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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
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Tail-anchored, or TA, proteins are estimated to make up to 3-5% of membrane proteins found in the eukaryotic cell. Such proteins have a single transmembrane domain located approximately 30 amino acid residues upstream from the C-terminal end. As a result, the signal recognition particle (SRP) cannot guide a TA protein to the ER membrane for cotranslational insertion. Hence, they are integrated into the ER membrane post-translationally using their C-terminal end as the anchor. TA proteins...
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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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Cotranslational Protein Translocation01:20

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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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The rough ER membrane synthesizes, assembles, and embeds transmembrane proteins in diverse topologies. These proteins function as transporters or channels and can remain in the ER membrane or are sent to the Golgi complex, lysosome, and cell membrane.
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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
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SND3 is the membrane insertase within a distinct SEC61 translocon complex.

Tzu-Jing Yang1, Saumyak Mukherjee2, Julian D Langer3,4

  • 1Membrane Protein Biogenesis Research Group, Max Planck Institute of Biophysics, Frankfurt am Main, Germany.

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The study identifies Chaetomium thermophilum SND3 as a membrane insertase, revealing its role in the poorly understood SRP-independent (SND) pathway for integral membrane protein (IMP) biogenesis.

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Area of Science:

  • Molecular Biology
  • Cell Biology
  • Structural Biology

Background:

  • Integral membrane proteins (IMPs) are crucial for cellular functions.
  • Their biogenesis involves insertion into the endoplasmic reticulum membrane via translocons.
  • The SRP-independent (SND) pathway remains poorly understood despite its broad IMP substrate range.

Purpose of the Study:

  • To elucidate the mechanism of the SND pathway in eukaryotic integral membrane protein biogenesis.
  • To characterize the structure and function of the SND3 translocon complex.

Main Methods:

  • Cryo-electron microscopy (cryo-EM) to determine the structure of the ribosome-associated SND3 translocon complex.
  • Molecular dynamics (MD) simulations to investigate the mechanism of IMP insertion.
  • Sequence and structural comparisons across different taxa.

Main Results:

  • Chaetomium thermophilum SND3 functions as a membrane insertase with an atypical fold.
  • The SND3 translocon complex comprises SEC61, CCDC47, and TRAPɑ, with SEC61β N-terminus and CCDC47 inhibiting substrate access.
  • SND3 utilizes a membrane-embedded hydrophilic groove to disrupt the lipid bilayer and facilitate IMP insertion.

Conclusions:

  • The SND3 translocon represents a distinct multipass translocon system in fungi and other eukaryotes.
  • This finding sheds light on the mechanism of IMP biogenesis via the understudied SND pathway.