Modular structure of the TIM23 preprotein translocase of mitochondria

Transmembrane Coordination of Preprotein Recognition and Motor Coupling by the Mitochondrial Presequence Receptor Tim50

Mitochondrial preproteins contain amino-terminal presequences directing them to the presequence translocase of the mitochondrial inner membrane (TIM23 complex). Depending on additional downstream import signals, TIM23 either inserts preproteins into the inner membrane or translocates them into the matrix. Matrix import requires the coupling of the presequence translocase-associated motor (PAM) to TIM23. The molecular mechanisms coordinating preprotein recognition by TIM23 in the intermembrane space (IMS) with PAM activation in the matrix are unknown. Here we show that subsequent to presequence recognition in the IMS, the Tim50 matrix domain facilitates the recruitment of the coupling factor Pam17. Next, the IMS domain of Tim50 promotes PAM recruitment to TIM23. Finally, the Tim50 transmembrane segment stimulates the matrix-directed import-driving force exerted by PAM. We propose that recognition of preprotein segments in the IMS and transfer of signal information across the inner membrane by Tim50 determine import motor activation.

Mitochondrial presequence import: Multiple regulatory knobs fine-tune mitochondrial biogenesis and homeostasis

Mitochondria are pivotal organelles for cellular signaling and metabolism, and their dysfunction leads to severe cellular stress. About 60-70% of the mitochondrial proteome consists of preproteins synthesized in the cytosol with an amino-terminal cleavable presequence targeting signal. The TIM23 complex transports presequence signals towards the mitochondrial matrix. Ultimately, the mature protein segments are either transported into the matrix or sorted to the inner membrane. To ensure accurate preprotein import into distinct mitochondrial sub-compartments, the TIM23 machinery adopts specific functional conformations and interacts with different partner complexes. Regulatory subunits modulate the translocase dynamics, tailoring the import reaction to the incoming preprotein. The mitochondrial membrane potential and the ATP generated via oxidative phosphorylation are key energy sources in driving the presequence import pathway. Thus, mitochondrial dysfunctions have rapid repercussions on biogenesis. Cellular mechanisms exploit the presequence import pathway to monitor mitochondrial dysfunctions and mount transcriptional and proteostatic responses to restore functionality.

Adaptation of a Genetic Screen Reveals an Inhibitor for Mitochondrial Protein Import Component Tim44

Diverse protein import pathways into mitochondria use translocons on the outer membrane (TOM) and inner membrane (TIM). We adapted a genetic screen, based on Ura3 mistargeting from mitochondria to the cytosol, to identify small molecules that attenuated protein import. Small molecule mitochondrial import blockers of the Carla Koehler laboratory (MB)-10 inhibited import of substrates that require the TIM23 translocon. Mutational analysis coupled with molecular docking and molecular dynamics modeling revealed that MB-10 binds to a specific pocket in the C-terminal domain of Tim44 of the protein-associated motor (PAM) complex. This region was proposed to anchor Tim44 to the membrane, but biochemical studies with MB-10 show that this region is required for binding to the translocating precursor and binding to mtHsp70 in low ATP conditions. This study also supports a direct role for the PAM complex in the import of substrates that are laterally sorted to the inner membrane, as well as the mitochondrial matrix. Thus, MB-10 is the first small molecule modulator to attenuate PAM complex activity, likely through binding to the C-terminal region of Tim44.

Tim62, a Novel Mitochondrial Protein in Trypanosoma brucei, Is Essential for Assembly and Stability of the TbTim17 Protein Complex

