Ribosome biogenesis governs protein synthesis. NIFK is transactivated by c-Myc, the key regulator of ribosome biogenesis. The biological function of human NIFK is not well established, except that it has been shown to interact with Ki67 and NPM1. Here we report that NIFK is required for cell cycle progression and participates in the ribosome biogenesis via its RNA recognition motif (RRM). We show that silencing of NIFK inhibits cell proliferation through a reversible p53-dependent G1 arrest, possibly by induction of the RPL5/RPL11-mediated nucleolar stress. Mechanistically it is the consequence of impaired maturation of 28S and 5.8S rRNA resulting from inefficient cleavage of internal transcribed spacer (ITS) 1, a critical step in the separation of pre-ribosome to small and large subunits. Complementation of NIFK silencing by mutants shows that RNA-binding ability of RRM is essential for the pre-rRNA processing and G1 progression. More specifically, we validate that the RRM of NIFK preferentially binds to the 5'-region of ITS2 rRNA likely in both sequence specific and secondary structure dependent manners. Our results show how NIFK is involved in cell cycle progression through RRM-dependent pre-rRNA maturation, which could enhance our understanding of the function of NIFK in cell proliferation, and potentially also cancer and ribosomopathies.
Keywords: 5S RNP, 5S ribonucleoprotein particle; CDK1, cyclin dependent kinase 1; DFC, dense fibrillar component; ETS/ITS, external/internal transcribed spacers; GSK3, glycogen synthase kinase 3; Ki67; Ki67FHAID, Ki67-FHA interaction domain; LSU, large subunit; MDM2, murine double minute 2; NIFK, Nucleolar protein Interacting with the FHA domain of pKi-67; NPM1/B23, nucleophosmin; Noprecipitation; PAR-CLIP, Photo-Activatable-Ribonucleoside-Enhanced Crosslinking and Immu-pre-rRNAs, rRNA precursors; REMSA, RNA electrophoresis mobility shift assay; RNA recognition motif; RNP1 and 2, ribonucleoprotein motif 1 and 2; RPL5 and RPL11, large ribosomal protein 5 and 11; RRM, RNA recognition motif; cell cycle; nucleolar stress; rNIFK, recombinant NIFK; ribosome biogenesis; snoRNP, small nucleolar ribonucleoprotein.