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Related Concept Videos

Overview of Protein Sorting and Transport01:45

Overview of Protein Sorting and Transport

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Eukaryotic cells have different membrane-bound organelles with distinct protein requirements. The process by which proteins are targeted to a specific organelle is called protein sorting.
Protein sorting can be of two types: signal-based sorting and vesicle-based trafficking. In signal-based sorting, specific amino acid sequences called sorting signals target proteins to the proper location inside the cell either via gated transport or by protein translocation.  In gated transport, folded...
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Cotranslational Protein Translocation01:20

Cotranslational Protein Translocation

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Translocation of proteins across membranes is an ancient process that occurs even in bacteria and archaebacteria. In fact, the components of the translocation machinery are still conserved between prokaryotes and eukaryotes.
Sec61 channel partners for cotranslational translocation
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Protein Translocation Machinery on the ER Membrane01:28

Protein Translocation Machinery on the ER Membrane

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The translocon complex situated on the ER membrane is the main gateway for the protein secretory pathway. It facilitates the transport of nascent peptides into the ER lumen and their insertion into the ER membrane.
Sec61 protein conducting channel
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Post-translational Translocation of Proteins to the RER01:27

Post-translational Translocation of Proteins to the RER

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A sizable fraction of proteins destined for ER are first synthesized in the cell cytosol and then transported across the ER membrane–a process called post-translational translocation. Similar to cotranslationally translocated proteins, these proteins also use the Sec translocon complex to enter the ER lumen.
Targeting proteins to the ER
Hsp40 and Hsp70 chaperone molecules bind the translated proteins in the cytosol to prevent their folding. The chaperone binding helps to keep the signal...
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Protein Transport to the Thylakoids01:22

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Thylakoids are membrane-bound sac-like structures within the chloroplast that serve as sites for photosynthesis. Thylakoid lumen contains many electron transport proteins and is enclosed by a thylakoid membrane rich in the light-harvesting complex. Proteins targeted to the thylakoids are transported as precursors and are sorted by the general TOC/TIC import pathway. Once the precursor reaches the stroma, stromal processing peptidases remove their transit signal and expose thylakoid signal...
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Transport Across the Golgi01:26

Transport Across the Golgi

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While it is unclear how molecules move between adjacent Golgi cisternae, it is apparent that the molecules move from cis- cisterna, the entry face, to the trans- cisterna, the exit face. Experiments initially suggested vesicles that bud from one cisterna and fuse with the next cisterna to transport proteins between the cisternae. This vesicular transport model describes the Golgi apparatus as a relatively static structure with a unique enzyme composition in each cisterna. Molecules are...
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Unconventional Protein Transport Across the Golgi Ribbon.

Jaakko Saraste1, Kristian Prydz2

  • 1Department of Biomedicine and Molecular Imaging Center, University of Bergen, Bergen, Norway. jaakko.saraste@uib.no.

Sub-Cellular Biochemistry
|November 15, 2025
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Summary

Unconventional protein secretion (UcPS) bypasses the Golgi apparatus using distinct vesicular pathways. The intermediate compartment (IC) acts as a key sorting hub, connecting ER-synthesized and cytoplasmic proteins to cell surface delivery routes.

Keywords:
Brefeldin A (BFA)ER–Golgi intermediate compartment (IC or ERGIC)Golgi bypassGolgi reassembly and stacking proteins (GRASPs or GORASPs)Golgi ribbonRab1Rab11Recycling endosome (RE)Unconventional protein secretion (UcPS)

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Area of Science:

  • Cell Biology
  • Molecular Biology
  • Biochemistry

Background:

  • The classical secretory pathway transports proteins via the endoplasmic reticulum (ER) and Golgi apparatus.
  • Unconventional protein secretion (UcPS) encompasses Golgi-independent routes for protein delivery to the cell surface.
  • The precise mechanisms and intermediates of UcPS pathways remain incompletely understood.

Purpose of the Study:

  • To elucidate the role of the intermediate compartment (IC) in UcPS pathways.
  • To investigate the sorting and trafficking of ER-synthesized and leaderless cargo via UcPS.
  • To explore the connection between classical and unconventional secretion routes.

Main Methods:

  • Investigated vesicular transport pathways.
  • Analyzed the function of the intermediate compartment (IC).
  • Examined the interaction between IC and the endocytic recycling system.

Main Results:

  • The IC serves as a crucial sorting station for two distinct UcPS pathways.
  • The IC mediates Golgi-independent trafficking of ER-synthesized and leaderless cytoplasmic cargo.
  • The IC interacts with the endocytic recycling system, facilitating cell surface protein delivery.

Conclusions:

  • The intermediate compartment (IC) is central to multiple unconventional protein secretion (UcPS) pathways.
  • UcPS routes diverge from or intersect with the classical secretory pathway at the Golgi ribbon level.
  • Understanding UcPS provides insights into protein trafficking beyond the Golgi apparatus.