Snapshot: Import and Sorting of Mitochondrial Proteins

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SnapShot: Import and Sorting of Mitochondrial Proteins Nils Wiedemann, Martin van der Laan, and Nikolaus Pfanner Institut für Biochemie und Molekularbiologie, ZBMZ, University of Freiburg, 79104 Freiburg, Germany

808 Cell 138, August 21, 2009 ©2009 Elsevier Inc. DOI 10.1016/j.cell.2009.08.004 See online version for legend and references. SnapShot: Import and Sorting of Mitochondrial Proteins Nils Wiedemann, Martin van der Laan, and Nikolaus Pfanner Institut für Biochemie und Molekularbiologie, ZBMZ, University of Freiburg, 79104 Freiburg, Germany

Mitochondria are organelles surrounded by two membranes. They contain ~1000 different proteins, of which 99% are encoded by DNA in the nucleus. These mitochondrial proteins are synthesized on cytosolic ribosomes as precursor proteins. They contain specific targeting signals that guide them to one of the four mitochondrial compartments: outer membrane (OM), intermembrane space (IMS), inner membrane (IM), and matrix. Transport of mitochondrial precursor proteins and their assembly into mature protein complexes requires several intricate molecular machines in the four mitochondrial compartments that are dedicated to transporting distinct classes of substrates. (A) The translocase of the outer membrane (TOM complex) is the general entry gate for virtually all nuclear-encoded mitochondrial precursor proteins. The core of the TOM complex consists of a protein-conducting channel, which is formed by Tom40, the central receptor Tom22, and three small proteins, Tom5, Tom6, and Tom7. Associated with the core complex are the primary receptors, Tom20 and Tom70. Mim1 is required for biogenesis of these α-helical outer membrane proteins, which are anchored at their N terminus. (B) The sorting and assembly machinery (SAM complex) of the outer membrane, which consists of Sam35, Sam37, Sam50, and Mdm10, is dedicated to the biogenesis of β-barrel membrane proteins, such as Tom40 and porin. These proteins are transferred from the TOM complex to the SAM complex with the help of small TIM chaperones in the intermembrane space. Sam35 recognizes the sorting signal at the C terminus of β-barrel proteins (β-signal) and transfers these precursors to Sam50, which integrates them into the lipid bilayer. (C) Many proteins in the intermembrane space are small cysteine-rich proteins, like the small Tim proteins Tim9 and Tim10. The proteins are synthesized in the cyto- sol with internal signal sequences and imported by the mitochondrial intermembrane space assembly machinery (MIA). Incoming substrates are bound by the Mia40 receptor in the intermembrane space via disulfide bonds, generating covalent import intermediates. The sulfhydryl oxidase Erv1 forms a disulfide relay with Mia40 and promotes the release and maturation of imported proteins. The zinc-binding protein Hot13 supports the activity of the MIA machinery. (D) Hydrophobic inner membrane proteins with internal targeting signals, like the metabolite carrier proteins, are guided through the intermembrane space by the small TIM chaperones (Tim9-Tim10, Tim8-Tim13). The carrier translocase of the inner membrane (TIM22 complex) consists of Tim18, Tim22, Tim54, and a membrane- bound chaperone complex composed of Tim9, Tim10, and Tim12. The precursor proteins are inserted into the inner membrane by the channel-forming Tim22 protein in a membrane potential (∆ψ)-dependent manner. (E) The presequence translocase of the inner membrane (TIM23 complex) imports precursor proteins with cleavable N-terminal signal sequences. The TIM23 core consists of the pore-forming Tim23 protein, which translocates preproteins in a ∆ψ-dependent manner, and the regulatory proteins Tim50 and Tim17. Tim50, Tim23, and another subunit, Tim21, cooperate with TOM in a TOM-TIM supercomplex that transfers preproteins across both mitochondrial membranes. The TIM23 complex switches between two distinct forms, which are dedicated to translocating preproteins into the matrix and lateral sorting of preproteins into the inner membrane. For inner membrane sorting, the TIM23 core remains associated with Tim21 that transiently connects TIM23 to the respiratory chain (bc1-complex and cytochrome c oxidase). For matrix translocation, Tim21 is released and the TIM23 complex is coupled to the ATP-driven import motor (PAM). PAM consists of the mitochondrial Hsp70 chaperone (Ssc1), the adaptor protein Tim44, the J-domain protein Pam18, the J-like protein Pam16, the regulatory component Pam17, and the nucleotide exchange factor Mge1. The presequences are cleaved off in the matrix by the mitochondrial processing protease (not shown). (F) A small number of hydrophobic respiratory chain proteins are encoded by mitochondrial DNA. They are synthesized on mitochondrial ribosomes and integrated into the inner membrane in a cotranslational manner. Oxa1 is a major translocase for mitochondrially encoded proteins. In addition, Mba1, as well as Mdm38, recruit ribosomes to the inner membrane. Cox18, Pnt1, and Mss2 are required for translocation of the C terminus of the Cox2 protein. Translocase components that are essential for cell viability are in bold. The standard nomenclature of the Saccharomyces Genome Database [SGD] is used. For alias names and additional gene information, see http://www.yeastgenome.org.

Abbreviations COX, cytochrome c oxidase; ∆ψ, membrane potential; Hsp70, heat shock protein 70; IM, inner mitochondrial membrane; IMS, intermembrane space; MIA, mitochondrial intermembrane space assembly machinery; OM, outer mitochondrial membrane; PAM, presequence translocase-associated motor; SAM, sorting and assembly machinery of outer membrane; TIM, translocase of inner membrane; TOM, translocase of outer membrane.

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