Lysine methylation is an important protein modification that regulates replication and transcription, the fundamental events of the central dogma. However, whether it plays a role in translation, the last step of the central dogma, remains unclear. Two recent studies revealed that lysine methyltransferase SMYD5 catalyzes the trimethylation of lysine 22 in ribosomal protein RPL40, enhances translation output, and promotes cancer progression.

Lysine methylation occurs on various proteins and the most well-studied substrates are histones. Therefore, the roles of histone lysine methylation in DNA replication, transcription, DNA repair, and DNA recombination have been extensively studied.1 Researches on non-histone lysine methylation events and their primary functions in modulating protein–protein interactions also gradually evolved. Non-histone lysine methylation has been implicated in numerous cellular processes, such as the regulation of protein stability, promoter binding, and subcellular localization.2

Ribosomal proteins are among the most abundant proteins in the cell. Several lysine methylation events at various ribosomal proteins, including RPL40, RPL4, RPL29, and RPL36A, have been documented.3,4,5 However, whether lysine methylation in ribosomal protein regulates translation, the ultimate function of the ribosome, remains unclear, most likely due to the lack of identified lysine methyltransferases specific to ribosomal proteins. Two recent articles made an important breakthrough in this direction,6,7 and insights from these studies will shed light on this underappreciated, but potentially highly important area.

Employing different methodologies, the above two studies reached the same conclusion that SMYD5 methylates the core ribosomal protein RPL40. Miao et al.6 utilized an in vitro methyltransferase assay to screen for SET domain-containing lysine methyltransferase capable of modifying the RPL40 protein. By contrast, Park et al.7 purified recombinant SMYD5 and conducted in vitro methyltransferase assays on cellular extracts from wild-type and SMYD5-depleted cells, searching for a bona fide substrate of SMYD5 through mass spectrometry analysis. Their findings indicate that RPL40 is the predominant substrate for SMYD5, and SMYD5 appears to be the sole enzyme responsible for RPL40 K22 methylation. Notably, RPL40 K22 is almost fully methylated in multiple cell lines and animal tissues, suggesting that RPL40 K22me3 is likely a constitutive event, which implies its functional importance. It is interesting to ask whether such an event can be dynamically regulated or impaired in certain physiological and pathological conditions. Additionally, although SMYD5 was previously reported to catalyze H3K36me3 and H4K20me3,8,9 these two studies demonstrated that SMYD5 is unlikely a histone methyltransferase due to its cytoplasmic localization and inability to methylate nucleosomal histones in biochemical assays. The authors further explored the functional implications of the SMYD5-RPL40 K22me3 axis in translation. SMYD5 depletion and RPL40 K22R mutation significantly reduced global protein synthesis. In addition, disruption of the SMYD5-RPL40 K22me3 axis altered the polysome profiles and resulted in perturbation of translation elongation. These results convincingly demonstrated the functional impact of the SMYD5-RPL40 K22me3 axis on translation, but the exact molecular mechanism by which RPL40 K22me3 regulates translation requires further investigation.

Elevated ribosome biogenesis and translation activity are important features of cancer cells, and translation machinery has been considered as a potential therapeutic target.10 To elucidate the role of SMYD5-RPL40 K22me3 axis in cancer progression, the two studies focused on hepatocellular carcinoma and gastric adenocarcinoma, respectively. Both studies observed that SMYD5 and RPL40 K22me3 levels are often elevated in these tumors and the upregulation of SMYD5 correlates with poor clinical outcomes. Using tissue-specific Smyd5 knockout mice, the researchers demonstrated that SMYD5 is critical for hepatocellular carcinoma and gastric adenocarcinoma tumorigenesis in vivo. Miao et al. conducted comparative drug screens and showed that SMYD5-depleted cancer cells exhibited increased sensitivity to mTOR inhibition. Combined inhibition of SMYD5 and mTOR cooperatively suppressed translation and cancer progression. Furthermore, Park et al. reported that combining MSLN-CAR-T therapy with PI3K-mTOR inhibition and SMYD5 depletion effectively suppressed aggressive metastatic gastric adenocarcinoma.

The above two elegant studies unveiled the first functionally important ribosomal protein lysine methylation (Fig. 1). These findings suggest that the other identified ribosomal protein methylation events are also worthy of further investigation. Interestingly, although RPL40 K22 appears to be almost completely methylated in many cell types, Smyd5 knockout mice exhibited minimal phenotypes, supporting that SMYD5 could be a drug target. One obvious interesting puzzle is how elevated SMYD5 can further increase the level of RPL40 K22me3 in cancer cells as reported.1,2 A potential answer is that RPL40 K22 may not be fully methylated in actively proliferating cancer cells with high levels of ribosome biogenesis, which provides a competitive edge for cancer cells with elevated SMYD5.

SET domain-containing methyltransferase SMYD5 catalyzes trimethylation of the core ribosomal protein RPL40 at lysine 22 (RPL40 K22me3), thereby enhancing translation output and promoting cancer progression. SMYD5 depletion or RPL40 K22R mutation reduces global protein synthesis. Targeting SMYD5 presents a promising therapeutic approach for hepatocellular carcinoma and gastric adenocarcinoma, especially with concurrent therapies, such as mTOR inhibition. The discovery of the SMYD5-RPL40 K22me3 axis as a translational regulatory mechanism provides new insights into the biological significance of lysine methylation in ribosome biology and translation. Certain features of this figure were drawn using Servier Medical Art licensed under CC BY 4.0.