Reader TTS

KEY POINTS * The vast majority of mitochondrial and chloroplast proteins are cytosolically synthesized and have to be translocated into the organelle. * Precursor proteins contain amino

acid-based signals. These signals supply information allowing the proteins to target, and interact with, the cytosolic chaperones that provide guidance to organelles. * Translocases at the

outer membrane of mitochondria and chloroplasts form the general entry gate into both organelles. * The translocases of both organelles consist of three receptors, which bind to the

multitude of different precursor proteins and deliver them to a translocation pore formed by a protein with β-barrel structure. * Despite similarities with respect to their composition, the

translocons differ with respect to signal length requirement and their energizing of the translocation event. * The mode of translocation is also distinct between the translocation

machineries: mitochondrial import across the outer membrane is affinity-driven, whereas the passage of precursor proteins into chloroplasts is modulated by GTP binding and hydrolysis, and by

phosphorylation events. ABSTRACT Mitochondria and chloroplasts import the vast majority of their proteins across two membranes, and use translocases of the outer membrane as an entry gate.

These translocases interact with the incoming precursor protein and guiding chaperone factors. Within the translocon, precursor-protein receptors dock to a central component that mediates

both transfer through a cation-selective channel and initial sorting towards internal subcompartments. Despite these similarities, the mode of translocation differs between the two

organelles: in chloroplasts, GTP-binding and hydrolysis by the receptors is required for transport, whereas in mitochondria passage of the preprotein is driven by its increasing affinity for

the translocase subunits. Access through your institution Buy or subscribe This is a preview of subscription content, access via your institution ACCESS OPTIONS Access through your

institution Subscribe to this journal Receive 12 print issues and online access $209.00 per year only $17.42 per issue Learn more Buy this article * Purchase on SpringerLink * Instant access

to full article PDF Buy now Prices may be subject to local taxes which are calculated during checkout ADDITIONAL ACCESS OPTIONS: * Log in * Learn about institutional subscriptions * Read

our FAQs * Contact customer support SIMILAR CONTENT BEING VIEWED BY OTHERS ARCHITECTURE OF CHLOROPLAST TOC–TIC TRANSLOCON SUPERCOMPLEX Article 26 January 2023 PEROXISOME BIOGENESIS INITIATED

BY PROTEIN PHASE SEPARATION Article 10 May 2023 IMPORT MECHANISM OF PEROXISOMAL PROTEINS WITH AN N-TERMINAL SIGNAL SEQUENCE Article Open access 09 May 2025 REFERENCES * Gray, M. W., Burger,

G. & Lang, B. F. Mitochondrial evolution. _Science_ 283, 1476–1481 (1999). CAS  PubMed  Google Scholar  * McFadden, G. I. Endosymbiosis and evolution of the plant cell. _Curr. Opin.

Plant Biol._ 2, 513–519 (1999). CAS  PubMed  Google Scholar  * Saraste, M. Oxidative phosphorylation at the _fin de siècle_. _Science_ 283, 1488–1493 (1999). CAS  PubMed  Google Scholar  *

Lill, R. & Mühlenhoff, U. Maturation of iron-sulfur proteins in eukaryotes: mechanisms, connected processes, and diseases. _Annu. Rev. Biochem._ 77, 669–700 (2008). CAS  PubMed  Google

Scholar  * López-Juez, E. Plastid biogenesis, between light and shadows. _J. Exp. Bot._ 58, 11–26 (2007). PubMed  Google Scholar  * Nelson, N. & Ben-Shem, A. The complex architecture of

oxygenic photosynthesis. _Nature Rev. Mol. Cell. Biol._ 5, 971–982 (2004). CAS  Google Scholar  * Sickmann, A. et al. The proteome of _Sacchoromyces cerevisiae_ mitochondria. _Proc. Natl

Acad. Sci. USA_ 103, 13207–13212 (2003). Google Scholar  * van Wijk, K. J. Plastid proteomics. _Plant Physiol. Biochem._ 42, 963–977 (2004). CAS  PubMed  Google Scholar  * Wickner, W. &

Schekman, R. Protein translocation across biological membranes. _Science_ 310, 1452–1456 (2005). CAS  PubMed  Google Scholar  * Grudnik, P., Bange, G. & Sinning, I. Protein targeting by

the signal recognition particle. _Biol. Chem._ 390, 775–782 (2009). CAS  PubMed  Google Scholar  * Schnell, D. J. & Hebert, D. N. Protein translocons: multifunctional mediators of

protein translocation across membranes. _Cell_ 112, 491–505 (2003). CAS  PubMed  Google Scholar  * Stewart, M. Molecular mechanism of the nuclear protein import cycle. _Nature Rev. Mol. Cell

Biol._ 8, 195–208 (2007). CAS  Google Scholar  * Carrie, C., Giraud, E. & Whelan, J. Protein transport in organelles: dual targeting of proteins to mitochondria and chloroplasts. _FEBS

J._ 276, 1187–1195 (2009). CAS  PubMed  Google Scholar  * Chacinska, A., Koehler, C. M., Milenkovic, D., Lithgow, T. & Pfanner, N. Importing mitochondrial proteins: machineries and

mechanisms. _Cell_ 138, 628–644 (2009). CAS  PubMed  PubMed Central  Google Scholar  * Neupert, W. & Herrmann, J. M. Translocation of proteins into mitochondria. _Annu. Rev. Biochem._

76, 723–749 (2007). CAS  PubMed  Google Scholar  * Bruce, B. D. The paradox of plastid transit peptides: conservation of function despite divergence in primary structure. _Biochim. Biophys.

