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EMC rectifies the topology of multipass membrane proteins.

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Smalinskaitė, Luka 


Most eukaryotic multipass membrane proteins are inserted into the membrane of the endoplasmic reticulum. Their transmembrane domains (TMDs) are thought to be inserted co-translationally as they emerge from a membrane-bound ribosome. Here we find that TMDs near the carboxyl terminus of mammalian multipass proteins are inserted post-translationally by the endoplasmic reticulum membrane protein complex (EMC). Site-specific crosslinking shows that the EMC's cytosol-facing hydrophilic vestibule is adjacent to a pre-translocated C-terminal tail. EMC-mediated insertion is mostly agnostic to TMD hydrophobicity, favored for short uncharged C-tails and stimulated by a preceding unassembled TMD bundle. Thus, multipass membrane proteins can be released by the ribosome-translocon complex in an incompletely inserted state, requiring a separate EMC-mediated post-translational insertion step to rectify their topology, complete biogenesis and evade quality control. This sequential co-translational and post-translational mechanism may apply to ~250 diverse multipass proteins, including subunits of the pentameric ion channel family that are crucial for neurotransmission.


Acknowledgements: We thank H. Wang and other Hegde Lab members for fruitful discussions, Y. Hooda for flow cytometry data, R. Judy for help with data curation, S. Juszkiewicz for plasmids, V. Paavilainen and K. McPhail for providing Apratoxin A and R. Keenan for discussions and reagents. This work was supported by the UK Medical Research Council (grant MC_UP_A022_1007 to RSH) and a European Molecular Biology Organization (EMBO) Postdoctoral Fellowship (ALTF 369-2021 to H.W.).


Animals, Membrane Proteins, Endoplasmic Reticulum, Protein Domains, Ribosomes, Protein Transport, Mammals

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Nat Struct Mol Biol

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Springer Science and Business Media LLC