Opinion
Mitochondrial protein import: from transport pathways to an integrated network

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Mitochondria, the powerhouses of the cell, import most of their proteins from the cytosol. It was originally assumed that mitochondria imported precursor proteins via a general pathway but recent studies have revealed a remarkable variety of import pathways and mechanisms. Currently, five different protein import pathways can be distinguished. However, the import machineries cooperate with each other and are connected to other systems that function in the respiratory chain, mitochondrial membrane organization, protein quality control and endoplasmic reticulum-mitochondria junctions. In this Opinion, we propose that mitochondrial protein import should not be seen as an independent task of the organelle and that a network of cooperating machineries is responsible for major mitochondrial functions.

Section snippets

Two classical import routes for mitochondrial proteins

Mitochondria are the powerhouses of eukaryotic cells and play important roles in cellular metabolism and regulation. They consist of two membranes, an intermembrane space and an inner compartment, the matrix. Although mitochondria contain a complete genetic and protein synthesis system in the matrix, derived from their prokaryotic ancestor, ∼99% of mitochondrial proteins are encoded in the nucleus of the cell and are synthesized as precursors in the cytosol 1, 2, 3, 4. The precursor proteins

Identification of new import components and pathways

In 2002, the new subunit Tim50 of the presequence translocase (TIM23 complex) was identified. Tim50 is essential for the presequence pathway and for viability of cells, which indicates that it is a major player in mitochondrial biogenesis 7, 8. Later, numerous new mitochondrial import components were identified. For example, important regulatory subunits that function in the TIM23 complex, the motor PAM or in preprotein processing were identified for the presequence pathway (Figure 1) 9, 11, 14

Two pathways for protein insertion into the mitochondrial outer membrane

The mitochondrial outer membrane contains two classes of proteins: β-barrel proteins, which are integrated into the membrane by multiple β-strands and are derived from the bacterial ancestor of mitochondria, and proteins with α-helical transmembrane segments, which probably derive from the eukaryotic host cell. Although the TOM complex is the main entry gate for the large majority of mitochondrial precursor proteins, further machineries are needed to insert proteins into the outer membrane.

In

Redox-regulated protein import into the intermembrane space

A few intermembrane space proteins are imported via the presequence pathway. The preproteins are laterally released from the TIM23 complex, and the mature proteins are cleaved off by an intermembrane space-exposed peptidase (Figure 1). However, the majority of intermembrane space proteins use the MIA pathway, during which the Mia40 protein interacts with the precursor proteins via disulfide bonds 21, 23. Many intermembrane space proteins contain characteristic cysteine motifs, and the mature

Integration of protein import machineries into a network of organellar and cellular functions

In addition to discovering new protein import routes into mitochondria, recent studies suggested unexpected connections of the preprotein translocases to various other mitochondrial systems. In this section, we summarize these observations and then propose a hypothesis on the organization of the protein import system.

Regarding the presequence pathway, the TIM23 complex is physically connected to the mitochondrial respiratory chain (Figure 1) 54, 55, 56. Tim21 binds to a supercomplex formed by

Concluding remarks

The analysis of mitochondrial protein import has provided exciting new insights into a highly complex system of organellar biogenesis and function. However, many more unexpected findings can be anticipated, as many open questions remain (Box 2). We speculate that further import pathways may exist; for example, some small proteins of the inner membrane are imported without a clear requirement for a membrane potential Δψ [78], whereas a Δψ is strictly needed for the two known translocases of the

Acknowledgments

This work has been supported by the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 746, Excellence Initiative of the German Federal & State Governments (EXC 294), Gottfried Wilhelm Leibniz Program, Landesforschungspreis Baden-Württemberg, and Bundesministerium für Bildung und Forschung.

Glossary

ERMES
The ER-mitochondria encounter structure (ERMES) tethers the endoplasmic reticulum (ER) to the outer membrane of mitochondria. This complex consists of the ER-localized protein Mmm1 and the mitochondria-bound proteins Mdm10, Mdm12, Mdm34 and Gem1.
MIA
The mitochondrial intermembrane space import and assembly (MIA) machinery mediates oxidative protein transport and folding. Mia40 cooperates with the sulfhydryl oxidase Erv1 in a disulfide relay to drive the import of cysteine-rich proteins into

References (78)

  • D. Mokranjac

    Role of Tim21 in mitochondrial translocation contact sites

    J. Biol. Chem.

    (2005)
  • M. Naoé

    Identification of Tim40 that mediates protein sorting to the mitochondrial intermembrane space

    J. Biol. Chem.

    (2004)
  • N. Mesecke

    A disulfide relay system in the intermembrane space of mitochondria that mediates protein import

    Cell

    (2005)
  • J.M. Hulett

    The transmembrane segment of Tom20 is recognized by Mim1 for docking to the mitochondrial TOM complex

    J. Mol. Biol.

    (2008)
  • H. Schägger et al.

