While lysine methylation is well-known for regulating gene expression transcriptionally, its implications in translation have been largely uncharted. Trimethylation at lysine 22 (K22me3) on RPL40, a core ribosomal protein located in the GTPase activation center, was first reported 27 years ago. Yet, its methyltransferase and role in translation remain unexplored. Here, we report that SMYD5 has robust in vitro activity toward RPL40 K22 and primarily catalyzes RPL40 K22me3 in cells. The loss of SMYD5 and RPL40 K22me3 leads to reduced translation output and disturbed elongation as evidenced by increased ribosome collisions. SMYD5 and RPL40 K22me3 are upregulated in hepatocellular carcinoma (HCC) and negatively correlated with patient prognosis. Depleting SMYD5 renders HCC cells hypersensitive to mTOR inhibition in both 2D and 3D cultures. Additionally, the loss of SMYD5 markedly inhibits HCC development and growth in both genetically engineered mouse and patient-derived xenograft (PDX) models, with the inhibitory effect in the PDX model further enhanced by concurrent mTOR suppression. Our findings reveal a novel role of the SMYD5 and RPL40 K22me3 axis in translation elongation and highlight the therapeutic potential of targeting SMYD5 in HCC, particularly with concurrent mTOR inhibition. This work also conceptually broadens the understanding of lysine methylation, extending its significance from transcriptional regulation to translational control.

虽然赖氨酸甲基化因转录调节基因表达而闻名，但其在翻译中的影响在很大程度上尚不清楚。 RPL40（一种位于 GTP 酶激活中心的核心核糖体蛋白）上赖氨酸 22 (K22me3) 的三甲基化于 27 年前首次被报道。然而，其甲基转移酶及其在翻译中的作用仍有待探索。在这里，我们报告 SMYD5 对 RPL40 K22 具有强大的体外活性，并主要催化细胞中的 RPL40 K22me3。 SMYD5 和 RPL40 K22me3 的丢失导致翻译输出减少和延伸受到干扰，核糖体碰撞增加证明了这一点。 SMYD5 和 RPL40 K22me3 在肝细胞癌 (HCC) 中表达上调，与患者预后呈负相关。在 2D 和 3D 培养物中，耗尽 SMYD5 会使 HCC 细胞对 mTOR 抑制过度敏感。此外，在基因工程小鼠和患者来源的异种移植物 (PDX) 模型中，SMYD5 的缺失显着抑制 HCC 的发展和生长，并且同时抑制 mTOR 进一步增强 PDX 模型中的抑制作用。我们的研究结果揭示了 SMYD5 和 RPL40 K22me3 轴在翻译延伸中的新作用，并强调了靶向 SMYD5 在 HCC 中的治疗潜力，特别是同时抑制 mTOR。这项工作还在概念上拓宽了对赖氨酸甲基化的理解，将其重要性从转录调控扩展到翻译控制。