2-Bromoadenosine-Substituted 2′,5′-Oligoadenylates Modulate Binding and Activation Abilities of Human Recombinant RNase L

2-5A induces a conformational change in the ankyrin-repeat domain of RNase L

RNase L is responsible for the 2-5A host defense system, an RNA degradation pathway present in cells of higher vertebrates that functions in both the antiviral and anticellular activities of interferon. The activity of RNase L is tightly regulated and is exerted only in the presence of 2-5A. The postulated mechanism of its regulation is as follows: the N-terminal half ankyrin-repeat domain masks the C-terminal half nuclease domain in the absence of 2-5A. On binding 2-5A at the ankyrin-repeat domain, RNase L forms a homodimer and removes the ankyrin-repeat domain from the nuclease domain to become the active form. A conformational change in the ankyrin-repeat domain is a key step in this hypothetical mechanism, but there is as yet no evidence for such a change. To clarify the events induced by 2-5A binding, we established procedures for expression and purification of the ankyrin-repeat domain of human RNase L. Fluorescence spectra of the protein showed clear difference in the presence and absence of 2-5A. The alterations in the spectra supported conformational changes of the protein. Time-resolved anisotropy measurements indicated that 2-5A binding led to a significant decrease in the rotational radius of the protein. In addition, 2-5A provided the domain with resistance to protease digestion as a result of a conformational change. These results indicated that the ankyrin-repeat domain of RNase L constricts its structure by binding of 2-5A. This observation suggests a revised model of the 2-5A-induced activation of RNase L.

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.

Contribution of Tyr712 and Phe716 to the activity of human RNase L

Ribonuclease L (RNase L) is a key enzyme in the 2-5A host defense system, and its activity is strictly regulated by an unusual 2',5'-linked oligoadenylate (2-5A). A bipartite model, in which the N-terminal half of RNase L is responsible for the 2-5A binding and the C-terminal half alone is able to hydrolyse the substrate RNA, has been proposed on the basis of the results of deletion mutant analyses [Dong, B. & Silverman, R.H. (1997) J. Biol. Chem.272, 22236-22242]. Above all, the region between Glu711 and His720 was revealed to be essential for RNA binding and/or hydrolysis. To dissect the function of the region, we performed scanning mutagenesis over the 10 residues of glutathione S-transferase (GST)-fusion RNase L. Among the single amino acid mutants examined, Y712A and F716A resulted in a significant decrease of RNase activity with a reduced RNA binding acitivity. The losses of the RNase activity were not restored by its conservative mutation, whereas the RNA binding activity was enhanced in the case of Y712F. These results indicate that both Tyr712 and Phe716 provide the enzyme with a RNA binding activity and catalytic environment.

2-Methyladenosine-Substituted 2',5'-oligoadenylates: conformations, 2-5A binding and catalytic activities with human ribonuclease L

2-Methyladenosine-substituted analogues of 2-5A, p5'A2'p5'A2'p5'(me2A), p5'(me2A)2'p5'A2'p5'A, and p5'(me2A) 2'p5'(me2A)2'pS'(me2A), were prepared via a modification of a lead ion-catalyzed ligation reaction. These 5'-monophosphates were subsequently converted into the corresponding 5'-triphosphates. Both binding and activation of human recombinant RNase L by various 2-methyladenosine-substituted 2-5A analogues were examined. Among the 2-5A analogues, p5'A2'p5'A2'p5'(me2A) showed the strongest binding affinity and was as effective as 2-5A itself as an activator of RNase L. The CD spectra of both p5'(me2A)2'p5'A2'p5'A and p5'A2'p5'A2'p5'(me2A) were superimposable on that of p5'A2'p5'A2'p5'A, indicative of an anti orientation about the base-glycoside bonds as in naturally occurring 2-5A.