The membrane targeting and insertion of proteins is facilitated by a conserved machinery that acts concomitantly with translation. Nascent proteins are recognized by the signal recognition particle (SRP) and transferred to the SRP receptor (SR) on the membrane. A series of conformational rearrangements of SRP and SR coordinates the handover of the nascent chain to the translocon, which inserts proteins into the membrane or facilitates their secretion. Here we used single particle cryo electron microscopy to obtain snapshots of co-translational protein targeting complexes at near atomic resolution [1]. We observe conformational changes of the SRP M domain induced by binding of the nascent chain, which are coupled to rearrangements of the SRP and SR. In addition we reconstructed a near atomic resolution structure of the translocon in complex with a translating ribosome. A detailed analysis of the ribosome interaction sites of SRP and translocon gives novel insight into the handover of the nascent chain to the translocon.

[1] Jomaa A, Boehringer D, Leibundgut M, Ban N. Structures of the E. coli translating ribosome with SRP and its receptor and with the translocon. 2016. Nat Commun. 7:10471

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EMC Abstracts - https://emc-proceedings.com/abstract/snapshots-of-co-translational-protein-targeting-complexes/