Trypanosoma brucei, the causative agent of human African trypanosomiasis, possesses non-canonical mitochondrial protein import machinery. Previously, we characterized the essential translocase of the mitochondrial inner membrane (TIM) consisting of Tim17 in T. brucei. TbTim17 is associated with TbTim62. Here we show that TbTim62, a novel protein, is localized in the mitochondrial inner membrane, and its import into mitochondria depends on TbTim17. Knockdown (KD) of TbTim62 decreased the steady-state levels of TbTim17 post-transcriptionally. Further analysis showed that import of TbTim17 into mitochondria was not inhibited, but its half-life was reduced >4-fold due to TbTim62 KD. Blue-native gel electrophoresis revealed that TbTim62 is present primarily in ∼150-kDa and also in ∼1100-kDa protein complexes, whereas TbTim17 is present in multiple complexes within the range of ∼300 to ∼1100 kDa. TbTim62 KD reduced the levels of both TbTim62 as well as TbTim17 protein complexes. Interestingly, TbTim17 was accumulated as lower molecular mass complexes in TbTim62 KD mitochondria. Furthermore, depletion of TbTim62 hampered the assembly of the ectopically expressed TbTim17-2X-myc into TbTim17 protein complex. Co-immunoprecipitation analysis revealed that association of TbTim17 with mHSP70 was markedly reduced in TbTim62 KD mitochondria. All together our results demonstrate that TbTim62, a unique mitochondrial protein in T. brucei, is required for the formation of a stable TbTim17 protein complex. TbTim62 KD destabilizes this complex, and unassembled TbTim17 degrades. Therefore, TbTim62 acts as a novel regulatory factor to maintain the levels of TIM in T. brucei mitochondria.

Mgr2 functions as lateral gatekeeper for preprotein sorting in the mitochondrial inner membrane

The majority of preproteins destined for mitochondria carry N-terminal presequences. The presequence translocase of the inner mitochondrial membrane (TIM23 complex) plays a central role in protein sorting. Preproteins are either translocated through the TIM23 complex into the matrix or are laterally released into the inner membrane. We report that the small hydrophobic protein Mgr2 controls the lateral release of preproteins. Mgr2 interacts with preproteins in transit through the TIM23 complex. Overexpression of Mgr2 delays preprotein release, whereas a lack of Mgr2 promotes preprotein sorting into the inner membrane. Preproteins with a defective inner membrane sorting signal are translocated into the matrix in wild-type mitochondria but are released into the inner membrane in Mgr2-deficient mitochondria. We conclude that Mgr2 functions as a lateral gatekeeper of the mitochondrial presequence translocase, providing quality control for the membrane sorting of preproteins.

Molecular chaperones are nanomachines that catalytically unfold misfolded and alternatively folded proteins

By virtue of their general ability to bind (hold) translocating or unfolding polypeptides otherwise doomed to aggregate, molecular chaperones are commonly dubbed “holdases”. Yet, chaperones also carry physiological functions that do not necessitate prevention of aggregation, such as altering the native states of proteins, as in the disassembly of SNARE complexes and clathrin coats. To carry such physiological functions, major members of the Hsp70, Hsp110, Hsp100, and Hsp60/CCT chaperone families act as catalytic unfolding enzymes or unfoldases that drive iterative cycles of protein binding, unfolding/pulling, and release. One unfoldase chaperone may thus successively convert many misfolded or alternatively folded polypeptide substrates into transiently unfolded intermediates, which, once released, can spontaneously refold into low-affinity native products. Whereas during stress, a large excess of non-catalytic chaperones in holding mode may optimally prevent protein aggregation, after the stress, catalytic disaggregases and unfoldases may act as nanomachines that use the energy of ATP hydrolysis to repair proteins with compromised conformations. Thus, holding and catalytic unfolding chaperones can act as primary cellular defenses against the formation of early misfolded and aggregated proteotoxic conformers in order to avert or retard the onset of degenerative protein conformational diseases.

Understanding the molecular mechanism of protein translocation across the mitochondrial inner membrane: still a long way to go

In order to reach the final place of their function, approximately half of the proteins in any eukaryotic cell have to be transported across or into one of the membranes in the cell. In this article, we present an overview of our current knowledge concerning the structural properties of the TIM23 complex and their relationship with the molecular mechanism of protein transport across the mitochondrial inner membrane. This article is part of a Special Issue entitled Protein translocation across or insertion into membranes.

The many faces of the mitochondrial TIM23 complex

The TIM23 complex in the inner membrane of mitochondria mediates import of essentially all matrix proteins and a large number of inner membrane proteins. Here we present an overview on the latest insights into the structure and function of this remarkable molecular machine.

