RNA helical packing in solution: NMR structure of a 30 kDa GAAA tetraloop-receptor complex.

Tertiary interactions are critical for proper RNA folding and ribozyme catalysis. RNA tertiary structure is often condensed through long-range helical packing interactions mediated by loop-receptor motifs. RNA structures displaying helical packing by loop-receptor interactions have been solved by X-ray crystallography, but not by NMR. Here, we report the NMR structure of a 30 kDa GAAA tetraloop-receptor RNA complex. In order to stabilize the complex, we used a modular design in which the RNA was engineered to form a homodimer, with each subunit containing a GAAA tetraloop phased one helical turn apart from its cognate 11-nucleotide receptor domain. The structure determination utilized specific isotopic labeling patterns (2H, 13C and 15N) and refinement against residual dipolar couplings. We observe a unique and highly unusual chemical shift pattern for an adenosine platform interaction that reveals a spectroscopic fingerprint for this motif. The structure of the GAAA tetraloop-receptor interaction is well defined solely from experimental NMR data, shows minor deviations from previously solved crystal structures, and verifies the previously inferred hydrogen bonding patterns within this motif. This work demonstrates the feasibility of using engineered homodimers as modular systems for the determination of RNA tertiary interactions by NMR.