Structural characterization of cationic lipid–tRNA complexes

Despite considerable interest and investigations on cationic lipid-DNA complexes, reports on lipid-RNA interaction are very limited. In contrast to lipid-DNA complexes where lipid binding induces partial B to A and B to C conformational changes, lipid-tRNA complexation preserves tRNA folded state. This study is the first attempt to investigate the binding of cationic lipid with transfer RNA and the effect of lipid complexation on tRNA aggregation and condensation. We examine the interaction of tRNA with cholesterol (Chol), 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP), dioctadecyldimethylammoniumbromide (DDAB) and dioleoylphosphatidylethanolamine (DOPE), at physiological condition, using constant tRNA concentration and various lipid contents. FTIR, UV-visible, CD spectroscopic methods and atomic force microscopy (AFM) were used to analyze lipid binding site, the binding constant and the effects of lipid interaction on tRNA stability, conformation and condensation. Structural analysis showed lipid-tRNA interactions with G-C and A-U base pairs as well as the backbone phosphate group with overall binding constants of K(Chol) = 5.94 (+/- 0.8) x 10(4) M(-1), K(DDAB) = 8.33 (+/- 0.90) x 10(5) M(-1), K(DOTAP) = 1.05 (+/- 0.30) x 10(5) M(-1) and K(DOPE) = 2.75 (+/- 0.50) x 10(4) M(-1). The order of stability of lipid-tRNA complexation is DDAB > DOTAP > Chol > DOPE. Hydrophobic interactions between lipid aliphatic tails and tRNA were observed. RNA remains in A-family structure, while biopolymer aggregation and condensation occurred at high lipid concentrations.