRNA in solution. These results further support our previous hypothesis, based on extensive gel-shift assays, that TRBP preferentially binds to sites of nearly ideal A-form structure while being excluded from sites of local deformation in the RNA helical structure. The implications of this mechanism for efficient micro-RNA processing will be discussed.