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Max Gemmer, Marten Chaillet, Mariska Groellers-Mulderij, Rodrigo Cuevas Arenas, Juliette Fedry, Friedrich Foerster (Structural Biochemistry, Bijvoet Center for Biomolecular Research, Utrecht University, Utrecht, The Netherlands)
Many proteins are secreted from the cell or anchored into the cell membrane. The journey of such proteins begins at the endoplasmic reticulum (ER) membrane, where nascent polypeptides are synthesized by the ribosome and co-translationally inserted into the ER by the membrane-embedded translocon complex. Thousands of different proteins - about 30% of the human proteome - are manufactured this way. To handle the large variety of protein substrates, specialized accessory components have evolved to facilitate translocation of specific substrate pools. We used cryo-electron tomography to visualize the molecular landscape of the ~135,000 ribosome from more than 800 ER vesicles. With the help of computational methods, we can dissect the entire machinery ‘from head to toe’. We detected 1 soluble population and 3 major translocon variants: the TRAP-, OSTA-, and multipass translocon. We analyzed their spatial distribution with polyribosomes and their sub-stoichiometric interaction partners. Sorting ribosome intermediate states (not shown) allows us to compare active and inactive translocons. Collectively, we visualize ER-bound polysomes with their coordinated downstream machinery.