Slicer Steps into the Limelight

Science's STKE  07 Sep 2004:
Vol. 2004, Issue 249, pp. tw320
DOI: 10.1126/stke.2492004tw320

During RNA interference, small interfering RNAs (siRNAs) generated by Dicer (or provided exogenously) are loaded onto the RNA-induced silencing complex (RISC), which then binds homologous target RNAs, cleaving and inactivating them. The major constituents of RISC are the single-stranded siRNA and any one of a number of different proteins of the Argonaute (Ago) family. Until now, the identity of the nuclease in RISC, nicknamed "Slicer," has remained a mystery (see the Perspective by Sontheimer and Carthew). Song et al. present the structure of the Ago protein from Pyrococcus furiosus, which consists of four domains; the PAZ and PIWI domains being the defining characteristics of Ago. The PfAgo PIWI domain is homologous to ribonuclease H (RNase H), including conserved catalytic residues, and the juxtaposition of PAZ and PIWI domains suggests a mechanism by which Ago might load and cleave target RNAs. Liu et al. show that, unlike other mouse Agos, only Ago2 can form a cleavage-competent RISC. Ago2 is also essential in vivo for RNAi, and is required for normal mouse development. Because the conserved catalytic residues in the RNase H-like PIWI domain are critical for RISC cleavage activity, it is likely that Ago2 is "Slicer."

J. J. Song, S. K. Smith, G. J. Hannon, L. Joshua-Tor, Crystal structure of Argonaute and its implications for RISC slicer activity. Science 305, 1434-1436 (2004). [Abstract] [Full Text]

J. Liu, M. A. Carmell, F. V. Rivas, C. G. Marsden, J. M. Thomson, J. J. Song, S. M. Hammond, L. Joshua-Tor, G. J. Hannon, Argonaute2 is the catalytic engine of mammalian RNAi. Science 305, 1437-1441 (2004). [Abstract] [Full Text]

E. J. Sontheimer, R. W. Carthew, Argonaute journeys into the heart of RISC. Science 305, 1409-1410 (2004). [Summary] [Full Text]