The GxxG loop is solvent exposed in the free protein and contacts the backbone of the RNA ligands. ( A ) Cartoon representation of the structure of eight KH domains. The side chains of amino acids of the GxxG loop are displayed in blue. ( B ) The Nova-2 KH3–RNA (top) and SF1 KH–RNA (bottom) complexes. The protein structure is displayed as a cartoon while the RNA is represented by a stick model. For SF1 the lowest energy NMR structure of the bundle is displayed. On the left are blow ups of the structures that detail the GxxG–RNA interaction. For the X-ray structure of Nova-2 we report the shortest distances between the RNA phosphates and the first and second variable amino acid of the GxxG loop. For the NMR structure of SF1 we report the equivalent distances measured over the bundle of ten structures and in the lowest energy structure (brackets).