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Related Concept Videos

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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Energy to Drive Translocation01:37

Energy to Drive Translocation

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Mitochondrial protein import is powered by two distinct energy sources: ATP hydrolysis and electrochemical potential across the inner membrane. Newly synthesized precursors are bound by cytosolic chaperones of the Hsp70 family, which guide them to the import receptors on the mitochondrial surface. Utilizing the energy of ATP hydrolysis, Hsp70 chaperones transfer these precursors to the TOM receptors on the mitochondrial outer membrane.
Generally, polypeptides are unfolded by two distinct...
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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
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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Protein Transport into the Inner Mitochondrial Membrane01:34

Protein Transport into the Inner Mitochondrial Membrane

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Nuclear encoded mitochondrial precursors are imported to the inner membrane in a multistep process involving two separate translocons, TIM22 and TIM23. TIM23 is a cation-selective pore that remains closed by the N terminal segment of the protein. Negative charges on the TIM23 act as a receptor for the incoming precursor, pulling the positively charged matrix-targeting sequence for peptide insertion and translocation.
Transport of mitochondrial precursors across the TIM23 channel is driven by...
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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
During cotranslational translocation, the Sec61 channel partners with the signal recognition particle (SRP), the signal recognition particle receptor (SR), and the ribosomes to transport the nascent polypeptide chain...
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Translocation of Proteins into the Mitochondria01:19

Translocation of Proteins into the Mitochondria

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Mitochondrial precursors are translocated to the internal subcompartments via independent mechanisms involving distinct protein machineries called translocases.
Sorting of outer membrane proteins:
Mitochondrial outer membrane proteins are of two types: the transmembrane, beta-barrel porins, and the membrane-anchored, alpha-helical proteins. Beta-barrel porin precursors are translocated by the TOM complex and inserted into the outer mitochondrial membrane by the SAM complex. In contrast,...
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Updated: May 23, 2025

Assessment of Submitochondrial Protein Localization in Budding Yeast Saccharomyces cerevisiae
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Stepwise ATP translocation into the endoplasmic reticulum by human SLC35B1.

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The endoplasmic reticulum (ER) imports ATP via the SLC35B1 transporter, not nucleotide sugars. Cryo-EM structures reveal SLC35B1

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Isolation of Physiologically Active Thylakoids and Their Use in Energy-Dependent Protein Transport Assays
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Area of Science:

  • Cellular Biology
  • Molecular Transport
  • Biochemistry

Background:

  • Mitochondria generate ATP, crucial for cellular functions.
  • The endoplasmic reticulum (ER) requires imported ATP for protein homeostasis and trafficking.
  • The ER-resident ATP transporter remained elusive, with SLC35B1 (AXER) recently proposed as a candidate.

Purpose of the Study:

  • To investigate the function of human SLC35B1 (AXER) as an ER ATP importer.
  • To elucidate the transport mechanism and structural basis of SLC35B1 activity.
  • To validate the physiological relevance of SLC35B1 in cellular growth.

Main Methods:

  • Biochemical assays to assess nucleotide binding and transport kinetics.
  • CRISPR-Cas9 gene editing to create SLC35B1 knockout cell lines.
  • Cryogenic electron microscopy (cryo-EM) to determine high-resolution structures of SLC35B1.

Main Results:

  • Human SLC35B1 binds ATP/ADP, not nucleotide sugars, and facilitates ATP import into ER microsomes.
  • SLC35B1 is essential for cell growth, confirmed by knockout studies.
  • Seven cryo-EM structures reveal ATP/ADP translocation via a stepwise mechanism involving vertical nucleotide repositioning within a flexible binding site.

Conclusions:

  • SLC35B1 functions as a dedicated ER ATP importer, crucial for cellular energy balance.
  • The stepwise translocation mechanism of SLC35B1 represents a novel mode of substrate transport for SLC transporters.
  • Understanding SLC35B1's function provides insights into ER energetics and cellular health.