Nuclear export of circular RNA
Circular RNAs (circRNAs), which can function as regulators of gene expression, are formed by back-splicing of precursor mRNAs in the nucleus. circRNAs are predominantly localized in the cytoplasm, indicating that they must be exported from the nucleus. Here, we uncover a pathway specific for nuclear export of circular RNA. This pathway requires Ran-GTP, Exportin-2 and IGF2BP1. Enhancing the nuclear Ran-GTP gradient by depletion or chemical inhibition of the major protein exporter, CRM1, selectively increases nuclear export of circRNAs, while reducing the nuclear Ran-GTP gradient selectively blocks circRNA export. Analysis of nuclear circRNA binding proteins reveals that interaction of IGF2BP1 with circRNA is enhanced by Ran-GTP, whereas its interaction with linear RNA is inhibited by Ran-GTP. Depletion or knockout of Exportin-2 specifically inhibits nuclear export of circRNA, while formation of an Exportin-2 circRNA export complex requires Ran-GTP and IGF2BP1. Our findings demonstrate that adaptors such as IGF2BP1 that bind directly to circular RNAs recruit Exportin-2 to export circRNAs in a mechanism analogous to protein export, rather than mRNA export. Overall design: The study is composed of 14 individually matched nuclear and cytoplasmic fractionated RNA samples, 28 samples in total. Of these, 12 samples were treated with either siRNA negative controls or IGF2BP1 siRNA to produce 3 matched replicate samples for each fraction and treatment. Likewise, the remaining 16 samples were treated with either siRNA negative controls or XPO2 siRNA to produce 4 matched replicate samples for each fraction and treatment