Structural basis for recognition of 2′,5′‐linked oligoadenylates by human ribonuclease L

An interferon‐induced endoribonuclease, ribonuclease L (RNase L), is implicated in both the molecular mechanism of action of interferon and the fundamental control of RNA stability in mammalian cells. RNase L is catalytically active only after binding to an unusual activator molecule containing a 5′‐phosphorylated 2′,5′‐linked oligoadenylate (2‐5A), in the N‐terminal half. Here, we report the crystal structure of the N‐terminal ankyrin repeat domain (ANK) of human RNase L complexed with the activator 2‐5A. This is the first structural view of an ankyrin repeat structure directly interacting with a nucleic acid, rather than with a protein. The ANK domain folds into eight ankyrin repeat elements and forms an extended curved structure with a concave surface. The 2‐5A molecule is accommodated at a concave site and directly interacts with ankyrin repeats 2–4. Interestingly, two structurally equivalent 2‐5A binding motifs are found at repeats 2 and 4. The structural basis for 2‐5A recognition by ANK is essential for designing stable 2‐5As with a high likelihood of activating RNase L.