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Mitochondrial protein import: from proteomics to functional mechanisms

Key Points

  • Proteomic studies revealed that mitochondria contain 1,000 (in yeast) to 1,500 (in humans) different proteins and led to the recent identification of new mitochondrial import pathways and components.

  • 99% of mitochondrial proteins are encoded by nuclear genes, synthesized on cytosolic ribosomes and imported into mitochondria. 1% of mitochondrial proteins are encoded by the mitochondrial genome and synthesized in the mitochondrial matrix.

  • The translocase of the outer membrane (TOM) complex is the main entry gate used by most nucleus-encoded mitochondrial precursor proteins. Presequence-carrying preproteins are then imported by the presequence translocase of the inner membrane (TIM23) complex and the presequences are proteolytically removed by specific processing enzymes.

  • The precursors of hydrophobic metabolite carriers of the inner membrane are imported by the TOM complex, bind to chaperone-like proteins in the intermembrane space (small TIM proteins) and are inserted by the carrier translocase of the inner membrane (TIM22) complex.

  • Many proteins of the mitochondrial intermembrane space are small and contain characteristic Cys motifs. Most of these proteins are imported and folded by the redox-dependent mitochondrial intermembrane space assembly (MIA) machinery.

  • The mitochondrial outer membrane contains α-helical proteins and β-barrel proteins. Whereas the import pathways of α-helical proteins are only partly understood, the pathway for β-barrel proteins has been characterized and shown to require the TOM complex, small TIM chaperones of the intermembrane space and the sorting and assembly machinery (SAM) complex of the outer membrane.

Abstract

Mitochondria contain 1,000 different proteins, most of which are imported from the cytosol. Two import pathways that direct proteins into the mitochondrial inner membrane and matrix have been known for many years. The identification of numerous new transport components in recent proteomic studies has led to novel mechanistic insight into these pathways and the discovery of new import pathways into the outer membrane and intermembrane space. Protein translocases do not function as independent units but are integrated into dynamic networks and are connected to machineries that function in bioenergetics, mitochondrial morphology and coupling to the endoplasmic reticulum.

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Figure 1: Biogenesis pathways of mitochondrial proteins.
Figure 2: Classical routes of protein import: the presequence pathway and the carrier pathway.
Figure 3: Oxidative import and folding in the intermembrane space.
Figure 4: Protein sorting to the outer mitochondrial membrane and connection to the ER.

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Acknowledgements

The authors' research is supported by the Deutsche Forschungsgemeinschaft, Excellence Initiative of the German Federal and State Governments (EXC 294 BIOSS; GSC-4 Spemann Graduate School), Trinationales Graduiertenkolleg GRK 1478, Bundesministerium für Bildung und Forschung, Sonderforschungsbereich 746, Gottfried Wilhelm Leibniz Program, Landesforschungspreis Baden-Württemberg and Fonds der Chemischen Industrie.

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SGD

Glossary

α-proteobacterium

A Gram-negative (outer membrane-containing) bacterium, such as Rickettsia spp., that is probably the closest living bacterial relative of mitochondria.

70 kDa heat shock protein

An ATP-dependent molecular chaperone that is essential in unstressed and stressed cells. The chaperones bind hydrophobic segments of unfolded proteins, preventing protein aggregation and promoting protein transport and folding.

Deafness dystonia syndrome

An X chromosome-linked neurodegenerative disease that includes deafness, cortical blindness and dystonia. It was the first human disease caused by a defect in the mitochondrial protein import machinery (specifically, in Tim8 (known as TIMM8A in humans)). It is also called Mohr–Tranebjaerg syndrome.

Porin

A pore-forming protein of the mitochondrial outer membrane that is permeable for many metabolites. It is the most abundant outer membrane protein and is also called voltage-dependent anion channel (VDAC).

Mitofusin

A mitochondrial outer membrane protein required for fusion of mitochondria and maintenance of mitochondrial morphology.

Cardiolipin

A large, dimeric phospholipid that is a characteristic of mitochondria and consists of two phosphatidyl moieties linked by glycerol.

Liposome

An artificial lipid vesicle that is typically formed by a phospholipid bilayer (membrane).

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Schmidt, O., Pfanner, N. & Meisinger, C. Mitochondrial protein import: from proteomics to functional mechanisms. Nat Rev Mol Cell Biol 11, 655–667 (2010). https://doi.org/10.1038/nrm2959

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