Volume 24, Issue 3, Pages 107-122 (September 2017) Regulation of mitochondrial structure and function by protein import: A current review  Kanchanjunga Prasai  Pathophysiology  Volume 24, Issue 3, Pages 107-122 (September 2017) DOI: 10.1016/j.pathophys.2017.03.001 Copyright © 2017 Elsevier B.V. Terms and Conditions

Fig. 1 Structure of a mitochondrion. A highly specialized double membrane encloses the mitochondrion. The MOM is enriched with VDAC and components of the protein import machinery (TOM complex). The IMS contains soluble proteins involved in different biological processes. The MIM is comprised of IBM and CM, which are connected via CJ. In addition to forming the structural basis of CJs, MICOS also connects cristae with protein components of the MOM at contact sites. Moreover, physical interaction between the TOM and the TIM complexes also occurs at contact sites. The IBM contains a large number of protein translocases (TIM complex), whereas the CM is enriched in respiratory chain components and ATP synthase. Several copies of mtDNA are localized in the protein-rich mitochondrial matrix. CJ, crista junction; CM, cristae membrane; IBM, inner boundary membrane; IMS, intermembrane space; MICOS, mitochondrial contact site and cristae organizing system; MOM, mitochondrial outer membrane; mtDNA, mitochondrial DNA; TIM, translocase of the inner membrane; TOM, translocase of the outer membrane; VDAC, voltage-dependent anion channel (Adapted from van der Laan et al. [19]). Pathophysiology 2017 24, 107-122DOI: (10.1016/j.pathophys.2017.03.001) Copyright © 2017 Elsevier B.V. Terms and Conditions

Fig. 2 The presequence pathway. Preproteins containing presequences are transferred to the central entry gate of mitochondria, the TOM complex, from where they are directed to the TIM23 complex. After exiting through the Tim23 channel, presequences are cleaved by the MPP in the matrix. Preproteins traversing through the TIM23 complex can take one of several different routes to reach their destined subcompartments. (i) Hydrophilic preproteins are pulled into the matrix with the help of the PAM, whose central motor component, mtHsp70, utilizes ATP to drive the process. (ii) Preproteins carrying a hydrophobic sorting signal following the presequence are released laterally into the lipid phase of the MIM. (iii) The IMP cleaves the hydrophobic sorting signal of some preproteins embedded in the MIM, thereby releasing them into the IMS. (iv) Some precursor proteins with bipartite signals (presequence followed by hydrophobic sorting signal) are first transported towards the matrix, following which they are eventually inserted into the MIM via the OXA machinery. The OXA machinery also catalyzes incorporation of proteins synthesized by mitochondrial ribosomes into the MIM. ΔΨ, mitochondrial membrane potential; IMP, inner membrane peptidase; IMS, intermembrane space; MIM, mitochondrial inner membrane; MOM, mitochondrial outer membrane; MPP, mitochondrial processing peptidase; mtHsp70, matrix heat shock protein 70; OXA, cytochrome oxidase activity machinery; PAM, presequence translocase-associated motor; TIM, translocase of the inner membrane; TOM, translocase of the outer membrane (Adapted from Horvath et al. [42] and Becker et al. [41]). Pathophysiology 2017 24, 107-122DOI: (10.1016/j.pathophys.2017.03.001) Copyright © 2017 Elsevier B.V. Terms and Conditions

Fig. 3 The internal targeting signal pathways for MOM proteins. (i) The precursors of β-barrel proteins are translocated through the TOM complex in the MOM, following which small Tim chaperone complexes in the IMS ferry the precursors to the SAM complex present in the MOM for membrane insertion. (ii) N-terminal signal containing precursors use the MIMC complex for membrane insertion. (iii) C-terminal membrane anchor proteins can either use the MIMC complex or be directly integrated into the lipid phase of the MOM. (iv) Preproteins with transmembrane segments in the middle region use TOM receptors before being relayed to the SAM complex, which then mediates membrane insertion. ΔΨ, mitochondrial membrane potential; IMS, intermembrane space; MIM, mitochondrial inner membrane; MIMC, mitochondrial import complex; MOM, mitochondrial outer membrane; SAM, sorting and assembly machinery; TOM, translocase of the outer membrane (Adapted from Schmidt et al. [7] and Horvath et al. [42]). Pathophysiology 2017 24, 107-122DOI: (10.1016/j.pathophys.2017.03.001) Copyright © 2017 Elsevier B.V. Terms and Conditions