Acta_ 1541, 2–21 (2001). CAS  PubMed  Google Scholar  * Li, H.-M. & Chiu, C.-C. Protein transport into chloroplasts. _Annu. Rev. Plant Biol._ 61, 157–180 (2010). CAS  PubMed  Google

Scholar  * Gakh, O., Cavadini, P. & Isaya, G. Mitochondrial processing peptidases. _Biochim. Biophys. Acta_ 1592, 63–77 (2002). CAS  PubMed  Google Scholar  * Richter, S. & Lamppa,

G. K. Structural properties of the chloroplast stromal processing peptidase required for its function in transit peptide removal. _J. Biol. Chem._ 278, 39497–39502 (2003). CAS  PubMed 

Google Scholar  * Huang, S., Taylor, N. L., Whelan, J. & Millar, A. H. Refining the definition of plant mitochondrial presequences through analysis of sorting signals, N-terminal

modifications, and cleavage motifs. _Plant Physiol._ 150, 1272–1285 (2009). CAS  PubMed  PubMed Central  Google Scholar  * Vögtle, F. N. et al. Global analysis of the mitochondrial

N-proteome identifies a processing peptidase critical for protein stability. _Cell_ 139, 428–439 (2009). PubMed  Google Scholar  * Martin, T. et al. A protein kinase family in _Arabidopsis_

phosphorylates chloroplast precursor proteins. _J. Biol. Chem._ 281, 40216–40223 (2006). CAS  PubMed  Google Scholar  * Schleiff, E. & Klösgen, R. B. Without a little help from

'my' friends: direct insertion of proteins into chloroplast membranes? _Biochim. Biophys. Acta_ 1541, 22–33 (2001). CAS  PubMed  Google Scholar  * van der Laan, M., Hutu, D. P.

& Rehling, P. On the mechanism of preprotein import by the mitochondrial presequence translocase. _Biochim. Biophys. Acta_ 1803, 732–739 (2010). CAS  PubMed  Google Scholar  * Kutik, S.

et al. Dissecting membrane insertion of mitochondrial β-barrel proteins. _Cell_ 132, 1011–1024 (2008). CAS  PubMed  Google Scholar  * Kleffmann, T. et al. The _Arabidopsis thaliana_

chloroplast proteome reveals pathway abundance and novel protein functions. _Curr. Biol._ 14, 354–362 (2004). CAS  PubMed  Google Scholar  * Armbruster, U. et al. Chloroplast proteins

without cleavable transit peptides: rare exceptions or a major constituent of the chloroplast proteome? _Mol. Plant_ 2, 1325–1335 (2009). CAS  PubMed  Google Scholar  * Hachiya, N. et al.

MSF, a novel cytoplasmic chaperone which functions in precursor targeting to mitochondria. _EMBO J._ 13, 5146–5154 (1994). CAS  PubMed  PubMed Central  Google Scholar  * May, T & Soll,

J. 14-3-3 proteins form a guidance complex with chloroplast precursor proteins in plants. _Plant Cell_ 12, 53–64 (2000). CAS  PubMed  PubMed Central  Google Scholar  * Yano, M., Terada, K.

& Mori, M. AIP is a mitochondrial import mediator that binds to both import receptor Tom20 and preproteins. _J. Cell. Biol._ 163, 45–56 (2003). CAS  PubMed  PubMed Central  Google

Scholar  * Young, J. C., Hoogenraad, N. J. & Hartl, F. U. Molecular chaperones Hsp90 and Hsp70 deliver preproteins to the mitochondrial import receptor Tom70. _Cell_ 112, 41–50 (2003).

THIS STUDY SHOWED THE DELIVERY OF HSP70- AND HSP90-GUIDED PRECURSOR PROTEINS TO TOM70. CAS  PubMed  Google Scholar  * Schemenewitz, A., Pollmann, S., Reinbothe, C. & Reinbothe, S. A

substrate-independent, 14-3-3 protein-mediated plastid import pathway of NADPH:protochlorophyllide oxidoreductase A. _Proc. Natl Acad. Sci. USA_ 104, 8538–8543 (2009). Google Scholar  *

Qbadou, S. et al. The molecular chaperone Hsp90 delivers precursor proteins to the chloroplast import receptor Toc64. _EMBO J._ 25, 1836–1847 (2006). THIS WORK DEFINED THE PATHWAY THAT

TARGETS HSP70- AND HSP90-GUIDED PRECURSORS TO TOC64. CAS  PubMed  PubMed Central  Google Scholar  * Oecking, C. & Jaspert, T. Plant 14-3-3 proteins catch up with their mammalian

orthologs. _Curr. Opin. Plant Biol._ 12, 760–765 (2009). CAS  PubMed  Google Scholar  * Beddoe, T. & Lithgow, T. Delivery of nascent polypeptides to the mitochondrial surface. _Biochim.

