Argonaute proteins make up a highly conserved family whose members have been implicated in RNAi and related phenomena in several organisms. 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 and 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.

Nuclear