Distinct forms of mitochondrial TOM-TIM supercomplexes define signal-dependent states of preprotein sorting

Mitochondrial import of cleavable preproteins occurs at translocation contact sites, where the translocase of the outer membrane (TOM) associates with the presequence translocase of the inner membrane (TIM23) in a supercomplex. Different views exist on the mechanism of how TIM23 mediates preprotein sorting to either the matrix or inner membrane. On the one hand, two TIM23 forms were proposed, a matrix transport form containing the presequence translocase-associated motor (PAM; TIM23-PAM) and a sorting form containing Tim21 (TIM23(SORT)). On the other hand, it was reported that TIM23 and PAM are permanently associated in a single-entity translocase. We have accumulated distinct transport intermediates of preproteins to analyze the translocases in their active, preprotein-carrying state. We identified two different forms of active TOM-TIM23 supercomplexes, TOM-TIM23(SORT) and TOM-TIM23-PAM. These two supercomplexes do not represent separate pathways but are in dynamic exchange during preprotein translocation and sorting. Depending on the signals of the preproteins, switches between the different forms of supercomplex and TIM23 are required for the completion of preprotein import.

Residues of Tim44 involved in both association with the translocon of the inner mitochondrial membrane and regulation of mitochondrial Hsp70 tethering

Translocation of proteins from the cytosol across the mitochondrial inner membrane is driven by the action of the import motor, which is associated with the translocon on the matrix side of the membrane. It is well established that an essential peripheral membrane protein, Tim44, tethers mitochondrial Hsp70 (mtHsp70), the core of the import motor, to the translocon. This Tim44-mtHsp70 interaction, which can be recapitulated in vitro, is destabilized by binding of mtHsp70 to a substrate polypeptide. Here we report that the N-terminal 167-amino-acid segment of mature Tim44 is sufficient for both interaction with mtHsp70 and destabilization of a Tim44-mtHsp70 complex caused by client protein binding. Amino acid alterations within a 30-amino-acid segment affected both the release of mtHsp70 upon peptide binding and the interaction of Tim44 with the translocon. Our results support the idea that Tim44 plays multiple roles in mitochondrial protein import by recruiting Ssc1 and its J protein cochaperone to the translocon and coordinating their interactions to promote efficient protein translocation in vivo.

Characterization of the mitochondrial inner membrane protein translocator Tim17 from Trypanosoma brucei

Mitochondrial protein translocation machinery in the kinetoplastid parasites, like Trypanosoma brucei, has been characterized poorly. In T. brucei genome database, one homolog for a protein translocator of mitochondrial inner membrane (Tim) has been found, which is closely related to Tim17 from other species. The T. brucei Tim17 (TbTim17) has a molecular mass 16.2kDa and it possesses four characteristic transmembrane domains. The protein is localized in the mitochondrial inner membrane. The level of TbTim17 protein is 6-7-fold higher in the procyclic form that has a fully active mitochondrion, than in the mammalian bloodstream form of T. brucei, where many of the mitochondrial activities are suppressed. Knockdown of TbTim17 expression by RNAi caused a cessation of cell growth in the procyclic form and reduced growth rate in the bloodstream form. Depletion of TbTim17 decreased mitochondrial membrane potential more in the procyclic than bloodstream form. However, TbTim17 knockdown reduced the expression level of several nuclear encoded mitochondrial proteins in both the forms. Furthermore, import of presequence containing nuclear encoded mitochondrial proteins was significantly reduced in TbTim17 depleted mitochondria of the procyclic as well as the bloodstream form, confirming that TbTim17 is critical for mitochondrial protein import in both developmental forms. Together, these show that TbTim17 is the translocator of nuclear encoded mitochondrial proteins and its expression is regulated according to mitochondrial activities in T. brucei.