Biophys. Acta_ 1592, 35–39 (2002). CAS  PubMed  Google Scholar  * Bae, W. et al. AKR2A-mediated import of chloroplast outer membrane proteins is essential for chloroplast biogenesis. _Nature

Cell Biol._ 10, 220–227 (2008). THIS STUDY REPORTS THE IDENTIFICATION OF THE FACTOR THAT TARGETS PROTEINS TO THE CHLOROPLAST OUTER ENVELOPE. CAS  PubMed  Google Scholar  * Dhanoa, P. K. et

al. Distinct pathways mediate the sorting of tail-anchored proteins to the plastid outer envelope. _PLoS ONE_ 5, e10098 (2010). PubMed  PubMed Central  Google Scholar  * Shen, G. et al.

ANKYRIN REPEAT-CONTAINING PROTEIN 2A is an essential molecular chaperone for peroxisomal membrane-bound ASCORBATE PEROXIDASE3 in _Arabidopsis_. _Plant Cell_ 22, 811–831 (2010). CAS  PubMed 

PubMed Central  Google Scholar  * Kellems, R. E., Allison, V. F. & Butow, R. A. Cytoplasmic type 80S ribosomes associated with yeast mitochondria. IV. Attachment of ribosomes to the

outer membrane of isolated mitochondria. _J. Cell Biol._ 65, 1–14 (1975). CAS  PubMed  Google Scholar  * Marc, P. et al. Genome-wide analysis of mRNAs targeted to yeast mitochondria. _EMBO

Rep._ 3, 159–164 (2002). CAS  PubMed  PubMed Central  Google Scholar  * Eliyahu, E. et al. Tom20 mediates localization of mRNAs to mitochondria in a translation-dependent manner. _Mol. Cell.

Biol._ 30, 284–294 (2010). CAS  PubMed  Google Scholar  * Villarejo, A. et al. Evidence for a protein transported through the secretory pathway _en route_ to the higher plant chloroplast.

_Nature Cell Biol._ 7, 1224–1231 (2005). PubMed  Google Scholar  * Neuspiel, M. et al. Cargo-selected transport from the mitochondria to peroxisomes is mediated by vesicular carriers. _Curr.

Biol._ 18, 102–108 (2008). CAS  PubMed  Google Scholar  * de Brito, O. M. & Scorrano, L. Mitofusin 2 tethers endoplasmic reticulum to mitochondria. _Nature_ 456, 605–610 (2008). PubMed

  Google Scholar  * Kormann, B. et al. An ER-mitochondria tethering complex revealed by a synthetic biology screen. _Science_ 325, 477–481 (2009). Google Scholar  * Andersson, M. X., Goksör,

M. & Sandelius, A. S. Optical manipulation reveals strong attracting forces at membrane contact sites between endoplasmic reticulum and chloroplasts. _J. Biol. Chem._ 282, 1170–1174

(2007). CAS  PubMed  Google Scholar  * Oreb, M., Tews, I. & Schleiff, E. Policing Tic 'n' Toc, the doorway to chloroplasts. _Trends Cell Biol._ 18, 19–27 (2008). CAS  PubMed 

Google Scholar  * Jarvis, P. Targeting of nucleus-encoded proteins to chloroplasts in plants. _New Phytol._ 179, 257–285 (2008). CAS  PubMed  Google Scholar  * Dekker, P. J. T. et al.

Preprotein translocase of the outer mitochondrial membrane: molecular dissection and assembly of the general import pore complex. _Mol. Cell. Biol._ 18, 6515–6524 (1998). CAS  PubMed  PubMed

Central  Google Scholar  * Sherman, E. L., Go, N. E. & Nargang, F. E. Functions of the small proteins in the TOM complex of _Neurospora crassa_. _Mol. Biol. Cell_ 16, 4172–4182 (2005).

CAS  PubMed  PubMed Central  Google Scholar  * Dietmeier, K. et al. Tom5 functionally links mitochondrial preprotein receptors to the general import pore. _Nature_ 388, 195–200 (1997). CAS 

PubMed  Google Scholar  * Becker, T. et al. Assembly of the mitochondrial protein import channel: role of Tom5 in two-stage interaction of Tom40 with the SAM complex. _Mol. Biol. Cell_ 21,

3106–3113 (2010). CAS  PubMed  PubMed Central  Google Scholar  * Jackson-Constan, D. & Keegstra, K. _Arabidopsis_ genes encoding components of the chloroplastic protein import apparatus.

_Plant Physiol._ 125, 1567–1576 (2001). CAS  PubMed  PubMed Central  Google Scholar  * Lister, R. et al. A transcriptomic and proteomic characterization of the _Arabidopsis_ mitochondrial

protein import apparatus and its response to mitochondrial dysfunction. _Plant Physiol._ 134, 777–789 (2004). CAS  PubMed  PubMed Central  Google Scholar  * Ivanova, Y., Smith, M. D., Chen,

K. & Schnell, D. J. Members of the Toc159 import receptor family represent distinct pathways for protein targeting to plastids. _Mol. Biol. Cell_ 15, 3379–3392 (2004). CAS  PubMed 

PubMed Central  Google Scholar  * Kubis, S. et al. Functional specialization amongst the _Arabidopsis_ Toc159 family of chloroplast protein import receptors. _Plant Cell_ 16, 2059–2077

(2004). REFERENCES 55 AND 56 DESCRIBE THE DIFFERENT FUNCTIONS OF THE DISTINCT TOC159 ISOFORMS. CAS  PubMed  PubMed Central  Google Scholar  * Inoue, H., Rounds, C. & Schnell, D. J. The

molecular basis for distinct pathways for protein import into _Arabidopsis_ chloroplasts. _Plant Cell_ 22, 1947–1960 (2010). CAS  PubMed  PubMed Central  Google Scholar  * Perry, A. J. et

al. Structure, topology and function of the translocase of the outer membrane of mitochondria. _Plant Physiol. Biochem._ 46, 265–274 (2008). CAS  PubMed  Google Scholar  * Ahting, U. et al.

