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  • Review Article
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Versatility of the mitochondrial protein import machinery

Nature Reviews Molecular Cell Biology volume 2, pages 339–349 (2001)Cite this article

Key Points

  • Mitochondria comprise 15–20% of the proteins of a cell. Only very few mitochondrial proteins are synthesized inside the organelle. Over 98% of mitochondrial proteins are made as preproteins in the cytosol.

  • The typical mitochondrial preproteins contain amino-terminal signal sequences (presequences) that are removed after import into the organelle. A large second class of hydrophobic preproteins do not carry cleavable presequences, but several internal signals.

  • The translocase of the outer mitochondrial membrane (TOM) contains receptors and a general import pore (GIP). Presequence-containing preproteins are recognized by the receptors Tom20 and Tom22, whereas preproteins with internal signals are preferentially recognized by Tom70. All preproteins are imported by the same GIP formed by the channel protein Tom40.

  • The mitochondrial inner membrane contains two distinct translocases. Presequence-containing preproteins are directed into the TIM23 complex in a membrane potential-dependent manner. Matrix Hsp70 interacts with the TIM23 complex through Tim44 and functions as ATP-dependent motor to drive import of the preproteins into the matrix.

  • The second pathway into the inner membrane is used by hydrophobic preproteins with several internal signals. They are guided through the intermembrane space by tiny Tim proteins (Tim9–Tim10) and dock to the membrane-integrated TIM22 complex. In a membrane potential-dependent manner, the preproteins are inserted into the TIM22 complex and are probably laterally released into the inner membrane.

  • Several variations in these pathways exist for special preproteins. Preproteins carrying additional sorting signals can diverge from the main pathways at distinct stages and are sorted to mitochondrial subcompartments.

Abstract

The vast majority of mitochondrial proteins are synthesized in the cytosol and are imported into mitochondria by protein machineries located in the mitochondrial membranes. It has become clear that hydrophilic as well as hydrophobic preproteins use a common translocase in the outer mitochondrial membrane, but diverge to two distinct translocases in the inner membrane. The translocases are dynamic, high-molecular-weight complexes that have to provide specific means for the recognition of preproteins, channel formation and generation of import-driving forces.

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Figure 1: The two main pathways of protein import into mitochondria.
Figure 2: The translocase of the inner mitochondrial membrane for presequence-carrying preproteins.
Figure 3: Import of a hydrophobic carrier protein into the inner mitochondrial membrane.
Figure 4: Variations on mitochondrial import pathways.

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

    Nikolaus Pfanner & Andreas Geissler

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DATABASE LINKS

porins

mtHsp70

MPP

Hsp60

Tom20

Tom22

Hsp10

Hmi1

cytochrome c1

cytochrome b2

Imp1

Imp2

Tom70

Tom40

Tom5

Tom6

Tom7

Tim23

Tim17

Tim44

Mge1

MSF

Tim9

Tim10

Tim8

Tim12

Tim13

Tim22

Tim54

Tim18

NADH-cytochrome b5 reductase

fumarase

deafness dystonia syndrome

Oxa1

Pnt1

FURTHER INFORMATION

Pfanner lab

ENCYCLOPEDIA OF LIFE SCIENCES

Protein translocation across membranes

Mitochondria: structure and role in respiration

Glossary

TRANSLOCASE

A system catalysing the transfer of a substrate across a barrier.

MOLECULAR CHAPERONES

Proteins that assist the correct folding, assembly or disassembly of other proteins in vivo, but are not components of the final functional structures.

TOM

(Translocase of the outer mitochondrial membrane). A protein complex that recognizes nuclear-encoded mitochondrial preproteins and mediates their translocation across the membrane.

GIP

(General import pore). Pore of the mitochondrial outer membrane translocase that translocates virtually all nuclear-encoded mitochondrial proteins.

TIM

(Translocases of the inner mitochondrial membrane). Protein complexes in the inner membrane and intermembrane space of mitochondria that mediate transport of preproteins into and across the membrane.

HEAT-SHOCK PROTEINS

Heat-shock proteins are synthesized in larger amounts when cells have been exposed to a temperature that is higher than normal. Many heat-shock proteins function as molecular chaperones and are crucial for cellular functions also under non-stress conditions.

CHAPERONINS

A subclass of molecular chaperones, including the chaperonin 60 family and the chaperonin 10 family.

GIP COMPLEX

Stable core complex of the preprotein translocase of the outer mitochondrial membrane, consisting of the channel and several associated proteins.

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Pfanner, N., Geissler, A. Versatility of the mitochondrial protein import machinery. Nat Rev Mol Cell Biol 2, 339–349 (2001). https://doi.org/10.1038/35073006

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