Multiple pathways for sorting mitochondrial precursor proteins

Abstract

Mitochondria import hundreds of different precursor proteins from the cytosol. More than 50% of mitochondrial proteins do not use the classical import pathway that is guided by amino-terminal presequences, but instead contain different types of internal targeting signals. Recent studies have revealed an unexpected complexity of the mitochondrial protein import machinery and have led to the discovery of new transport pathways. Here, we review the versatility of mitochondrial protein import and its connection to mitochondrial morphology, redox regulation and energetics.

Figure 1

Two main protein import pathways of mitochondria. Presequences direct proteins through the TOM complex, TIM23 complex and motor PAM to the matrix; the mitochondrial processing peptidase (MPP) removes the presequences. Cleavable inner membrane proteins are laterally released from the TIM23 complex. Carrier precursors with internal targeting signals are recognized by the receptor Tom70, and translocated by the TOM complex and the Tim9–Tim10 chaperone of the intermembrane space. The TIM22 complex promotes insertion of carrier proteins into the inner membrane. MtHsp70, matrix heat shock protein 70; PAM, presequence translocase-associated motor; TIM, translocase of the inner membrane; TOM, translocase of the outer membrane.

Figure 2

Mitochondrial intermembrane space import and assembly machinery. Precursors of small intermembrane space (IMS) proteins are translocated through the TOM complex and bound by Mia40 through disulphide bonds. The sulphydryl oxidase Erv1 cooperates with Mia40 in the oxidation of precursor proteins and their assembly into oligomeric complexes. Further factors such as Hot13 support assembly of the protein complexes. Erv1, Essential for respiration and viability 1; Hot13, Helper of TIM13; Mia40, mitochondrial intermembrane space import and assembly; TIM, translocase of the inner membrane; TOM, translocase of the outer membrane.

Figure 3

Sorting and assembly machinery of the outer mitochondrial membrane. The precursors of β-barrel proteins are initially imported through the TOM complex, interact with small TIM chaperones (Tim9–Tim10 complex, Tim8–Tim13 complex) in the intermembrane space, and are inserted into the outer membrane by the SAM complex. Other outer membrane proteins—the MDM complex and Mim1—support assembly of β-barrel proteins. Mdm, mitochondrial distribution and morphology; Mim1, mitchondrial import 1; Mmm1, maintenance of mitochondrial morphology 1; SAM, sorting and assembly machinery; TIM, translocase of the inner memebrane; TOM, translocase of the outer membrane.

Natalia Bolender

Albert Sickmann

Richard Wagner

Chris Meisinger

Nikolaus Pfanner