Awaking TIM22, a Dynamic Ligand-gated Channel for Protein Insertion in the Mitochondrial Inner Membrane

Aqueous channels are at the core of the translocase of the outer membrane (TOM) and the translocase of the inner membrane for the transport of preproteins (TIM23), the translocases mediating the transport of proteins across the outer and inner mitochondrial membranes. Yet, the existence of a channel associated to the translocase of the inner membrane for the insertion of multitopic protein (TIM22) complex has been arguable, as its function relates to the insertion of multispanning proteins into the inner membrane. For the first time, we report conditions for detecting a channel activity associated to the TIM22 translocase in organelle, i.e. intact mitoplasts. An internal signal peptide in the intermembrane space of mitochondria is a requisite to inducing this channel, which is otherwise silent. The channel showed slightly cationic and high conductance activity of 1000 pS with a predominant half-open substate. Despite their different composition, the channels of the three mitochondrial translocases were thus remarkably similar, in agreement with their common task as pores transiently trapping proteins en route to their final destination. The opening of the TIM22 channel was a step-up process depending on the signal peptide concentration. Interestingly, low membrane potentials kept the channel fully open, providing a threshold level of the peptide is present. Our results portray TIM22 as a dynamic channel solely active in the presence of its cargo proteins. In its fully open conformation, favored by the combined action of internal signal peptide and low membrane potential, the channel could embrace the in-transit protein. As insertion progressed and initial interaction with the signal peptide faded, the channel would close, sustaining its role as a shunt that places trapped proteins into the membrane.

Association of the Tim14.Tim16 subcomplex with the TIM23 translocase is crucial for function of the mitochondrial protein import motor

Tim14 and Tim16 are essential components of the import motor of the mitochondrial TIM23 preprotein translocase. Tim14 contains a J domain in the matrix space that is anchored in the inner membrane by a transmembrane segment. Tim16 is a J-related protein with a moderately hydrophobic segment at its N terminus. The J and J-like domains function in the regulation of the ATPase activity of the Hsp70 chaperone of the import motor. We report here on the role of the hydrophobic segments of Tim16 and Tim14 in the TIM23 translocase. Yeast cells lacking the hydrophobic N-terminal segment in either Tim16 or Tim14 are viable but show growth defects and decreased import rates of matrix-targeted preproteins into mitochondria. The interaction of the Tim14.Tim16 complex with the core complex of the TIM23 translocase is destabilized in these cells. In particular, the N-terminal domain of Tim16 is crucial for the interaction of the Tim14.Tim16 complex with the TIM23 preprotein translocase. Deletion of hydrophobic segments in both, Tim16 and Tim14, is lethal. We conclude that import into the matrix space of mitochondria requires association of the co-chaperones Tim16 and Tim14 with the TIM23 preprotein translocase.

A fluorescence assay for peptide translocation into mitochondria

Translocation of the presequence is an early event in import of preproteins across the mitochondrial inner membrane by the TIM23 complex. Import of signal peptides, whose sequences mimic mitochondrial import presequences, was measured using a novel, qualitative, fluorescence assay in about 1h. This peptide assay was used in conjunction with classical protein import analyses and electrophysiological approaches to examine the mechanisms underlying the functional effects of depleting two TIM23 complex components. Tim23p forms, at least in part, the pore of this complex while Tim44p forms part of the translocation motor. Depletion of Tim23p eliminates TIM23 channel activity, which interferes with both peptide and preprotein translocation. In contrast, depletion of Tim44p disrupts preprotein but not peptide translocation, which has no effect on TIM23 channel activity. Two conclusions were made. First, this fluorescence peptide assay was validated as two different mutants were accurately identified. Hence, this assay could provide a rapid means of screening mutants to identify those that fail an initial step in import, i.e., translocation of the presequence. Second, translocation of signal peptides required normal channel activity and disruption of the presequence translocase-associated motor complex did not modify TIM23 channel activity nor prevent presequence translocation.

Tim17p regulates the twin pore structure and voltage gating of the mitochondrial protein import complex TIM23

The TIM23 complex mediates import of preproteins into mitochondria, but little is known of the mechanistic properties of this translocase. Here patch clamping reconstituted inner membranes allowed for first time insights into the structure and function of the preprotein translocase. Our findings indicate that the TIM23 channel has "twin pores" (two equal sized pores that cooperatively gate) thereby strikingly resembling TOM, the translocase of the outer membrane. Tim17p and Tim23p are homologues, but their functions differ. Tim23p acts as receptor for preproteins and may largely constitute the preprotein-conducting passageway. Conversely depletion of Tim17p induces a collapse of the twin pores into a single pore, whereas N terminus deletion or C terminus truncation results in variable sized pores that cooperatively gate. Further analysis of Tim17p mutants indicates that the N terminus is vital for both voltage sensing and protein sorting. These results suggest that although Tim23p is the main structural unit of the pore Tim17p is required for twin pore structure and provides the voltage gate for the TIM23 channel.