The TOMcore complex: the general import pore of the outer membrane of mitochondria. _J. Cell Biol._ 147, 959–968 (1999). CAS  PubMed  PubMed Central  Google Scholar  * Schleiff, E., Soll,

J., Küchler, M., Kühlbrandt, W. & Harrer, R. Characterization of the translocon of the outer envelope of chloroplasts. _J. Cell Biol._ 160, 541–551 (2003). CAS  PubMed  PubMed Central 

Google Scholar  * Künkele, K. P. et al. The preprotein translocation channel of the outer membrane of mitochondria. _Cell_ 93, 1009–1019 (1998). REFERENCES 60 AND 61 PROVIDE THE FIRST

STRUCTURAL AND FUNCTIONAL ANALYSIS OF PURIFIED TOM AND TOC COMPLEXES. PubMed  Google Scholar  * Model, K. et al. Protein translocase of the outer mitochondrial membrane: role of import

receptors in the structural organization of the TOM complex. _J. Mol. Biol._ 316, 657–666 (2002). CAS  PubMed  Google Scholar  * Model, K., Meisinger, C. & Kühlbrandt, W. Cryo-electron

microscopy structure of a yeast mitochondrial preprotein translocase. _J. Mol. Biol._ 383, 1049–1057 (2008). CAS  PubMed  Google Scholar  * van Wilpe, S. et al. Tom22 is a multifunctional

organizer of the mitochondrial preprotein translocase. _Nature_ 401, 485–489 (1999). THIS STUDY IDENTIFIED THE MULTIPLE FUNCTIONS OF TOM22 IN PRECURSOR-PROTEIN BINDING, MOLECULAR

ORGANIZATION OF THE TRANSLOCASE AND PORE REGULATION. CAS  PubMed  Google Scholar  * Becker, T. et al. Preprotein recognition by the Toc complex. _EMBO J._ 23, 520–530 (2004). THE FIRST

MECHANISTIC ANALYSIS OF TRANSLOCATION STEPS IN THE TOC COMPLEX. CAS  PubMed  PubMed Central  Google Scholar  * Lee, J., Wang, F. & Schnell, D. J. Toc receptor dimerization participates

in the initiation of membrane translocation during protein import into chloroplasts. _J. Biol. Chem._ 284, 31130–31141 (2009). CAS  PubMed  PubMed Central  Google Scholar  * Jarvis, P. et

al. An _Arabidopsis_ mutant defective in the plastid general protein import apparatus. _Science_ 282, 100–103 (1998). THIS STUDY DESCRIBES THE PHENOTYPE AND ULTRASTRUCTURAL CHANGES OF THE

FIRST IDENTIFIED MUTANT OF THE TOC MACHINERY, THE _PPI1_ MUTANT. CAS  PubMed  Google Scholar  * Oreb, M., Höfle, A., Mirus, O. & Schleiff, E. Phosphorylation regulates the assembly of

chloroplast import machinery. _J. Exp. Bot._ 59, 2309–2316 (2008). CAS  PubMed  PubMed Central  Google Scholar  * Aronsson, H., Combe, J., Patel, R. & Jarvis, P. _In vivo_ assessment of

the significance of phosphorylation of the _Arabidopsis_ chloroplast protein import receptor, atToc33. _FEBS Lett._ 580, 649–655 (2006). CAS  PubMed  Google Scholar  * Oreb, M. et al.

Phospho-mimicry mutant of atToc33 affects early development of _Arabidopsis thaliana_. _FEBS Lett._ 581, 5945–5951 (2007). CAS  PubMed  Google Scholar  * Saitoh, T. et al. Tom20 recognizes

mitochondrial presequences through dynamic equilibrium among multiple bound states. _EMBO J._ 26, 4777–4787 (2007). CAS  PubMed  PubMed Central  Google Scholar  * Mirus, O. & Schleiff,

E. The evolution of tetratricopeptide repeat domain containing receptors involved in protein translocation. _Endocytobiosis Cell Res._ 19, 31–50 (2009). Google Scholar  * Wu, Y. & Sha,

B. Crystal structure of yeast outer membrane translocon member Tom70p. _Nature Struct. Mol. Biol._ 13, 589–593 (2006). CAS  Google Scholar  * Qbadou, S. et al. Toc64 — a preprotein-receptor

at the outer membrane with bipartide function. _J. Mol. Biol._ 367, 1330–1346 (2007). CAS  PubMed  Google Scholar  * Chew, O. et al. A plant outer mitochondrial membrane protein with high

amino acid sequence identity to a chloroplast protein import receptor. _FEBS Lett._ 557, 109–114 (2004). CAS  PubMed  Google Scholar  * Yamamoto, H. et al. Role of Tom70 in import of

presequence-containing mitochondrial proteins. _J. Biol. Chem._ 284, 31625–31646 (2009). Google Scholar  * Hines, V. et al. Protein import into yeast mitochondria is accelerated by the outer

