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Structure of Porins01:21

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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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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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Mitochondrial precursors are translocated to the internal subcompartments via independent mechanisms involving distinct protein machineries called translocases.
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The mitochondrial cristae membrane is the primary site for the oxidative phosphorylation (OXPHOS) process of energy conversion mediated through respiratory complexes I to V. These complexes have been widely studied for decades, and it has been proven that they form supramolecular structures called respiratory supercomplexes (SC). These higher-order complexes may be crucial in maintaining the biochemical structure and improving the physiological activity of the individual complexes while...
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A single mitochondrion is a bean-shaped organelle enclosed by a double-membrane system. The outer membrane of mitochondria is smooth and contains many porins - the integral membrane transporters. Porins enable free diffusion of ions and small uncharged molecules through the outer mitochondrial membrane but limit the transport of molecules larger than 5000 Daltons. Further, the outer mitochondrial membrane forms a unique structure called membrane contact sites with other subcellular organelles,...
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Related Experiment Video

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Rapid Isolation of the Mitoribosome from HEK Cells
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Parallel Structural Evolution of Mitochondrial Ribosomes and OXPHOS Complexes.

Eli O van der Sluis1, Heike Bauerschmitt2, Thomas Becker3

  • 1Gene Center and Center for integrated Protein Science Munich (CiPSM), Department of Biochemistry, University of Munich, Germany vandersluis@lmb.uni-muenchen.de beckmann@lmb.uni-muenchen.de.

Genome Biology and Evolution
|April 12, 2015
PubMed
Summary

Mitochondrial ribosomes and oxidative phosphorylation complexes evolved through constructive and reductive phases, recruiting nuclear-encoded proteins to compensate for mutations in mitochondrial components. This explains their complex structure and function.

Keywords:
cryo-electron microscopymitochondrial evolutionnonadaptive evolutionribosome

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Area of Science:

  • Mitochondrial biology
  • Evolutionary biology
  • Structural biology

Background:

  • Oxidative phosphorylation (OXPHOS) complexes and mitochondrial ribosomes (mitoribosomes) in eukaryotes have more subunits than their bacterial counterparts, but the evolutionary mechanisms are unclear.
  • Previous hypotheses that mitoribosome protein content compensates for low ribosomal RNA (rRNA) amount have been disproven by structural data.

Purpose of the Study:

  • To investigate the evolutionary mechanisms behind the structural complexity of mitoribosomes and OXPHOS complexes.
  • To elucidate the roles of newly recruited nuclear-encoded proteins in mitochondrial evolution.

Main Methods:

  • Cryo-electron microscopy of the Neurospora crassa 73S mitoribosome.
  • Genomic and proteomic analyses of mitoribosome composition across eukaryotes.

Main Results:

  • Mitoribosomes and OXPHOS complexes evolved via two phases: a constructive phase (early eukaryote evolution) adding ~75 subunits, and a reductive phase (metazoan evolution) shortening rRNA and protein components.
  • These evolutionary phases are explained by accumulation of deleterious mutations in mitochondrial-encoded genes.
  • Recruited nuclear-encoded proteins structurally compensate for mutationally destabilized mitochondrial components.

Conclusions:

  • The recruitment of nuclear-encoded proteins was initially nonadaptive, serving to stabilize mitochondrial components.
  • This compensatory role provided a selective advantage and potentially enabled new mitochondrion-specific functions.
  • This framework explains the structural evolution of mitoribosomes and OXPHOS complexes and associated protein functions.