Sequential processing of a mitochondrial tandem protein: insights into protein import in Schizosaccharomyces pombe

The sequencing of the genome of Schizosaccharomyces pombe revealed the presence of a number of genes encoding tandem proteins, some of which are mitochondrial components. One of these proteins (pre-Rsm22-Cox11) consists of a fusion of Rsm22, a component of the mitochondrial ribosome, and Cox11, a factor required for copper insertion into cytochrome oxidase. Since in Saccharomyces cerevisiae, Cox11 is physically attached to the mitochondrial ribosome, it was suggested that the tandem organization of Rsm22-Cox11 is used to covalently tie the mitochondrial ribosome to Cox11 in S. pombe. We report here that pre-Rsm22-Cox11 is matured in two subsequent processing events. First, the mitochondrial presequence is removed. At a later stage of the import process, the Rsm22 and Cox11 domains are separated by cleavage of the mitochondrial processing peptidase at an internal processing site. In vivo data obtained using a tagged version of pre-Rsm22-Cox11 confirmed the proteolytic separation of Cox11 from the Rsm22 domain. Hence, the tandem organization of pre-Rsm22-Cox11 does not give rise to a persistent fusion protein but rather might be used to increase the import efficiency of Cox11 and/or to coordinate expression levels of Rsm22 and Cox11 in S. pombe.

Protein import into mitochondria

Mitochondria comprise approx. 1000–3000 different proteins, almost all of which must be imported from the cytosol into the organelle. So far, six complex molecular machines, protein translocases, were identified that mediate this process. The TIM23 complex is a major translocase in the inner mitochondrial membrane. It uses two energy sources, namely membrane potential and ATP, to facilitate preprotein translocation across the inner membrane and insertion into the inner membrane. Recent research has led to the discovery of a number of new constituents of the TIM23 complex and to the unravelling of the mechanisms of preprotein translocation.

Conformational properties of bacterial DnaK and yeast mitochondrial Hsp70

Among the eukaryotic members of the Hsp70 family, mitochondrial Hsp70 shows the highest degree of sequence identity with bacterial DnaK. Although they share a functional mechanism and homologous co-chaperones, they are highly specific and cannot be exchanged between Escherichia coli and yeast mitochondria. To provide a structural basis for this finding, we characterized both proteins, as well as two DnaK/mtHsp70 chimeras constructed by domain swapping, using biochemical and biophysical methods. Here, we show that DnaK and mtHsp70 display different conformational and biochemical properties. Replacing different regions of the DnaK peptide-binding domain with those of mtHsp70 results in chimeric proteins that: (a) are not able to support growth of an E. coli DnaK deletion strain at stress temperatures (e.g. 42 degrees C); (b) show increased accessibility and decreased thermal stability of the peptide-binding pocket; and (c) have reduced activation by bacterial, but not mitochondrial co-chaperones, as compared with DnaK. Importantly, swapping the C-terminal alpha-helical subdomain promotes a conformational change in the chimeras to an mtHsp70-like conformation. Thus, interaction with bacterial co-chaperones correlates well with the conformation that natural and chimeric Hsp70s adopt in solution. Our results support the hypothesis that a specific protein structure might regulate the interaction of Hsp70s with particular components of the cellular machinery, such as Tim44, so that they perform specific functions.

Role of Tim21 in mitochondrial translocation contact sites

Translocation of preproteins with N-terminal presequences into mitochondria requires the cooperation of the translocase of the outer membrane (TOM complex) and the presequence translocase of the inner membrane (TIM23 complex). However, the molecular nature of the translocation contact sites is poorly understood. We have identified a novel component of the TIM23 translocase, Tim21, which is involved in their formation. Tim21 is anchored in the mitochondrial inner membrane by a single transmembrane domain and exposes its C-terminal domain into the intermembrane space. The purified C-terminal domain of Tim21 appears not to bind to any of the TIM23 components but rather specifically interacts with the TOM complex. We propose that Tim21 binds to the trans site of the TOM complex thus keeping the two translocases in close contact.

