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Mitochondrial import and the twin-pore translocase

Nature Reviews Molecular Cell Biology volume 5, pages 519–530 (2004)Cite this article

Key Points

  • The membrane-embedded multiprotein complexes of mitochondria mediate the transport of nuclear-encoded proteins across and into the outer or inner mitochondrial membranes.

  • The TOM (translocase of the outer mitochondrial membrane) complex consists of cytosol-exposed receptors and a pore-forming core, and it mediates the transport of proteins from the cytosol across and into the outer mitochondrial membrane. A novel protein complex in the outer membrane of mitochondria, called the SAM complex (sorting and assembly machinery), is involved in the biogenesis of β-barrel proteins of the outer membrane.

  • Two translocases of the inner mitochondrial membrane (TIM complexes) mediate protein transport at the inner membrane. The TIM23 complex (a presequence translocase) mediates the transport of presequence-containing proteins across and into the inner membrane. The TIM22 complex (a twin-pore carrier translocase) catalyses the insertion of multispanning proteins that have internal targeting signals into the inner membrane.

  • The TIM23 complex requires the PAM complex (presequence-translocase-associated motor complex) and a membrane potential

    (Δψ) for the complete transport of proteins into the mitochondrial matrix. The insertion of presequence-containing inner-membrane proteins can be mediated by the TIM23 complex alone or also requires PAM in cases in which the protein enters the matrix space first and is exported into the inner membrane.

  • The TIM22 complex mediates the membrane insertion of multispanning inner-membrane proteins that have internal targeting signals, and it uses a Δψ as an external driving force. Membrane insertion by the TIM22 complex is a multistep process, in which the preprotein initially tethers to the translocase in a step that is independent of Δψ. Subsequently, in steps that require a Δψ, positive charges in the matrix-exposed loops of the precursor protein allow docking in the twin-pore translocase and, eventually, the precursor inserts into the inner membrane.

Abstract

The mitochondrial inner membrane is rich in multispanning integral membrane proteins, most of which mediate the vital transport of molecules between the matrix and the intermembrane space. The correct transport and membrane insertion of such proteins is essential for maintaining the correct exchange of molecules between mitochondria and the rest of the cell. Mitochondria contain several specific complexes — known as translocases — that translocate precursor proteins. Recent analysis of the inner-membrane, twin-pore protein translocase (TIM22 complex) allows a glimpse of the molecular mechanisms by which this machinery triggers protein insertion using the membrane potential as an external driving force.

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Figure 1: Protein-import pathways for mitochondrial proteins.
Figure 2: A hypothetical model for membrane insertion by the twin-pore translocase.
Figure 3: The transport of multispanning, inner-membrane proteins that have a presequence.

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Acknowledgements

We are grateful to A. E. Frazier for critical comments on the manuscript. Work from the authors' laboratory is supported by the Deutsche Forschungsgemeinschaft, the Sonderforschungsbereich 388 Freiburg, Max Planck Research Award, Alexander von Humboldt Foundation, Bundesministerium für Bildung und Forschung and the Fonds der Chemischen Industrie.

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Authors and Affiliations

  1. Institut für Biochemie und Molekularbiologie, Universität Freiburg, Hermann-Herder-Straße 7, Freiburg, D-79104, Germany

    Peter Rehling, Katrin Brandner & Nikolaus Pfanner

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  1. Peter Rehling
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  2. Katrin Brandner
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  3. Nikolaus Pfanner
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Correspondence to Peter Rehling.

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DATABASES

Saccharomyces genome database

mtHsp70

Pam16

Pam18

Sam35

Sam50

Tim8

Tim9

Tim10

Tim12

Tim13

Tim17

Tim18

Tim22

Tim23

Tim50

Tim54

Tom20

Tom22

Tom40

Tom70

Glossary

MEMBRANE POTENTIAL

The mitochondrial respiratory chain generates a proton gradient across the inner mitochondrial membrane that leads to the formation of an electrochemical potential. This potential is comprised of two components — a chemical potential (ΔpH) and an electrical membrane potential (Δψ). Δψ is the part of the electrochemical potential that functions as a driving force in protein translocation.

HSP90 CHAPERONES

Homodimer-forming chaperones that are composed of subunits that have a molecular mass of ∼90 kDa. Each subunit has an ATPase domain. Hsp90 proteins recognize specific substrate proteins and stabilize intermediate folding states of the protein.

HSP70 CHAPERONES

Molecular chaperones of about 70 kDa that are composed of a substrate-binding domain and an ATPase domain. Hsp70 molecules interact with hydrophobic segments in unfolded proteins in an ATP-dependent manner and assist in protein folding through consecutive rounds of substrate binding and release.

ZINC-FINGER MOTIF

A motif in proteins that contains conserved cysteine residues. The sulphydryl groups of the cysteines coordinate a Zn2+ ion.

DNAJ-LIKE PROTEINS

Proteins that show similarity to a portion of the Escherichia coli DnaJ protein (the so-called J-domain) and that activate the ATPase activity of Hsp70 chaperones. The tripeptide His-Pro-Asp (HPD motif) is crucial for the ATPase-stimulating activity of the J-domain.

IONOPHORES

Molecules that bind ions and that allow their passage across a membrane barrier by surrounding the charges during the passage of the ion across the lipophilic phase. Ionophores are usually specific for a defined set of ions — for example, protonophores are ionophores that are specific for H+.

ELECTROPHORETIC FORCE

Movement of a protein across the inner mitochondrial membrane can be driven by the electric field that is generated by the membrane potential (Δψ). Positive charges in the protein move towards the negatively charged side of the membrane due to the driving force of the field.

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Rehling, P., Brandner, K. & Pfanner, N. Mitochondrial import and the twin-pore translocase. Nat Rev Mol Cell Biol 5, 519–530 (2004). https://doi.org/10.1038/nrm1426

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