Nuclear export of circular RNA
Circular RNAs (circRNAs), which are increasingly being implicated in a variety of functions in normal and cancerous cells1-5, are formed by back-splicing of precursor mRNAs in the nucleus6-10. circRNAs are predom¬inantly 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. Depletion or knockout of Exportin-2 specifically inhibits nuclear export of circRNA. Analysis of nuclear circRNA binding proteins reveals that interaction of IGF2BP1 with circRNA is enhanced by Ran-GTP.  Formation of circRNA export complexes in the nucleus is promoted by Ran-GTP through its interactions with IGF2BP1, Exportin-2 and circRNA. Our findings demonstrate that adaptors such as IGF2BP1 that bind directly to circular RNAs recruit Ran-GTP and Exportin-2 to export circRNAs in a mechanism analogous to protein export, rather than mRNA export. Overall design: HITS-CLIP was performed on endogenous Ran from CAL51 cells using a Ran specific antibody.