The mitochondrial intermembrane space: the most constricted mitochondrial sub-compartment with the largest variety of protein import pathways

Abstract

The mitochondrial intermembrane space (IMS) is the most constricted sub-mitochondrial compartment, housing only about 5% of the mitochondrial proteome, and yet is endowed with the largest variability of protein import mechanisms. In this review, we summarize our current knowledge of the major IMS import pathway based on the oxidative protein folding pathway and discuss the stunning variability of other IMS protein import pathways. As IMS-localized proteins only have to cross the outer mitochondrial membrane, they do not require energy sources like ATP hydrolysis in the mitochondrial matrix or the inner membrane electrochemical potential which are critical for import into the matrix or insertion into the inner membrane. We also explore several atypical IMS import pathways that are still not very well understood and are guided by poorly defined or completely unknown targeting peptides. Importantly, many of the IMS proteins are linked to several human diseases, and it is therefore crucial to understand how they reach their normal site of function in the IMS. In the final part of this review, we discuss current understanding of how such IMS protein underpin a large spectrum of human disorders.

Diverse Functions of Tim50, a Component of the Mitochondrial Inner Membrane Protein Translocase

“…There are parallel studies from plants [16] and most recent advances are made from other eukaryotic supergroups including excavatae that includes many pathogenic protists [17][18][19][20]. Previous studies from fungi, mammals, and plants revealed that at least eight protein complexes are required to traffic proteins to different locations in mitochondria (Figure 1); (1) TOM complex imports most of the mitochondrial proteins [21], (2) the sorting and assembly (SAM) complex in the MOM imports and assembles MOM β-barrel proteins [22], (3) MOM protein import (MIM) complex imports other MOM proteins with α-helical TMDs [23], (4) mitochondrial IMS assembly (MIA) complex is required for import and folding of several IMS-proteins [12], (5) TIM23 and (6) TIM22 are two major translocases of the MIM that imports proteins to the matrix, MIM and IMS [24,25], (7) presequence translocase associated motor (PAM) complex mediates translocation of preproteins to the matrix at expense of ATP hydrolysis [26], and (8) cytochrome oxidase assembly (OXA) complex inserts/imports some MIM proteins [27] (Table 1). Most of these translocases are multi-protein complexes (Table 1).…”

“…Most of the mitochondrial proteins are in the MIM and the matrix thus require crossing both membranes [8]. The MOM and IMS also possess several important proteins [11,12]. Proteins are translocated through TOM and TIMs either as fully or partially unfolded conditions [8][9][10].…”

Interplay between Mitochondrial Protein Import and Respiratory Complexes Assembly in Neuronal Health and Degeneration

“…Proteins destined for the IMS take the route of the MIA pathway ( Figure 1 and Table 1 ), which has been reviewed in great detail previously [ 54 ]. This class of proteins lack an MTS, are generally small, and share a conserved coiled coil-helix1-coiled coil-helix 2 domain (CHCHD).…”

“…This class of proteins lack an MTS, are generally small, and share a conserved coiled coil-helix1-coiled coil-helix 2 domain (CHCHD). These cysteine-rich proteins contain two pairs of cysteines separated by three or nine amino acid residues (Cx 3 C or Cx 9 C) in the helices [ 54 ]. The small TIM chaperones of the IMS, important for translocase of the inner mitochondrial membrane 22 (TIM22)-dependent translocation described below, and assembly factors of IMM proteins, such as the respiratory complexes (see below and Table S1 ), are some examples of MIA substrates.…”

Targeting and Insertion of Membrane Proteins in Mitochondria

“…Distinct conformations of the TIM23 complex have been identified that aid either the translocation of large hydrophilic domains across the inner membrane to the matrix, or the lateral diffusion of transmembrane α-helices into the lipid bilayer (Schendzielorz et al, 2018;Edwards et al, 2021). Key to these differing conformations are the TIM23 accessory proteins Pam18 and Mgr2.…”