In mammalian cells, both microRNAs (miRNAs) and small interfering RNAs (siRNAs) are thought to be loaded into the same RNA-induced silencing complex (RISC), where they guide mRNA degradation or translation silencing, depending on the complementarity of the target. Argonaute proteins make up a highly conserved family whose members are believed to form the major part of the RISC complex and as such have been implicated as the 'effectors' of RNAi and related phenomena in several organisms including Saccharomyces spp., Drosophila and mammals.
The roles of Argonaute 1 and 2 (named Ago1 and Ago2 in Drosophila) have been most extensively defined. In flies, Ago2 has been clearly identified as part of the RISC complex and it is an essential component for siRNA-directed RNAi response (Okamura et al. 2004). Ago2 is required for the unwinding of siRNA duplex and thus assembly of siRNA into RISC in Drosophila embryos. It is noteable that Drosophila embryos lacking Ago2, which are siRNA-directed RNAi-defective, are still capable of miRNA-directed target RNA cleavage. This suggests multiple proteins may carry out the Ago2 nuclease role. In contrast, Ago1, which is dispensable for siRNA-directed target RNA cleavage, is required for mature miRNA production that impacts on miRNA-directed RNA cleavage (Okamura et al. 2004). The association of Ago1 with Dicer-1 and pre-miRNA also suggests that Ago1 is involved in miRNA biogenesis.
In addition to roles in RNAi-like mechanisms, Argonaute proteins influence development, and at least a subset are involved in stem cell fate determination. Argonaute proteins are ~100 kD highly basic proteins that contain two common domains, namely PAZ and PIWI domains (Cerutti et al. 2000). The PAZ domain, consisting of 130 amino acids, has been identified in Argonaute proteins and in Dicer (Bernstein et al. 2001a). Although it has no defined function, PAZ is thought to be a protein-protein interaction domain, potentially mediating both homo- and heterodimerization (Cerutti et al. 2000).
There are several reports that Argonaute proteins, being highly basic, might bind RNA. Miwi, a murine homolog of Piwi, exists in a complex with its target mRNAs in vivo (Deng & Lin, 2002). Furthermore, Drosophila Ago1 has been shown to bind ribohomopolymers in vitro (Kataoka et al. 2001). Of course, the involvement of Argonaute proteins in RNAi and related phenomena presupposes an interaction with RNA, either directly or indirectly. However, detailed studies of Argonaute structure and biochemistry will be required to reveal its precise function in RNA metabolism.