membrane protein MAS70. _EMBO J._ 9, 3191–3200 (1990). CAS  PubMed  PubMed Central  Google Scholar  * Aronsson, H. et al. Toc64/OEP64 is not essential for the efficient import of proteins

into chloroplasts in _Arabidopsis thaliana_. _Plant J._ 52, 53–68 (2007). CAS  PubMed  Google Scholar  * Ramage, L., Junne, T., Hahne, K., Lithgow, T. & Schatz, G. Functional cooperation

of mitochondrial protein import receptors in yeast. _EMBO J._ 12, 4115–4123 (1993). CAS  PubMed  PubMed Central  Google Scholar  * Moczko, M. et al. Deletion of the receptor MOM19 strongly

impairs import of cleavable preproteins into _Sacchoromyces cerevisae_ mitochondria. _J. Biol. Chem._ 269, 9045–9051 (1994). CAS  PubMed  Google Scholar  * Constan, D., Patel, R., Keegstra,

K. & Jarvis, P. An outer envelope membrane component of the plastid protein import apparatus plays an essential role in _Arabidopsis_. _Plant J._ 38, 93–106 (2004). CAS  PubMed  Google

Scholar  * Kessler, F. & Schnell, D. J. Chloroplast biogenesis: diversity and regulation of the protein import apparatus. _Curr. Opin. Cell Biol._ 21, 494–500 (2009). CAS  PubMed  Google

Scholar  * Yamano, K. et al. Tom20 and Tom22 share the common signal recognition pathway in mitochondrial protein import. _J. Biol. Chem._ 283, 3799–3807 (2008). CAS  PubMed  Google Scholar

  * Brix, J., Dietmeier, K. & Pfanner, N. Differential recognition of preproteins by the purified cytosolic domains of the mitochondrial import receptors Tom20, Tom22 and Tom70. _J.

Biol. Chem._ 272, 20730–20735 (1997). CAS  PubMed  Google Scholar  * Abe, Y. et al. Structural basis of presequence recognition by the mitochondrial protein import receptor Tom20. _Cell_

100, 551–560 (2000). THIS STUDY DESCRIBES FOR THE FIRST TIME THE STRUCTURAL BASIS OF PRECURSOR RECOGNITION BY A RECEPTOR OF THE TOM TRANSLOCON. CAS  PubMed  Google Scholar  * Keil, P. &

Pfanner, N. Insertion of MOM22 into mitochondrial outer membrane strictly depends on surface receptors. _FEBS Lett._ 321, 197–200 (1993). CAS  PubMed  Google Scholar  * Wallas, T. R., Smith,

M. D., Sanchez-Nieto, S. & Schnell, D. J. The roles of Toc34 and Toc75 in targeting the Toc159 preprotein receptor to chloroplasts. _J. Biol. Chem._ 278, 44289–44297 (2003). CAS  PubMed

  Google Scholar  * Bauer, J. et al. The major protein import receptor of plastids is essential for chloroplast biogenesis. _Nature_ 403, 203–207 (2000). CAS  PubMed  Google Scholar  * Hill,

K. et al. Tom40 forms the hydrophilic channel of the mitochondrial import pore for preproteins. _Nature_ 395, 516–521 (1998). CAS  PubMed  Google Scholar  * Hinnah, S. C., Hill, K., Wagner,

R., Schlicher, T. & Soll, J. Reconstitution of a chloroplast protein import channel. _EMBO J._ 16, 7351–7360 (1997). REFERENCES 89 AND 90 PRESENT THE FIRST DESCRIPTION OF THE

RECONSTITUTION OF TOC75 AND TOM40, AND THEIR ELECTROPHYSIOLOGICAL CHARACTERIZATION. CAS  PubMed  PubMed Central  Google Scholar  * Hinnah, S. C., Wagner, R., Sveshnikova, N., Harrer, R.

& Soll, J. The chloroplast protein import channel Toc75: pore properties and interaction with transit peptides. _Biophys. J._ 83, 899–911 (2002). CAS  PubMed  PubMed Central  Google

Scholar  * Becker, L. et al. Preprotein translocase of the outer mitochondrial membrane: reconstituted Tom40 forms a characteristic TOM pore. _J. Mol. Biol._ 353, 1011–1020 (2005). CAS 

PubMed  Google Scholar  * Poynor, M., Eckert, R. & Nussberger, S. Dynamics of the preprotein translocation channel of the outer membrane of mitochondria. _Biophys. J._ 95, 1511–1522

(2008). CAS  PubMed  PubMed Central  Google Scholar  * Schleiff, E. & McBride, H. The central matrix loop drives import of uncoupling protein 1 into mitochondria. _J. Cell Sci._ 113,

2267–2272 (2000). CAS  PubMed  Google Scholar  * Wiedemann, N., Pfanner, N. & Ryan, M. T. The three modules of ADP/ATP carrier cooperate in receptor recruitment and translocation into

mitochondria. _EMBO J._ 20, 951–960 (2001). CAS  PubMed  PubMed Central  Google Scholar  * Dolezal, P., Likic, V., Tachezy, J. & Lithgow, T. Evolution of the molecular machines for

protein import into mitochondria. _Science_ 313, 314–318 (2006). CAS  PubMed  Google Scholar  * Bayrhuber, M. et al. Structure of the human voltage-dependent anion channel. _Proc. Natl Acad.

