Regulation of translational fidelity by repression of C/D box SNoRNPs during UPR

Endoplasmic reticulum (ER) is the major cellular compartment where the folding and maturation of secretory and membrane proteins take place. When protein folding demands exceed the protein handling capacity of ER, the unfolded protein response (UPR) pathway is activated. UPR reduces the client protein load in the ER by inhibiting protein translation and increases protein handling capacity by upregulating genes encoding ER-resident molecular chaperones. The main pathway for translational repression in response to ER stress has been the phosphorylation of eIF2 by PERK, which also suppresses rRNA synthesis. rRNA methylation and pseudouridylation can fine-tune protein synthesis. Small nucleolar ribonucleoproteins (SnoRNPs) are mainly separated into two subtypes: C/D box and H/ACA box. C/D box SNoRNPs guides rRNA 2-O-methylation, while H/ACA SNoRNPs catalyze the pseudouridylation modifications of rRNAs. We hypothesize that the UPR may regulate translational fidelity by regulating the expression of core proteins of C/D and H/ACA box SnoRNPs. In this study, we show that the expressions of NHP2L1, NOP56 and FBL were significantly downregulated by UPR. In addition, FBL knockdown reduced cell proliferation and colony formation. Furthermore, we show the effects of UPR and FBL knockdown on rRNA methylation and translation fidelity, including alterations in nonsense suppression, frameshifts, ribosome pausing and translation initiation by IRES. In conclusion, this study reveals the regulation of translational fidelity during UPR by change in rRNA methylation due to reduced function of C/D box SnoRNPs.