METTL18-mediated histidine methylation of RPL3 modulates translation elongation for proteostasis maintenance.

Protein methylation occurs predominantly on lysine and arginine residues, but histidine also serves as a methylation substrate. However, a limited number of enzymes responsible for this modification have been reported. Moreover, the biological role of histidine methylation has remained poorly understood to date. Here, we report that human METTL18 is a histidine methyltransferase for the ribosomal protein RPL3 and that the modification specifically slows ribosome traversal on Tyr codons, allowing the proper folding of synthesized proteins. By performing an in vitro methylation assay with a methyl donor analog and quantitative mass spectrometry, we found that His245 of RPL3 is methylated at the τ-N position by METTL18. Structural comparison of the modified and unmodified ribosomes showed stoichiometric modification and suggested a role in translation reactions. Indeed, genome-wide ribosome profiling and an in vitro translation assay revealed that translation elongation at Tyr codons was suppressed by RPL3 methylation. Because the slower elongation provides enough time for nascent protein folding, RPL3 methylation protects cells from the cellular aggregation of Tyr-rich proteins. Our results reveal histidine methylation as an example of a ribosome modification that ensures proteome integrity in cells.

Suppression of RPL34 Inhibits Tumor Cell Proliferation and Promotes Apoptosis in Glioblastoma.

Accumulating evidence indicates Ribosomal protein 34 (RPL34) promotes tumor malignance and its expression is associated with poor prognosis in multiple cancer cells. However, the physiological role and biological mechanism of RPL34 in glioblastoma (GBM) remain unclear. Hence, this study aimed to investigate the expression and the role of RPL34 in GBM. A total of 59 glioma samples and 12 normal brains for epilepsy surgery were used to determine the underlying mechanisms and the biological behaviors of RPL34 in GBM. In this study, we identified that RPL34 expression was significantly (p < 0.05) enriched in GBM tumors compared with low-grade glioma and normal brain, and its expression was associated with poor survival. Additionally, RPL34 was functionally required for tumor proliferation in vitro. Mechanically, inhibition of RPL34 induced glioma cell apoptosis by activation of Bad/Caspase7/PARP signaling pathway. The RPL34 promotes cell survival in GBM and could be a potential therapeutic target for GBM.

The RNA helicase Dbp7 promotes domain V/VI compaction and stabilization of inter-domain interactions during early 60S assembly.

Early pre-60S ribosomal particles are poorly characterized, highly dynamic complexes that undergo extensive rRNA folding and compaction concomitant with assembly of ribosomal proteins and exchange of assembly factors. Pre-60S particles contain numerous RNA helicases, which are likely regulators of accurate and efficient formation of appropriate rRNA structures. Here we reveal binding of the RNA helicase Dbp7 to domain V/VI of early pre-60S particles in yeast and show that in the absence of this protein, dissociation of the Npa1 scaffolding complex, release of the snR190 folding chaperone, recruitment of the A3 cluster factors and binding of the ribosomal protein uL3 are impaired. uL3 is critical for formation of the peptidyltransferase center (PTC) and is responsible for stabilizing interactions between the 5' and 3' ends of the 25S, an essential pre-requisite for subsequent pre-60S maturation events. Highlighting the importance of pre-ribosome remodeling by Dbp7, our data suggest that in the absence of Dbp7 or its catalytic activity, early pre-ribosomal particles are targeted for degradation.