Sci. USA_ 105, 15370–15375 (2008). CAS  PubMed  PubMed Central  Google Scholar  * Bredemeier, R. et al. Functional and phylogenetic properties of the pore-forming β-barrel transporters of

the Omp85 family. _J. Biol. Chem._ 282, 1882–1890 (2007). A DISSECTION OF THE ELECTROPHYSIOLOGICAL PROPERTIES OF SAM50-LIKE AND TOC75-LIKE OMP85 PROTEINS. CAS  PubMed  Google Scholar  *

Bohnsack, M. T. & Schleiff, E. The evolution of protein targeting and translocation systems. _Biochim. Biophys. Acta_ 1803, 1115–1130 (2010). CAS  PubMed  Google Scholar  * Eckart, K. et

al. A Toc75-like protein import channel is abundant in chloroplasts. _EMBO Rep._ 3, 557–562 (2002). CAS  PubMed  PubMed Central  Google Scholar  * Patel, R., Hsu, S. C., Bedard, J., Inoue,

K. & Jarvis, P. The Omp85-related chloroplast outer envelope protein OEP80 is essential for viability in _Arabidopsis_. _Plant. Physiol._ 148, 235–245 (2008). CAS  PubMed  PubMed Central

  Google Scholar  * Kim, S. et al. Structure and function of an essential component of the outer membrane protein assembly machine. _Science_ 317, 961–964 (2007). CAS  PubMed  Google Scholar

  * Sánchez-Pulido, L., Devos, D., Genevrois, S., Vicente, M. & Valencia, A. POTRA: a conserved domain in the FtsQ family and a class of β-barrel outer membrane proteins. _Trends

Biochem. Sci._ 28, 523–526 (2003). PubMed  Google Scholar  * Koenig, P. et al. Conserved properties of polypeptide transport-associated (POTRA) domains derived from cyanobacterial Omp85. _J.

Biol. Chem._ 285, 18016–18024 (2010). CAS  PubMed  PubMed Central  Google Scholar  * Clantin, B. et al. Structure of the membrane protein FhaC: a member of the Omp85-TpsB transporter

superfamily. _Science_ 317, 957–961 (2007). CAS  PubMed  Google Scholar  * Ertel, F. et al. The evolutionarily related β-barrel polypeptide transporters from _Pisum sativum_ and _Nostoc_

PCC7120 contain two distinct functional domains. _J. Biol. Chem._ 280, 28281–28289 (2005). CAS  PubMed  Google Scholar  * Dembowski, M., Künkele, K. P., Nargang, F. E., Neupert, W. &

Rapaport, D. Assembly of Tom6 and Tom7 into the TOM core complex of _Neurospora crassa_. _J. Biol. Chem._ 276, 17679–17685 (2001). CAS  PubMed  Google Scholar  * Esaki, M. et al. Tom40

protein import channel binds to non-native proteins and prevents their aggregation. _Nature Struct. Mol. Biol._ 10, 988–994 (2003). CAS  Google Scholar  * Schmitt, S. et al. Role of Tom5 in

maintaining the structural stability of the TOM complex of mitochondria. _J. Biol. Chem._ 280, 14499–14506 (2005). CAS  PubMed  Google Scholar  * Schleiff, E., Jelic, M. & Soll, J. A

GTP-driven motor moves proteins across the outer envelope of chloroplasts. _Proc. Natl Acad. Sci. USA_ 100, 4604–4609 (2003). CAS  PubMed  PubMed Central  Google Scholar  * Komiya, T. et al.

Interaction of mitochondrial targeting signals with acidic receptor domains along the protein import pathway: evidence for the 'acid chain' hypothesis. _EMBO J._ 17, 3886–3898

(1998). CAS  PubMed  PubMed Central  Google Scholar  * Bolliger, L., Junne, T., Schatz, G. & Lithgow, T. Acidic receptor domains on both sides of the outer membrane mediate translocation

of precursor proteins into yeast mitochondria. _EMBO J._ 14, 6318–6326 (1997). Google Scholar  * Kouranov, A. & Schnell, D. J. Analysis of the interactions of preproteins with the

import machinery over the course of protein import into chloroplasts. _J. Cell Biol._ 29, 1677–1685 (1997). Google Scholar  * Jelic, M., Soll, J. & Schleiff, E. Two Toc34 homologues with

different properties. _Biochemistry_ 42, 5906–5916 (2003). CAS  PubMed  Google Scholar  * Smith, M. D. et al. atToc159 is a selective transit peptide receptor for the import of

nucleus-encoded chloroplast proteins. _J. Cell Biol._ 165, 323–334 (2004). THIS STUDY EXPLORES THE RECEPTOR FUNCTION OF TOC159. CAS  PubMed  PubMed Central  Google Scholar  * Lee, K. H.,

Kim, S. J., Lee, Y. J., Jin, J. B. & Hwang, I. The M domain of atToc159 plays an essential role in the import of proteins into chloroplasts and chloroplasts biogenesis. _J. Biol. Chem._

