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Related Concept Videos

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
In eukaryotes, the translocon complex comprises a core heterotrimeric translocator channel called the Sec61 complex. This channel includes three transmembrane proteins, Sec61α, Sec61β, and Sec61γ, and is the largest subunit of the...
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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.
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Post-translational Translocation of Proteins to the RER01:27

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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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Membrane Asymmetry Regulating Transporters01:19

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Enzymes like flippase, floppase, and scramblase transfer phospholipids from one layer to another in the membrane, thereby affecting membrane asymmetry.
Flippase
Eukaryotic flippases are type-IV P-type ATPases or P4-ATPases belonging to P-type ATPase family proteins that are membrane-bound pumps involved in the ATP-mediated transport of ions and molecules across the membrane. Flippases flip specific phospholipids from the outer to the inner leaflet of a membrane. All P4-ATPases have one...
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Insertion of Single-pass Transmembrane Proteins in the RER01:26

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Integral membrane proteins are proteins adhered to the lipid bilayer of a cell organelle or membrane. They can be of two types: transmembrane integral proteins that span the lipid bilayer and monotopic proteins that are attached to either side of the membrane but do not pass through it.
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Assembly of the Lipid Bilayer in the ER01:28

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Biological membranes are more than just a barrier separating cell cytoplasm from the outside environment. They are highly dynamic and help maintain the integrity and physiological stability of the cells as well as membrane-bound organelles. Membranes also play vital roles in cell-to-cell and intracellular communication.
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Updated: May 19, 2025

Isolation of Physiologically Active Thylakoids and Their Use in Energy-Dependent Protein Transport Assays
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Lipid Scrambling Pathways in the Sec61 Translocon Complex.

Matti Javanainen1,2, Jan Šimek3,4, Dale Tranter2

  • 1Unit of Physics, University of Tampere, FI-33720 Tampere, Finland.

Journal of the American Chemical Society
|May 6, 2025
PubMed
Summary
This summary is machine-generated.

The translocon-associated protein (TRAP) complex facilitates lipid scrambling across the endoplasmic reticulum membrane, independent of the Sec61 protein

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Area of Science:

  • Cellular Biology
  • Membrane Biophysics
  • Protein-Lipid Interactions

Background:

  • Cellular homeostasis relies on ATP-independent lipid translocation across the endoplasmic reticulum (ER) membrane.
  • Scramblases are membrane proteins facilitating lipid translocation; ER-resident scramblases are recently identified.
  • Previous structures suggested the Sec61/translocon-associated protein (TRAP) complex might mediate lipid scrambling due to observed membrane thinning.

Purpose of the Study:

  • To investigate the role of the Sec61/TRAP complex in lipid scrambling.
  • To identify the specific component and mechanism responsible for lipid translocation within the complex.
  • To determine the lipid specificity and regulation of this scrambling activity.

Main Methods:

  • Reconstitution of translocon complexes for fluorescence spectroscopy assays.
  • Inhibition assays using Sec61 inhibitors.
  • Molecular dynamics simulations.
  • Kinetic and thermodynamic analyses.

Main Results:

  • Translocon complexes exhibited nonselective lipid scrambling activity.
  • Scrambling activity persisted even when Sec61's lateral gate was inhibited, suggesting an alternative pathway.
  • Molecular dynamics simulations revealed TRAP facilitates lipid translocation via a "credit card" mechanism.
  • Membrane thinning enhanced scrambling efficiency; TRAP and Sec61 preferentially scrambled phosphatidylcholine over phosphatidylethanolamine and phosphatidylserine.

Conclusions:

  • The trimeric TRAP subunit provides an alternative, Sec61-independent pathway for lipid scrambling.
  • This TRAP-mediated lipid translocation is insensitive to the functional state of the Sec61 translocon.
  • The findings identify TRAP as a key player in metazoan lipid homeostasis.