Abstract
Translation elongation is a universally conserved step in protein synthesis, relying on elongation factors that engage the ribosomal L7/L12 stalk to mediate aminoacyl-tRNA delivery, accommodation, and ribosomal translocation. Using in organello cryo-electron microscopy, we reveal how the mitochondrial translation accelerator TACO1 promotes efficient elongation on human mitoribosomes. TACO1 binds the mitoribosomal region typically bound by elongation factor Tu (mtEF-Tu), bridging the large and small subunits via contacts with 16S rRNA, bL12m, A-site tRNA, and uS12m. While active throughout elongation, TACO1 is especially critical when translating polyproline motifs. Its absence prolongs mtEF-Tu residence in A/T states, causes persistent mitoribosomal stalling and premature subunit dissociation. Structural analyses indicate that TACO1 competes with mtEF-Tu for mitoribosome binding, stabilizes A-site tRNA, and enhances peptidyl transfer through a mechanism distinct from EF-P and eIF5A. These findings suggest that bacterial TACO1 orthologs may serve analogous roles, highlighting an evolutionarily conserved strategy for maintaining elongation efficiency during challenging translation events.
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Data availability
The cryo-EM density maps of the mitoribosome–TACO1 complex generated in this study have been deposited in the Electron Microscopy Data Bank (EMDB) under the following accession codes: EMD-70621 (consensus map of the WT 55S mitoribosome), EMD-70620 (focused mtSSU map), EMD-70619 (focused mtLSU map), and EMD-70592 (composite map of the WT 55S mitoribosome). The corresponding atomic model has been deposited in the Protein Data Bank (PDB) under accession code 9OLF (55S mitoribosome in complex with LRPPRC, SLIRP, OXA1L, and TACO1). Cryo-EM maps of distinct mitoribosome states from WT and TACO1-KO cells have been deposited in the EMDB under accession codes EMD-71630 [https://www.ebi.ac.uk/emdb/EMD-70630], EMD-71634 [https://www.ebi.ac.uk/emdb/EMD-70634], EMD-71829, EMD-71623, EMD-71633, EMD-71797, EMD-71802, EMD-71809, EMD-71811, EMD-71815, EMD-71818, EMD-71635, EMD-71825, and EMD-71828. The corresponding atomic models have been deposited in the PDB under accession codes 9PGF, 9PGL, 9PSM, 9PG8, 9PGI, 9PR4, 9PRA, 9PRD, 9PRQ, 9PRX, 9PS0, 9PGM, 9PS7, and 9PSI. A complete list of accession codes is provided in Supplementary Tables 1-4. Mass spectrometry data have been deposited to the ProteomeXchange Consortium via the PRIDE partner repository with the dataset identifier PXD064026. Source data are provided with this paper.
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Acknowledgements
We thank V. Singh, A. Khawaja, and J. Rorbach at Karolinka Institutet for helpful discussions and providing purified mtEF-Tu. We thank J. Lin, K. Zhou, and S. Wu of the Yale Cryo-EM Resource for expert training and assistance with grid screening and data collection. We are grateful to N. Grigorieff and J. Elferich from University of Massachusetts for sharing the latest version of cisTEM. We also thank all members of the Xiong laboratory for assistance and encouragement throughout this project. Special thanks to S. Tang, A. Didychuk, I. Lomakin, and J. Wang for insightful discussions and suggestions. This work was supported by startup funds from Yale University to Y.X., National Institute of General Medicine (NIGMS) grant R35-GM118141 to A.B., Muscular Dystrophy Association Research grant 1069392 to A.B., Muscular Dystrophy Association Development grant 22-1288334 to M.B. and AFM-Téléthon Trampoline grant 28651 to M.B.
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Y.X. and A.B. conceived and supervised this study. S.W. performed cell culture and mitochondria isolation. S.W., C.W., and W.Z. carried out cryo-EM grid screening and data collection. S.W. and Y.X. processed the cryo-EM data. S.W. and Y.Z. identified the TACO1 and built the structural models. M.B. performed the GTP-binding assays, sucrose gradient sedimentation analyses and mass spectrometry experiments. S.W., Y.X., M.B., and A.B. contributed to drafting the manuscript. All authors participated in data analysis and manuscript review.
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Wang, S., Brischigliaro, M., Zhang, Y. et al. Structural basis of TACO1-mediated efficient mitochondrial translation. Nat Commun (2026). https://doi.org/10.1038/s41467-026-69156-y
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DOI: https://doi.org/10.1038/s41467-026-69156-y
