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Related Concept Videos

Translocation of Proteins into the Mitochondria01:19

Translocation of Proteins into the Mitochondria

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,...
Mitochondrial Precursor Proteins01:39

Mitochondrial Precursor Proteins

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 precursors...
Porin Insertion in the Outer Mitochondrial Membrane01:12

Porin Insertion in the Outer Mitochondrial Membrane

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.
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Animal Mitochondrial Genetics02:59

Animal Mitochondrial Genetics

Among all the organelles in an animal cell, only mitochondria have their own independent genomes. Animal mitochondrial DNA is a double-stranded, closed-circular molecule with around 20,000 base pairs. Mitochondrial DNA is unique in that one of its two strands, the heavy, or H, -strand is guanine rich, whereas the complementary strand is cytosine rich and called the light, or L, -strand. Compared to nuclear DNA, mitochondrial DNA has a very low percentage of non-coding regions and is marked by...
Mitochondria01:37

Mitochondria

Mitochondria are eukaryotic cellular organelles that are known to produce energy through a process called oxidative phosphorylation. Besides their primary function, mitochondria are involved in various cellular processes, including cell growth, differentiation, signaling, metabolism, and senescence. Age-related changes cause a decline in mitochondrial quality and integrity due to increased mitochondrial mutations and oxidative damage. Thus, aging can severely impact mitochondrial functions,...
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Mitochondrial Protein Sorting

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.
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Understanding the Changes in Mitochondrial Morphology through Dynamic and Three-dimensional Fluorescence Micrographs
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Terminating human mitochondrial protein synthesis: a shift in our thinking.

Robert N Lightowlers1, Zofia M A Chrzanowska-Lightowlers

  • 1Institute for Ageing and Health, Newcastle University, Newcastle upon Tyne, UK.

RNA Biology
|May 12, 2010
PubMed
Summary

Human mitochondria translation termination is not unusual. AGA/AGG codons trigger -1 frameshifting, allowing standard stop codons and a single release factor to terminate all mitochondrial gene products.

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Area of Science:

  • Molecular Biology
  • Genetics
  • Biochemistry

Background:

  • Human mitochondria were thought to use four stop codons (UAA, UAG, AGA, AGG) for translation termination.
  • The reassignment of arginine codons (AGA/AGG) as stop codons presented a puzzle regarding termination factor recognition.

Purpose of the Study:

  • To investigate the mechanism of translation termination for AGA/AGG codons in human mitochondria.
  • To determine the factors involved in recognizing these non-universal stop codons.

Main Methods:

  • Analysis of mitochondrial translation termination mechanisms.
  • Investigation of ribosomal frameshifting events.
  • Assessment of mitochondrial release factor (mtRF1a) sufficiency.

Main Results:

  • Human mitochondria do not utilize AGA/AGG as direct stop codons.
  • Absence of a cognate release factor or tRNA for AGA/AGG triggers -1 ribosomal frameshifting.
  • This frameshift repositions a standard UAG stop codon in the ribosomal A-site.
  • The universal stop codons UAA and UAG are sufficient for human mitochondrial translation termination.
  • A single mitochondrial release factor, mtRF1a, is sufficient to terminate all 13 encoded mitochondrial genes.

Conclusions:

  • Human mitochondrial translation termination relies on a conserved mechanism involving ribosomal frameshifting, not novel stop codons.
  • The mitochondrial genetic code employs a simplified termination system utilizing only UAA and UAG, recognized by mtRF1a.
  • This finding resolves the previous conundrum of AGA/AGG codon usage in human mitochondrial translation.