Fig. 4 The internal targeting signal pathway for MIM proteins. Cytosolic chaperones bind and direct carrier precursors to the Tom70 receptor, following which they are translocated through the Tom40 channel. IMS-localized small Tim chaperones bind the incoming precursors and guide them to the TIM22 complex, which drives their membrane insertion in a ΔΨ-dependent mechanism. ΔΨ, mitochondrial membrane potential; IMS, intermembrane space; MIM, mitochondrial inner membrane; MOM, mitochondrial outer membrane; TIM, translocase of the inner membrane; TOM, translocase of the outer membrane (Adapted from Schmidt et al. [7]). Pathophysiology 2017 24, 107-122DOI: (10.1016/j.pathophys.2017.03.001) Copyright © 2017 Elsevier B.V. Terms and Conditions

Fig. 5 The internal targeting signal pathway for IMS proteins. IMS precursors containing characteristic Cys motifs traverse through the TOM complex. Mia40 binds the incoming precursor via a transient disulfide bond, which is subsequently transferred to the precursor. Erv1 then reoxidizes Mia40, and is reduced in the process. Electrons removed during protein oxidation flow from the precursor via Mia40 to Erv1 and finally to the respiratory chain through cytochrome c. ΔΨ, mitochondrial membrane potential; COX, cytochrome c oxidase; Cyt c, cytochrome c; Erv1, essential for respiration and viability; IMS, intermembrane space; MIA, mitochondrial intermembrane space import and assembly machinery; MIM, mitochondrial inner membrane; MOM, mitochondrial outer membrane; TOM, translocase of the outer membrane (Adapted from Becker et al. [41]). Pathophysiology 2017 24, 107-122DOI: (10.1016/j.pathophys.2017.03.001) Copyright © 2017 Elsevier B.V. Terms and Conditions

Fig. 6 The OXPHOS system. The mitochondrial OXPHOS machinery includes proton-pumping complexes (I, III, and IV), which generate electrochemical proton gradient that in turn drives complex V for ATP synthesis. Complex I is the entry point for electrons donated by NADH, whereas complex II receives electrons from succinate oxidation. Membrane-integrated ubiquinone and soluble cytochrome c mediate electron transfer between different complexes. Electrons are finally donated to molecular oxygen to form water in a reaction catalyzed by complex IV. ΔΨ, mitochondrial membrane potential; c, cytochrome c; IMS, intermembrane space; MIM, mitochondrial inner membrane; Q, ubiquinone (Adapted from Schon et al. [156]). Pathophysiology 2017 24, 107-122DOI: (10.1016/j.pathophys.2017.03.001) Copyright © 2017 Elsevier B.V. Terms and Conditions

Fig. 7 Mechanism of the Q-cycle. In the figure, thick arrows represent substrate transformation or movement, whereas thin arrows represent electron transfer. cyt c, cytochrome c; cyt c1, cytochrome c1; Q, ubiquinone; Q, ubisemiquinone; QH2, ubiquinol; QN and QP, ubiquinone and ubiquinol binding sites in cytochrome b located at the negative (matrix) and positive (IMS) sides, respectively (Adapted from Schultz and Chan [134]). Pathophysiology 2017 24, 107-122DOI: (10.1016/j.pathophys.2017.03.001) Copyright © 2017 Elsevier B.V. Terms and Conditions

Fig. 8 Human mitochondrial complex V. Complex V consists of two functional units, F1 and F0. F1 is situated in the matrix and consists of five distinct subunits, whereas F0 is integrated in the MIM and is composed of ten different subunits. F1 subunits γ, δ, and ε form the central stalk, while the peripheral stalk is made up of F0 subunits b, d, F6 and OSCP. In the figure, one α-subunit is omitted to visualize the central stalk. ΔΨ, mitochondrial membrane potential; MIM, mitochondrial inner membrane (Redrawn from Jonckheere et al. [137]). Pathophysiology 2017 24, 107-122DOI: (10.1016/j.pathophys.2017.03.001) Copyright © 2017 Elsevier B.V. Terms and Conditions