Electrophysiological approaches to the study of protein translocation in mitochondria

Electrophysiological techniques have been integral to our understanding of protein translocation across various membranes, and, in particular, the mitochondrial inner and outer membranes. Descriptions of various methodologies (for example, patch clamp, planar bilayers, and tip dip, and their past and potential contributions) are detailed within. The activity of protein import channels of native mitochondrial inner and outer membranes can be studied by directly patch clamping mitochondria and mitoplasts (mitochondria stripped of their outer membrane by French pressing) from various genetically manipulated strains of yeast and mammalian tissue cultured cells. The channel activities of TOM, TIM23, and TIM22 complexes are compared with those reconstituted in proteoliposomes and with those of the recombinant proteins Tom40p, Tim23p, and Tim22p, which play major roles in protein translocation. Studies of the mechanism(s) and the role of channels in protein translocation in mitochondria are prototypes, as the same principles are likely followed in all biological membranes including the endoplasmic reticulum and chloroplasts. The ability to apply electrophysiological techniques to these channels is now allowing investigations into the role of mitochondria in diverse fields such as neurotransmitter release, long-term potentiation, and apoptosis.

Alternative topogenesis of Mgm1 and mitochondrial morphology depend on ATP and a functional import motor

Mitochondrial morphology and inheritance of mitochondrial DNA in yeast depend on the dynamin-like GTPase Mgm1. It is present in two isoforms in the intermembrane space of mitochondria both of which are required for Mgm1 function. Limited proteolysis of the large isoform by the mitochondrial rhomboid protease Pcp1/Rbd1 generates the short isoform of Mgm1 but how this is regulated is unclear. We show that near its NH2 terminus Mgm1 contains two conserved hydrophobic segments of which the more COOH-terminal one is cleaved by Pcp1. Changing the hydrophobicity of the NH2-terminal segment modulated the ratio of the isoforms and led to fragmentation of mitochondria. Formation of the short isoform of Mgm1 and mitochondrial morphology further depend on a functional protein import motor and on the ATP level in the matrix. Our data show that a novel pathway, to which we refer as alternative topogenesis, represents a key regulatory mechanism ensuring the balanced formation of both Mgm1 isoforms. Through this process the mitochondrial ATP level might control mitochondrial morphology.

The J domain-related cochaperone Tim16 is a constituent of the mitochondrial TIM23 preprotein translocase

Mitochondria import the vast majority of their proteins from the cytosol. The mitochondrial import motor of the TIM23 translocase drives the translocation of precursor proteins across the outer and inner membrane in an ATP-dependent reaction. Tim44 at the inner face of the translocation pore recruits the chaperone mtHsp70, which binds the incoming precursor protein. This reaction is assisted by the cochaperones Tim14 and Mge1. We have identified a novel essential cochaperone, Tim16. It is related to J-domain proteins and forms a stable subcomplex with the J protein Tim14. Depletion of Tim16 has a marked effect on protein import into the mitochondrial matrix, impairs the interaction of Tim14 with the TIM23 complex and leads to severe structural changes of the import motor. In conclusion, Tim16 is a constituent of the TIM23 preprotein translocase, where it exerts crucial functions in the import motor.

