CryoEM reveals that ribosomes in microsporidian spores are locked in a dimeric hibernating state.

Acknowledgements: R.C. and P.G.D. were funded by a Wellcome Trust Seed Award (212439/Z/18/Z) awarded to B.D. L.G. and M.M. were funded by an ERC starting grant (from the European Research Council under the European Union’s Horizon 2020 research and innovation programme, grant agreement no. 803894) awarded to B.D. M.M. was also funded by a BBSRC New Investigator Research Grant (BB/R008639/1) to V.G. S.C. was supported a Ministerio de Economía Y Competitividad Grant (MINECO; CTQ201782222-R). We acknowledge Diamond Light Source for access and support of the cryoEM facilities at the UK’s national eBIC at Diamond Light Source (under proposals BI25452 and EM18258), funded by the Wellcome Trust, MRC and BBRSC. We acknowledge access and support at the GW4 Facility for High-Resolution Electron Cryo-Microscopy, funded by the Wellcome Trust (202904/Z/16/Z and 206181/Z/17/Z) and BBSRC (BB/R000484/1). We are grateful to U. Borucu of the GW4 Regional Facility for High-Resolution Electron Cryo-Microscopy for help with screening and K. Heesom of University of Bristol Proteomics Facility for the mass spectrometry analysis. For the purpose of open access, the author has applied a CC BY public copyright licence to any author accepted manuscript version arising from this submission.Translational control is an essential process for the cell to adapt to varying physiological or environmental conditions. To survive adverse conditions such as low nutrient levels, translation can be shut down almost entirely by inhibiting ribosomal function. Here we investigated eukaryotic hibernating ribosomes from the microsporidian parasite Spraguea lophii in situ by a combination of electron cryo-tomography and single-particle electron cryo-microscopy. We show that microsporidian spores contain hibernating ribosomes that are locked in a dimeric (100S) state, which is formed by a unique dimerization mechanism involving the beak region. The ribosomes within the dimer are fully assembled, suggesting that they are ready to be activated once the host cell is invaded. This study provides structural evidence for dimerization acting as a mechanism for ribosomal hibernation in microsporidia, and therefore demonstrates that eukaryotes utilize this mechanism in translational control