AUF1 promotes let-7b loading on Argonaute 2

Given that RNA-binding proteins (RBPs) are potent post-transcriptional regulators of gene expression (Glisovic et al. 2008), there is great interest in identifying comprehensively the functions of RBPs. The RBP AU-binding factor 1 (AUF1) comprises four isoforms—p37, p40, p42, and p45—and has been closely linked to physiological and pathological processes, including cellular senescence, myogenesis, inflammation, aging, and cancer (Zucconi and Wilson 2011; Pont et al. 2012; Panda et al. 2014). Although all isoforms contain two RNA recognition motifs (RRMs) followed by a glutamine-rich domain, p37 and p40 lack exon 7, while p37 and p42 lack exon 2. The four isoforms can bind to similar RNA sequences but display distinct subcellular localization profiles, affinities for target mRNAs, and impacts on the fate of mRNA targets (White et al. 2013). We recently used the method PAR-CLIP (photoactivatable ribonucleoside-enhanced cross-linking and immunoprecipitation) (Hafner et al. 2010, 2012) to identify comprehensively the RNA targets of AUF1 (Yoon et al. 2014). The analysis revealed that AUF1 primarily recognized U/GU-rich sequences in target mRNAs and long noncoding RNAs (lncRNAs) and was capable of destabilizing some target RNAs, as previously described (Zucconi and Wilson 2011; Pont et al. 2012), but also stabilized other target transcripts and altered the translation of a subset of mRNAs (Yoon et al. 2014). By increasing the expression of several target mRNAs encoding genomic maintenance proteins, AUF1 was found to help preserve DNA integrity; this function is in keeping with the ability of AUF1 to prevent premature cellular senescence and aging (Pont et al. 2012; White et al. 2013; Yoon et al. 2014). Earlier evidence indicated a functional connection between AUF1 and Argonaute (AGO) proteins, the catalytic components of the RNA-induced silencing complex (RISC) (Chang et al. 2010; Wu et al. 2013). The main function of the argonaute proteins is to bind microRNAs, a family of short (∼22-nucleotide [nt]) noncoding RNAs that form partial hybrids with target mRNAs, often through the “seed” region (nucleotides 2–7) of the microRNA (Ambros 2004; Bartel 2004). AGO–microRNA complexes in turn recruit the RISC to an mRNA and silence the mRNA by reducing its stability and/or translation (Filipowicz 2005; Joshua-Tor 2006; Chekulaeva and Filipowicz 2009). Here, we report that AUF1 p37 displayed strong affinity for let-7b, enhanced the loading of let-7b on AGO2 in vitro, and promoted AGO2–let-7-mediated mRNA decay. Our findings underscore a new mechanism by which microRNA transfer from AUF1 p37 to AGO2 facilitates microRNA-elicited gene silencing.