MYCN is an RNA-binding accessory protein of the nuclear exosome
The MYCN oncoprotein broadly binds promoters in a heterodimer with its partner protein MAX. MYCN also interacts with the nuclear exosome, a 3'-5'exoribonuclease complex, to facilitate progression through the S phase hinting at an RNA-centric function. Here we show that MYCN forms stable high molecular weight complexes with multiple RNA-binding proteins including the nuclear exosome. RNA-binding assays reveal that MYCN directly binds to thousands of predominantly intronic RNA sites via a short, highly conserved sequence motif termed MYCBoxI. Exosome depletion results in MYCN globally re-localizing from promoters to intronic RNAs. The vacant promoters are then occupied by the MXD transcription repressor protein MNT, which inhibits MYCN-dependent transcription. MYCN fosters the degradation of its bound introns via tight association with the nuclear exosome targeting complex (NEXT). Our data demonstrate that MYCN is an RNA-binding protein that controls RNA turnover and argue that the competition between MYCN's RNA- and DNA-bound states links the dynamics of the MYCN/MAX/MXD network to mRNA processing. Overall design: CUT&RUN, ChIP and ChIP-Rx DNA-sequencing was done with SH-EP NMYC-ER cells where the NMYC expression can be induced with 4-OHT. For all CUT&Run sequencing and some ChIP-seq and ChIP-Rx sequencing experiments, SH-EP NMYC-ER cells were used that carry a doxycycline inducible shRNA against TF3C5. For CUT&Run sequencing samples, the immunoprecipitation of digested DNA was done for EXOSC5 with or without NMYC expression and with or without expression of shTF3C5. As control, DNA was sequenced following immunoprecipitation with an unspecific IgG antibody. For ChIP-seq and ChIP-Rx sequencing experiments, the immunopresipitated antibodies are listed below. All experiments were done with or without NMYC expression and where indicated, a shRNA targeting TF3C5 was induced via doxycycline or one of the below listed inhibitors was applied.