278, 36794–36805 (2003). CAS  PubMed  Google Scholar  * Wang, F., Agne, B., Kessler, F. & Schnell, D. J. The role of GTP binding and hydrolysis at the atToc159 preprotein receptor during

protein import into chloroplasts. _J. Cell Biol._ 183, 87–99 (2008). CAS  PubMed  PubMed Central  Google Scholar  * Agne, B. et al. A Toc159 import receptor mutant, defective in hydrolysis

of GTP, supports preprotein import into chloroplasts. _J. Biol. Chem._ 284, 8670–8679 (2009). CAS  PubMed  PubMed Central  Google Scholar  * Becker, T. et al. Toc12, a novel subunit of the

intermembrane space preprotein translocon of chloroplasts. _Mol. Biol. Cell_ 15, 5130–5144 (2004). CAS  PubMed  PubMed Central  Google Scholar  * Matouschek, A. et al. Active unfolding of

precursor proteins during mitochondrial protein import. _EMBO J._ 16, 6727–6736 (1997). CAS  PubMed  PubMed Central  Google Scholar  * Gaume, B. et al. Unfolding of preproteins upon import

into mitochondria. _EMBO J._ 17, 6497–6507 (1998). CAS  PubMed  PubMed Central  Google Scholar  * Hageman, J. et al. Protein import into and sorting inside the chloroplast are independent

processes. _Plant Cell_ 2, 479–494 (1990). CAS  PubMed  PubMed Central  Google Scholar  * Bionda, T. et al. Chloroplast import signals: the length requirement for translocation _in vitro_

and _in vivo_. _J. Mol. Biol._ 402, 510–523 (2010). CAS  PubMed  Google Scholar  * Sato, T., Esaki, M., Fernandez, J. M. & Endo, T. Comparison of the protein-unfolding pathways between

mitochondrial protein import and atomic-force microscopy measurements. _Proc. Natl Acad. Sci. USA_ 102, 17999–18004 (2005). THIS STUDY COMPARES THE MITOCHONDRIAL TRANSLOCATION ROUTES OF

N-TERMINAL- AND C-TERMINAL-FUSED SIGNALS WITH THE BIOPHYSICAL PROPERTIES OF THE PASSENGER. CAS  PubMed  PubMed Central  Google Scholar  * Oguro, T. et al. Structural stabilities of different

regions of the titin I27 domain contribute differently to unfolding upon mitochondrial protein import. _J. Mol. Biol._ 385, 811–819 (2009). CAS  PubMed  Google Scholar  * Ruprecht, M. et

al. On the impact of precursor unfolding during protein import into chloroplasts. _Mol. Plant_ 3, 499–508 (2010). THIS STUDY COMPARES THE TRANSLOCATION ROUTE OF N-TERMINAL CHLOROPLAST

SIGNALS WITH THE BIOPHYSICAL PROPERTIES OF THE PASSENGER. CAS  PubMed  Google Scholar  * Fulgosi, H. & Soll, J. The chloroplast protein import receptors Toc34 and Toc159 are

phosphorylated by distinct protein kinases. _J. Biol. Chem._ 277, 8934–8940 (2002). CAS  PubMed  Google Scholar  * Agne, B. et al. The acidic A-domain of _Arabidopsis_ TOC159 occurs as a

hyperphosphorylated protein. _Plant Physiol._ 153, 1016–1030 (2010). CAS  PubMed  PubMed Central  Google Scholar  * Esaki, M. et al. Mitochondrial protein import. Requirement of presequence

elements and TOM components for precursor binding to the TOM complex. _J. Biol. Chem._ 279, 45701–45707 (2004). CAS  PubMed  Google Scholar  * Gabriel, K., Egan, B. & Lithgow, T. Tom40,

the import channel of the mitochondrial outer membrane, plays an active role in sorting proteins. _EMBO J._ 22, 2380–2386 (2003). CAS  PubMed  PubMed Central  Google Scholar  * Moczko, M. et

al. The intermembrane space domain of mitochondrial Tom22 functions as a trans binding site for preproteins with N-terminal targeting sequences. _Mol. Cell. Biol._ 17, 6574–6584 (1997). CAS

  PubMed  PubMed Central  Google Scholar  * Chacinska, A. et al. Mitochondrial presequence translocase: switching between TOM tethering and motor recruitment involves Tim21 and Tim17. _Cell_

120, 817–829 (2005). CAS  PubMed  Google Scholar  * Chacinska, A. et al. Mitochondrial translocation contact sites: separation of dynamic and stabilizing elements of a TOM-TIM preprotein

supercomplex. _EMBO J._ 22, 5370–5381 (2003). CAS  PubMed  PubMed Central  Google Scholar  * Nielsen, E., Akita, M., Davila-Aponte, J. & Keegstra, K. Stable association of chloroplastic

precursors with protein translocation complexes that contain proteins from both envelope membranes and a stromal Hsp100 molecular chaperone. _EMBO J._ 16, 935–946 (1997). CAS  PubMed  PubMed

Central  Google Scholar  * Akita, M., Nielsen, E. & Keegsta, K. Identification of protein complexes in the chloroplastic envelope membranes via chemical cross-linking. _J. Cell Biol._

136, 983–994 (1997). REFERENCES 133, 134, 135 DESCRIBE THE INITIAL IDENTIFICATION OF THE TOM–TIM23 AND TOC–TIC SUPERCOMPLEXES. CAS  PubMed  PubMed Central  Google Scholar  * Yi, L. &

Dalbey, R. E. Oxa1/Alb3/YidC system for insertion of membrane proteins in mitochondria, chloroplasts and bacteria. _Mol. Membr. Biol._ 22, 101–111 (2005). CAS  PubMed  Google Scholar  *

