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Incoming links: Cotranslational protein translocation
Cotranslational protein translocation 2026-10-18
Cotranslational protein translocation is passage of a growing polypeptide chain across a membrane during biological translation. SRP recognizes an exposed hydrophobic ER signal sequence. The SRP receptor enables handoff to the Sec61 complex. Biological translation can slow during delivery; the ribosome remains on the cytosol side.[ref]
Membrane Protein Insertion at the Endoplasmic Reticulum by Sichen Shao and Ramanujan S. Hegde. Shao and Hegde, Membrane Protein Insertion at the Endoplasmic Reticulum, 2011; “Cotranslational Targeting”, Figure 3
The useful engineering feature is coordination: a targeting instruction can become accessible while the rest of its cargo is still being made. Recognition, docking and crossing remain distinct operations. A defect in any one can prevent successful import even when the protein primary structure contains an appropriate signal.
This route is not universal. Many tail-anchored proteins use delivery after biological translation, and their membrane-spanning segment lies near the C-terminus. Thus protein targeting signals need not be at the N-terminus.[ref]
Membrane Protein Insertion at the Endoplasmic Reticulum by Sichen Shao and Ramanujan S. Hegde. Shao and Hegde, 2011; “Posttranslational Membrane Protein Topogenesis”, Figure 6
Organelle protein import Created 2026-10-13 Updated 2026-10-18
Organelle protein import brings externally synthesized proteins into an organelle. Many plastid proteins made from nuclear genes carry targeting information recognized by machinery that transports them across the envelope membranes. This delivery enables nuclear genes to supply functions inside a plastid, but the presence of targeting information is not itself evidence that a gene was acquired through endosymbiotic gene transfer. [ref]
Origin and Evolution of Plastids and Photosynthesis in Eukaryotes by Geoffrey I. McFadden. “How to tame your symbiont”: targeting, membrane passage and host-derived proteins
For the endoplasmic reticulum, cotranslational protein translocation illustrates how organelle protein import can begin before a polypeptide chain is complete. Import into the ER lumen and insertion as a transmembrane protein are different outcomes; neither alone establishes subsequent secretion from the biological cell.
Protein targeting Created 2026-10-13 Updated 2026-10-18
Protein targeting directs a protein to its cellular destination. Synthesis in the cytosol does not imply that the protein works there.
For entry into the endoplasmic reticulum, distinguish three questions: does the nascent chain reach the membrane, does part of it cross into the ER lumen, and does a segment remain embedded as a transmembrane protein? Cotranslational protein translocation couples these events to biological translation. A signal peptide can initiate delivery without specifying the eventual destination of the mature protein.
Protein targeting to the endoplasmic reticulum: delivery and three possible outcomes
. Original pathway and membrane topology schematic. The upper row separates SRP recognition, membrane docking and passage through a translocon. Lower cartoons stipulate a soluble cargo, a cleaved signal peptide plus retained stop-transfer sequence, and an uncleaved internal signal anchor. Each drawn terminus is labeled; the internal-anchor orientation is one possibility. Shapes and lengths are illustrative, with no molecular coordinates or measured timings. Details.
This distinction matters experimentally. Finding a protein in a membrane-containing centrifugation pellet could reflect surface binding or protein aggregation. The microsomal protein import experiment instead asks whether an intact membrane prevents an external protease from reaching the product. That tests accessibility across a barrier, rather than proximity to it.