Selectivity of Exportin 5 binding to human precursor microRNAs

Exportin 5 (Exp5, XPO5) is a nuclear export factor that functions in the microRNA (miRNA) biogenesis pathway to transport precursor miRNAs (pre-miRNAs) from the nucleus to the cytoplasm. Most of our current understanding of the Exp5 and pre-miRNA interaction is based on the investigation of how Exp5 binds to human pre-miR-30a (pre-miR-30 for short). As there are hundreds of human miRNA genes, how representative pre-miR-30 is, whether or how Exp5 interacts with distinct cargoes differentially, or whether Exp5 regulates miRNA expression, is unknown. Here we examined and compared the interactions between Exp5 and 157 human pre-miRNAs. We found that Exp5 binds distinct pre-miRNAs with modest variations in efficiencies, with the 3ʹ overhang and the apical loop in pre-miRNAs being the two major discriminating factors. Exp5 binding efficiencies do not significantly correlate with endogenous miRNA expression, suggesting that Exp5 activity does not contribute to differential miRNA expression <i>in vivo</i>. Nonetheless, in human cells with reduced Exp5 levels, preferential Exp5 binding correlated with miRNA expression changes. Thus, our study provides a global picture of how Exp5 interacts with human pre-miRNAs and its role in regulating miRNA expression. <b>Abbreviations:</b> Exp5: Exportin 5; miRNA: microRNA; pri-miRNA: primary microRNA; pre-miRNA: precursor microRNA; nt: nucleotide