How to slice: snapshots of Argonaute in action

Genetic and biochemical studies first implicated Argonaute (or Ago) as a key component of the mechanisms of RNA silencing in eukaryotes [1]. Argonaute proteins fall mainly into two subfamilies (Ago and Piwi), defined initially on the basis of sequence similarity [1], with an additional subfamily specific to Caenorhabditis elegans and outliers, which include the prokaryotic Argonautes (eubacterial and archaeal), though the latter category displays some similarity to the Piwi subfamily. Structural studies have revealed the molecular functions of Argonaute, showing that Argonaute is 'Slicer' [2,3], and that it provides anchor sites for the 5' and 3' ends of the guide RNA strand [4-10]. Thus, Argonaute is the principal protein component of RNA silencing. The structural studies also show that three-dimensional Argonaute is roughly a bi-lobal protein, with an N-terminal lobe composed of an N-domain, L1 linker region and PAZ domain (highly mobile) and a C-terminal lobe composed of MID and PIWI domains. Patel et al. now provide the first pictures of full length Argonaute in complex with guide and target strands [11-13], captured at multiple distinct phases of the catalytic cycle, revealing the molecular mechanisms of this slicing machine. As with other structural studies on full length Argonaute, the protein stems from a prokaryote (Thermus thermophilus). This is because eukaryotic Argonautes from any subfamily are difficult to obtain in the quantities required for X-ray crystallography. Previous studies showed that prokaryotic Argonautes display a preference for a DNA guide strand [9,14] (their in vivo function is still unknown) and, accordingly, Patel and colleagues crystallized the complexes with a DNA guide and RNA targets.The structures and the stages in the Argonaute cycle that they most closely represent are summarized below and shown in Figure 1:？ The binary complex [11], consisting of Thermus thermophilus Argonaute (TtAgo) and a 21 mer DNA guide strand. The structu