Autoinhibition of angiogenins: insights from the X-ray structure of RNase 2 from Atlantic salmon

Recently, the superfamily of animal, extracellular, pyrimidine-specific RNases, often called the RNase A superfamily, has been shown to include not only tetrapod enzymes, but also fish enzymes [1]. In particular, five RNases from zebrafish (Danio rerio) [1-3] and two from the Atlantic salmon (Salmo salar) [4] have been reported to have a very low RNase activity and to be endowed, like RNase 5 (human angiogenin), with powerful angiogenic activity. We have determined the X-ray structure of two zebrafish RNases [3]. In these proteins, like in human angiogenin, the putative binding subsite B1 of the pyrimidine base is partially obstructed by the side chain of Glu located in the C-terminal segment of the protein, and this structural feature well account for their low catalytic activity. More recently, the crystal structure of RNase-2 from Salmo salar (Ss2) has been also determined. Surprisingly, within an essentially unmodified RNase folding, the enzyme presents an extensive reorganization of the active site region with respect to other pancreatic RNases. In particular, although it has the highest catalytic activity among fish RNases, it presents an active site fully obstructed by a peptide segment at C-terminal region (CTR), and with the two catalytic histidines in direct contact. Thus the enzyme appears to be auto-inhibited in a completely different manner compared to the other angiogenins. Comparison of the structure of Ss2 with those of RNase complexes with substrate analogs suggests that Ss2 could adopt two distinct conformations: a closed form with the CTR blocking the substrate binding cleft (observed in the crystal structure) and an open conformation, where the CTR swings out forming an open cleft with the active site exposed. Overall, these data provide novel structural insights into the mechanism that modulates RNase activity of angiogenins. [1] E. Pizzo, P. Buonanno, A. Di Maro, S. Ponticelli, S. De Falco, N. Quarto, M.V. Cubellis and G. D'Alessio, J Biol Chem, 281, 2006, 27454. [2] S. Cho and J. Zhang, Mol Biol Evol, 24, 2007, 1259. [3] E. Pizzo, A. Merlino, M. Turano, I. Russo Krauss, F. Coscia, A. Zanfardino, M. Varcamonti, A. Furia, C. Giancola, L. Mazzarella, F. Sica, G. D'Alessio, Biochem J., 433(2), 2010, 345 [4] E. Pizzo, M. Varcamonti, A. Di Maro, A. Zanfardino, C. Giancola, G. D'Alessio, FEBS J, 275, 2008, 1283