microRNAs (miRNAs) bind to Argonaute (Ago) proteins and inhibit translation or promote degradation of mRNA targets. Human let-7 miRNA inhibits translation initiation of mRNA targets in an m(7)G cap-dependent manner and also appears to block protein production, but the molecular mechanism(s) involved is unknown and the role of Ago proteins in translational regulation remains elusive. Here we identify a motif (MC) within the Mid domain of Ago proteins, which bears significant similarity to the m(7)G cap-binding domain of eIF4E, an essential translation initiation factor. We identify conserved aromatic residues within the MC motif of human Ago2 that are required for binding to the m(7)G cap and for translational repression but do not affect the assembly of Ago2 with miRNA or its catalytic activity. We propose that Ago2 represses the initiation of mRNA translation by binding to the m(7)G cap of mRNA targets, thus likely precluding the recruitment of eIF4E.

Piwi family proteins are essential for germline development and bind piwi-interacting RNAs (piRNAs 1 2 3). The grandchildless gene aub of Drosophila melanogaster encodes the piRNA-binding protein Aub that is essential for formation of primordial germ cells (PGCs) 4. Here we report that mouse, Xenopus laevis and Drosophila melanogaster Piwi family proteins contain symmetrical dimethylarginines (sDMAs). We find that Piwi proteins are expressed in X. laevis oocytes and we identify numerous X. laevis piRNAs. We report that the Drosophila homolog of protein methyltransferase 5 (dPRMT5, csul/dart5), which is also the product of a grandchildless gene 5, 6, is required for arginine methylation of Drosophila Piwi, Ago3 and Aub proteins, in vivo. Loss of dPRMT5 activity leads to reduction of piRNAs and in particular of Ago3 and Aub protein levels and accumulation of retrotransposons in the Drosophila ovary. Our studies explain the relationship between aub and dPRMT5 (csul/dart5) genes by demonstrating that dPRMT5 is the enzyme that methylates Aub. Our findings underscore the significance of sDMA modification of Piwi proteins in the germline and suggest an interacting pathway of genes that are required for piRNA function and PGC specification.

Argonaute (Ago) proteins bind to microRNA (miRNAs) and short interfering RNAs (siRNAs) and form the core components of effector complexes that mediate miRNA and siRNA function. Currently, there is a paucity of reliable antibodies against mammalian Ago proteins, thus precluding studies of endogenous Ago proteins from tissues. Here we report the development of 2A8, a novel anti-Ago monoclonal antibody that recognizes human and mouse Ago proteins and efficiently immunoprecipitates miRNAs. We report the characterization of 2A8 and its use to clone miRNAs from human brain and from preparations of human polymorphonuclear leukocytes (neutrophils), which revealed a prevalent miRNA with unusual features.

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