Herrmann, J. M. & Köhl, R. Catch me if you can! Oxidative protein trapping in the intermembrane space of mitochondria. _J. Cell Biol._ 176, 559–563 (2007). CAS  PubMed  PubMed Central 

Google Scholar  * Soll, J. & Schleiff, E. Protein import into chloroplasts. _Nature Rev. Mol. Cell Biol._ 5, 198–208 (2004). CAS  Google Scholar  * Kovacheva, S., Bédard, J., Wardle, A.,

Patel, R. & Jarvis, P. Further _in vivo_ studies on the role of the molecular chaperone, Hsp93, in plastid protein import. _Plant J._ 50, 364–379 (2007). CAS  PubMed  Google Scholar  *

Shi, L.-X. & Theg, S. M. A stromal heat shock protein 70 system functions in protein import into chloroplasts in the moss _Physcomitrella patens_. _Plant Cell_ 22, 205–220 (2010). CAS 

PubMed  PubMed Central  Google Scholar  * Su, P.-H. & Li, H. Stromal Hsp70 is important for protein translocation into pea and _Arabidopsis_ chloroplasts. _Plant Cell_ 22, 1516–1531

(2010). CAS  PubMed  PubMed Central  Google Scholar  Download references ACKNOWLEDGEMENTS T.B. thanks N. Pfanner for support. The work was supported by Baden-Württemberg Stiftung (T.B.), the

Deutsche Forschungsgemeinschaft (DFG) in the frame of the Sonderforschungsbereich SFB746 (T.B.), the Volkswagen-foundation (E.S.) and the DFG in the frame of the Sonderforschungsbereich

SFB807/P17 (E.S.). AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * Department of Biosciences, Goethe University, Cluster of Excellence 'Macromolecular Complexes', Centre of Membrane

Proteomics, Molecular Cell Biology of Plants, Max-von-Laue Str. 9, Frankfurt, D-60438, Germany Enrico Schleiff * Institut für Biochemie und Molekularbiologie, Zentrum für Biochemie und

Molekulare Zellforschung, Universität Freiburg, Freiburg, 79104, Germany Thomas Becker Authors * Enrico Schleiff View author publications You can also search for this author inPubMed Google

Scholar * Thomas Becker View author publications You can also search for this author inPubMed Google Scholar ETHICS DECLARATIONS COMPETING INTERESTS The authors declare no competing

financial interests. RELATED LINKS RELATED LINKS FURTHER INFORMATION Thomas Becker's homepage Enrico Schleiff's homepage GLOSSARY * Endosymbiosis Endosymbiosis is the process in

which a free-living bacteria — the ancestral endosymbiont — was enclosed by a cell and, during evolution, became integrated into the cellular network. By transfer of most of its genetic

content to the host, the nucleus lost its independence and became an organelle. * Thylakoid membrane A component of chloroplasts, the thylakoid membrane is a specialized membranous

compartment where photosynthesis occurs. * Oxygenic photosynthesis Oxygenic photosynthesis is the conversion of carbon dioxide and water into organic compounds, especially sugars, and oxygen

by the thylakoid and stromal enzymes, including the photosystems. * Amphiphilic α-helix An amphiphilic α-helix is a helix in which one side is composed of hydrophobic amino acids and the

other of hydrophilic amino acids. * β-barrel proteins β-barrel proteins are membrane proteins that are typically found in the outer membrane of mitochondria, of chloroplasts and of

Gram-negative bacteria. These proteins form a membrane-inserted barrel composed of β-strands. * 14-3-3 proteins Proteins that are expressed in eukaryotic cells and that bind preferentially

to phosphorylated regions in diverse proteins involved in signal transduction and protein translocation. * Tetratricopeptide repeat (TPR). A structural motif, found in a wide variety of

proteins, that is composed of 34 amino acids. TPRs are involved in intra- and inter-molecular interactions. * Ankyrin Ankyrin repeats are structurally but not functionally conserved units of

33 amino acids that consist of two α-helices separated by a loop, and comprise one of the most common structural motifs identified in bacterial, archaeal and eukaryotic proteins. * ERMES

complex (Endoplasmic reticulummitochondria encounter structure complex). This is the complex that tethers mitochondria and the endoplasmic reticulum. It is composed of the two mitochondrial

membrane proteins Mdm34 and Mdm10, the integral endoplasmic reticulum membrane protein Mmm1 and the peripheral protein Mdm12. * Chemical crosslinking Chemical crosslinking is the

introduction of synthetic bonds that link two proteins in close proximity by chemical molecules — for example, by maleimide, which reacts with the thiol group of cysteines. RIGHTS AND

PERMISSIONS Reprints and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE Schleiff, E., Becker, T. Common ground for protein translocation: access control for mitochondria and chloroplasts.

_Nat Rev Mol Cell Biol_ 12, 48–59 (2011). https://doi.org/10.1038/nrm3027 Download citation * Published: 08 December 2010 * Issue Date: January 2011 * DOI: https://doi.org/10.1038/nrm3027

SHARE THIS ARTICLE Anyone you share the following link with will be able to read this content: Get shareable link Sorry, a shareable link is not currently available for this article. Copy to

clipboard Provided by the Springer Nature SharedIt content-sharing initiative