A transcriptomic and proteomic characterization of the Arabidopsis mitochondrial protein import apparatus and its response to mitochondrial dysfunction

Mitochondria import hundreds of cytosolically synthesized proteins via the mitochondrial protein import apparatus. Expression analysis in various organs of 19 components of the Arabidopsis mitochondrial protein import apparatus encoded by 31 genes showed that although many were present in small multigene families, often only one member was prominently expressed. This was supported by comparison of real-time reverse transcriptase-polymerase chain reaction and microarray experimental data with expressed sequence tag numbers and massive parallel signature sequence data. Mass spectrometric analysis of purified mitochondria identified 17 import components, their mitochondrial sub-compartment, and verified the presence of TIM8, TIM13, TIM17, TIM23, TIM44, TIM50, and METAXIN proteins for the first time, to our knowledge. Mass spectrometry-detected isoforms correlated with the most abundant gene transcript measured by expression data. Treatment of Arabidopsis cell culture with mitochondrial electron transport chain inhibitors rotenone and antimycin A resulted in a significant increase in transcript levels of import components, with a greater increase observed for the minor isoforms. The increase was observed 12 h after treatment, indicating that it was likely a secondary response. Microarray analysis of rotenone-treated cells indicated the up-regulation of gene sets involved in mitochondrial chaperone activity, protein degradation, respiratory chain assembly, and division. The rate of protein import into isolated mitochondria from rotenone-treated cells was halved, even though rotenone had no direct effect on protein import when added to mitochondria isolated from untreated cells. These findings suggest that transcription of import component genes is induced when mitochondrial function is limited and that minor gene isoforms display a greater response than the predominant isoforms.

Tim14, a novel key component of the import motor of the TIM23 protein translocase of mitochondria

The TIM23 translocase mediates the deltaPsi- and ATP-dependent import of proteins into mitochondria. We identified Tim14 as a novel component of the TIM23 translocase. Tim14 is an integral protein of the inner membrane with a typical J-domain exposed to the matrix space. TIM14 genes are present in the genomes of virtually all eukaryotes. In yeast, Tim14 is essential for viability. Mitochondria from cells depleted of Tim14 are deficient in the import of proteins mediated by the TIM23 complex. In particular, import of proteins that require the action of mtHsp70 is affected. Tim14 interacts with Tim44 and mtHsp70 in an ATP-dependent manner. A mutation in the HPD motif of the J-domain of Tim14 is lethal. Thus, Tim14 is a constituent of the mitochondrial import motor. We propose a model in which Tim14 is required for the activation of mtHsp70 and enables this chaperone to act in a rapid and regulated manner in the Tim44-mediated trapping of unfolded preproteins entering the matrix.

Control of mitochondrial protein import by pH

Protein import into mitochondria is inhibited by protons (IC(50) pH 6.5). The channels of the import machinery were examined to further investigate this pH dependence. TOM and TIM23 are the protein translocation channels of the mitochondrial outer and inner membranes, respectively, and their single channel behaviors at various pHs were determined using patch-clamp techniques. While not identical, increasing H(+) concentration decreases the open probability of both TIM23 and TOM channels. The pattern of the pH dependences of protein import and channel properties suggests TIM23 open probability can limit import of nuclear-encoded proteins into the matrix of yeast mitochondria.

Tim50, a novel component of the TIM23 preprotein translocase of mitochondria

The preprotein translocase of the inner membrane of mitochondria (TIM23 complex) is the main entry gate for proteins of the matrix and the inner membrane. We isolated the TIM23 complex of Neurospora crassa. Besides Tim23 and Tim17, it contained a novel component, referred to as Tim50. Tim50 spans the inner membrane with a single transmembrane segment and exposes a large hydrophilic domain in the intermembrane space. Tim50 is essential for viability of yeast. Mitochondria from cells depleted of Tim50 displayed strongly reduced import kinetics of preproteins using the TIM23 complex. Tim50 could be cross-linked to preproteins that were halted at the level of the translocase of the outer membrane (TOM complex) or spanning both TOM and TIM23 complexes. We suggest that Tim50 plays a crucial role in the transfer of preproteins from the TOM complex to the TIM23 complex through the intermembrane space.

