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.

Exon junction complexes (EJCs) are deposited onto mRNAs during splicing, serve as positional landmarks for the intron exon structure of genes, and direct posttranscriptional processes in the cytoplasm. EJC removal and recycling by translation are ill understood and have been attributed to ribosomal passage. This work identifies the ribosome-associated protein PYM as an EJC disassembly factor and defines its mechanism of function. Whereas EJC assembly intermediates are resistant to PYM, fully assembled EJCs are dissociated from spliced mRNAs by PYM. This disassembly involves PYM binding to the EJC proteins MAGOH-Y14. PYM overexpression in cells disrupts EJC association with spliced mRNA and inhibits nonsense-mediated mRNA decay. In cells depleted of PYM, EJCs accumulate on spliced mRNAs and EJC protein recycling is impaired. Hence, PYM is an EJC disassembly factor that acts both in vitro and in living cells, and that antagonizes important EJC functions.

Protein complexes deposited on messenger RNAs during their maturation are able to recruit components of a cellular RNA surveillance pathway, thereby linking RNA maturation to subsequent steps in RNA quality control.