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Abstract

The synthesis of cellular proteins requires a significant portion of energy and cellular resources. Thus, protein translation is highly regulated to adapt the proteome output to environmental changes. In mammalian cells, the kinase mTORC1 is activated by environmental inputs from growth factors and nutrients. In turn, mTORC1 enhances anabolic processes, such as synthesizing proteins and lipids, while suppressing protein catabolism. Although the molecular mechanisms by which mTORC1 regulates protein synthesis are well studied, it is still unclear how cells can adapt to the inactivation of mTORC1. In my PhD project, I performed genome-wide CRISPR screens to systematically identify genes crucial for adapting mammalian cells to the inactivation of mTORC1. Particularly striking hits were tRNA-modifying enzymes that form mcm5s2 modifications at wobble uridines of tRNAs (U34-enzymes): Elongator and Ctu1/2. Under mTORC1 inhibition, Elongator and Ctu1/2 became essential for cell proliferation in vitro and in tumors in vivo. As the functional role of tRNA wobble modifications in mammalian cells was poorly understood, I first characterized the impact of U34-enzymes on protein synthesis. By integrating nascent proteomics, steady-state proteomics, and ribosome profiling, I found that tRNA wobble modifications globally affect protein synthesis by promoting efficient decoding of VAA codons. Notably, U34-enzymes enhanced the synthesis of ribosomal proteins, but steady-state protein levels were only slightly affected. Finally, I assessed the impact of mTORC1 inhibition in U34-enzyme-deficient cells and found that the simultaneous suppression of mTORC1 and U34-enzymes depleted cells of ribosomal proteins, resulting in global translation inhibition. Hence, my PhD project revealed a molecular mechanism by which mTORC1 and tRNA wobble modification cooperate to generate the protein synthesis machinery and thus maintain cellular translation capacity and cell proliferation.