Molecular chaperones as essential mediators of mitochondrial biogenesis

Chaperone proteins have been initially identified by their ability to confer cellular resistance to various stress conditions. However, molecular chaperones participate also in many constitutive cellular processes. Mitochondria contain several members of the major chaperone families that have important functions in maintaining mitochondrial function. The major Hsp70 of the mitochondrial matrix (mtHsp70) is essential for the translocation of cytosolic precursor proteins across the two mitochondrial membranes. MtHsp70 interacts with the preprotein in transit in an ATP-dependent reaction as it emerges from the translocation channel of the inner membrane. Together with two essential partner proteins, Tim44 and Mge1, mtHsp70 forms a membrane-associated import motor complex responsible for vectorial polypeptide movement and unfolding of preprotein domains. Folding of newly imported proteins in the matrix is assisted by the soluble chaperone system formed by mtHsp70 and its partner protein Mdj1. For certain substrate proteins, the protected folding environment that is offered by the large oligomeric Hsp60 complex facilitates further folding reactions. The mitochondrial Hsp70 Ssq1 is involved in the assembly of mitochondrial Fe/S clusters together with another member of the DnaJ family, Jac1. Chaperones of the Clp/Hsp100 family mediate the prevention of aggregation under stress conditions and eventually the degradation of mitochondrial proteins. Together, the chaperones of the mitochondrial matrix form a complex interdependent chaperone network that is essential for most reactions of mitochondrial protein biogenesis.

The protein import motor of mitochondria

Proteins that are destined for the matrix of mitochondria are transported into this organelle by two translocases: the TOM complex, which transports proteins across the outer mitochondrial membrane; and the TIM23 complex, which gets them through the inner mitochondrial membrane. Two models have been proposed to explain how this protein-import machinery works -- a targeted Brownian ratchet, in which random motion is translated into vectorial motion, or a 'power stroke', which is exerted by a component of the import machinery. Here, we review the data for and against each model.

Mitochondrial protein import: two membranes, three translocases

Most mitochondrial proteins are synthesised in the cytosol and must be translocated across one or two membranes to reach their functional destination inside mitochondria. Dynamic protein complexes in the outer and inner membranes function as specific machineries that recognise the various kinds of precursor proteins and promote their translocation through protein-conducting channels. At least three major translocase complexes with a high flexibility and versatility are needed to ensure the proper import of precursor proteins into mitochondria.

The protein import motor of mitochondria: a targeted molecular ratchet driving unfolding and translocation

Unfolding and import of preproteins into mitochondria are facilitated by a molecular motor in which heat shock protein 70 (Hsp70) in the matrix plays an essential role. Here we present two different experimental approaches to analyze mechanisms underlying this function of Hsp70. First, preproteins containing stretches of glutamic acid (polyE) or glycine (polyG) repeats in front of folded domains were imported into mitochondria. This occurred although Hsp70 cannot pull on these stretches to unfold the folded domains, since it does not bind to polyE and polyG. Secondly, preproteins containing titin immunoglobulin (Ig)-like domains were imported into mitochondria, despite the fact that forces of >200 pN are required to mechanically unfold these domains. Since molecular motors generate forces of approximately 5 pN, Hsp70 could not promote unfolding of the Ig-like domains by mechanical pulling. Our observations suggest that Hsp70 acts as an element of a Brownian ratchet, which mediates unfolding and translocation of preproteins across the mitochondrial membranes.

The Hsp70 peptide-binding domain determines the interaction of the ATPase domain with Tim44 in mitochondria

Ssc1, a molecular chaperone of the Hsp70 family, drives preprotein import into the mitochondrial matrix by a specific interaction with the translocase component Tim44. Two other mitochondrial Hsp70s, Ssc3 (Ecm10) and Ssq1, show high sequence homology to Ssc1 but fail to replace Ssc1 in vivo, possibly due to their inability to interact with Tim44. We analyzed the structural basis of the Tim44 interaction by the construction of chimeric Hsp70 proteins. The ATPase domains of all three mitochondrial Hsp70s were shown to bind to Tim44, supporting the active motor model for the Hsp70 mechanism during preprotein translocation. The peptide-binding domain of Ssc1 sustained binding of Tim44, while the peptide-binding domains of Ssc3 and Ssq1 exerted a negative effect on the interaction of the ATPase domains with Tim44. A mutation in the peptide-binding domain of Ssc1 resulted in a similar negative effect not only on the ATPase domain of Ssc1, but also of Ssq1 and Ssc3. Hence, the determination of a crucial Hsp70 function via the peptide-binding domain suggests a new regulatory principle for Hsp70 domain cooperation.