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Related Concept Videos

Porin Insertion in the Outer Mitochondrial Membrane01:12

Porin Insertion in the Outer Mitochondrial Membrane

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Porins are beta-barrel proteins translocated to the mitochondrial outer membrane through the TOM complex into the intermembrane space. Porin precursors bind TIM chaperones within the intermembrane space and are guided to the Sorting and Assembly Machinery complex or SAM complex on the outer mitochondrial membrane.
Three models describe the assembly of porins by the SAM complex and their insertion into the outer membrane. Model 1 suggests that porins are assembled outside the SAM channel as the...
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Structure of Porins01:21

Structure of Porins

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Mitochondria, chloroplasts, and gram-negative bacteria have transmembrane, beta-barrel proteins called porins to mediate the free diffusion of ions and metabolites across the membrane. Mitochondrial porin precursors contain conserved amino acid sequences called beta signals at their C-terminal. Beta signals have a  motif of PoXGXXHyXHy (Po-Polar, X-Any amino acid, G-Glycine, Hy-LargeHydrophobic), which are crucial for precursor recognition to initiate precursor assembly. Beta-barrel...
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Mitochondrial Protein Sorting01:39

Mitochondrial Protein Sorting

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Mitochondria are double-membrane organelles of the eukaryotes involved in cellular metabolism, signaling, ATP synthesis, and programmed cell death.  Each of these processes requires specific proteins and enzymes that must be correctly sorted to the right mitochondrial subcompartment for the proper functioning of the organelle.
Most of these mitochondrial proteins are encoded by the nucleus and imported to the mitochondria as unfolded or loosely folded precursors. Mitochondrial precursors...
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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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Mitochondrial Precursor Proteins01:39

Mitochondrial Precursor Proteins

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Mitochondrial precursors are partially unfolded or loosely folded polypeptide chains. Newly synthesized precursors are inhibited from spontaneously folding into their native conformation by the cytosolic chaperones, heat shock proteins 70 (Hsp70), and mitochondrial import stimulation factors (MSFs). Precursors bound to MSFs are guided to the TOM70-TOM37 receptors, while precursors bound to Hsp70  chaperones are targetted to TOM20-TOM22 receptor complexes.
Most of the mitochondrial...
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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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Related Experiment Video

Updated: Dec 16, 2025

Author Spotlight: Unveiling Mitochondrial Contact Sites and Architectural Insights
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Author Spotlight: Unveiling Mitochondrial Contact Sites and Architectural Insights

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Structural insight into mitochondrial β-barrel outer membrane protein biogenesis.

Kathryn A Diederichs1, Xiaodan Ni2, Sarah E Rollauer1,3,4

  • 1Laboratory of Molecular Biology, National Institute of Diabetes & Digestive & Kidney Diseases, National Institutes of Health, 9000 Rockville Pike, Bethesda, MD, 20892, USA.

Nature Communications
|July 5, 2020
PubMed
Summary

The mitochondrial Sorting and Assembly Machinery (SAM) complex

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Area of Science:

  • Mitochondrial biology
  • Protein import and assembly
  • Structural biology

Background:

  • Mitochondrial outer membrane proteins, including beta-barrel proteins, are crucial for various cellular functions.
  • The Sorting and Assembly Machinery (SAM) complex facilitates the folding and insertion of these proteins.

Purpose of the Study:

  • To determine the cryo-electron microscopy (cryo-EM) structures of the SAM complex from Myceliophthora thermophila.
  • To elucidate the structural organization and mechanism of the SAM complex in beta-barrel protein insertion.

Main Methods:

  • Cryo-electron microscopy (cryo-EM) to obtain high-resolution structures.
  • Structural analysis of the Myceliophthora thermophila SAM complex.

Main Results:

  • The SAM complex comprises Sam50, Sam35, and Sam37 in a 1:1:1 stoichiometry.
  • Sam50 forms a 16-stranded transmembrane beta-barrel with a POTRA domain.
  • Sam35 and Sam37, located on the cytosolic side, interact with each other and Sam50, exhibiting a GST-like fold distinct from bacterial homologs.
  • Structural insights reveal how the Sam50 beta-barrel opens a lateral gate for substrate accommodation.

Conclusions:

  • The study provides high-resolution structures of the fungal SAM complex, revealing unique features of its components.
  • The findings offer a mechanistic understanding of how the SAM complex facilitates beta-barrel protein insertion into the mitochondrial outer membrane.