    Blue native electrophoresis for isolation of membrane protein complexes in enzymatically active form

    Anal. Biochem.

    (1991)
  • D.J. Pagliarini

    A mitochondrial protein compendium elucidates complex I disease biology

    Cell

    (2008)
  • S. Kutik

    Dissecting membrane insertion of mitochondrial β-barrel proteins

    Cell

    (2008)
  • V. Kozjak

    An essential role of Sam50 in the protein sorting and assembly machinery of the mitochondrial outer membrane

    J. Biol. Chem.

    (2003)
  • C. Meisinger

    The mitochondrial morphology protein Mdm10 functions in assembly of the preprotein translocase of the outer membrane

    Dev. Cell

    (2004)
  • T. Becker

    Biogenesis of mitochondria: dual role Tom7 in modulating assembly of the preprotein translocase of the outer membrane

    J. Mol. Biol.

    (2011)
  • M. Bien

    Mitochondrial disulfide bond formation is driven by intersubunit electron transfer in Erv1 and proofread by glutathione

    Mol. Cell

    (2010)
  • M. van der Laan

    A role for Tim21 in membrane-potential-dependent preprotein sorting in mitochondria

    Curr. Biol.

    (2006)
  • T. Tatsuta et al.

    AAA proteases in mitochondria: diverse functions of membrane-bound proteolytic machines

    Res. Microbiol.

    (2009)
  • K. von der Malsburg

    Dual role of mitofilin in mitochondrial membrane organization and protein biogenesis

    Dev. Cell

    (2011)
  • J. Xie

    The mitochondrial inner membrane protein mitofilin exists as a complex with SAM50, metaxins 1 and 2, coiled-coil-helix coiled-coil-helix domain-containing protein 3 and 6 and DnaJC11

    FEBS Lett.

    (2007)
  • M. Darshi

    ChChd3, an inner mitochondrial membrane protein, is essential for maintaining crista integrity and mitochondrial function

    J. Biol. Chem.

    (2011)
  • R.E. Jensen

    Control of mitochondrial shape

    Curr. Opin. Cell Biol.

    (2005)
  • D.A. Stroud

    Composition and topology of the endoplasmic reticulum-mitochondria encounter structure

    J. Mol. Biol.

    (2011)
  • O. Schmidt

    Regulation of mitochondrial protein import by cytosolic kinases

    Cell

    (2011)
  • K. Wagner

    Mitochondrial F1Fo-ATP synthase: the small subunits e and g associate with monomeric complexes to trigger dimerization

    J. Mol. Biol.

    (2009)
  • P. Dolezal

    Evolution of the molecular machines for protein import into mitochondria

    Science

    (2006)
  • W. Neupert et al.

    Translocation of proteins into mitochondria

    Annu. Rev. Biochem.

    (2007)
  • P. Rehling

    Protein insertion into the mitochondrial inner membrane by a twin-pore translocase

    Science

    (2003)
  • A.E. Frazier

    Pam16 has an essential role in the mitochondrial protein import motor

    Nat. Struct. Mol. Biol.

    (2004)
  • C. Kozany

    The J domain-related cochaperone Tim16 is a constituent of the mitochondrial TIM23 preprotein translocase

    Nat. Struct. Mol. Biol.

    (2004)
  • M. van der Laan

    Pam17 is required for architecture and translocation activity of the mitochondrial protein import motor

    Mol. Cell. Biol.

    (2005)
  • N. Wiedemann

    Machinery for protein sorting and assembly in the mitochondrial outer membrane

    Nature

    (2003)
  • S.A. Paschen

    Evolutionary conservation of biogenesis of β-barrel membrane proteins

    Nature

    (2003)
  • I. Gentle

    The Omp85 family of proteins is essential for outer membrane biogenesis in mitochondria and bacteria

    J. Cell Biol.

    (2004)
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