Mitochondrial GrpE modulates the function of matrix Hsp70 in translocation and maturation of preproteins

Pam16 and Pam18 were repurposed during Trypanosoma brucei evolution to regulate the replication of mitochondrial DNA

Protein import and genome replication are essential processes for mitochondrial biogenesis and propagation. The J-domain proteins Pam16 and Pam18 regulate the presequence translocase of the mitochondrial inner membrane. In the protozoan Trypanosoma brucei, their counterparts are TbPam16 and TbPam18, which are essential for the procyclic form (PCF) of the parasite, though not involved in mitochondrial protein import. Here, we show that during evolution, the 2 proteins have been repurposed to regulate the replication of maxicircles within the intricate kDNA network, the most complex mitochondrial genome known. TbPam18 and TbPam16 have inactive J-domains suggesting a function independent of heat shock proteins. However, their single transmembrane domain is essential for function. Pulldown of TbPam16 identifies a putative client protein, termed MaRF11, the depletion of which causes the selective loss of maxicircles, akin to the effects observed for TbPam18 and TbPam16. Moreover, depletion of the mitochondrial proteasome results in increased levels of MaRF11. Thus, we have discovered a protein complex comprising TbPam18, TbPam16, and MaRF11, that controls maxicircle replication. We propose a working model in which the matrix protein MaRF11 functions downstream of the 2 integral inner membrane proteins TbPam18 and TbPam16. Moreover, we suggest that the levels of MaRF11 are controlled by the mitochondrial proteasome.

Intermembrane space-localized TbTim15 is an essential subunit of the single mitochondrial inner membrane protein translocase of trypanosomes

All mitochondria import >95% of their proteins from the cytosol. This process is mediated by protein translocases in the mitochondrial membranes, whose subunits are generally highly conserved. Most eukaryotes have two inner membrane protein translocases (TIMs) that are specialized to import either presequence-containing or mitochondrial carrier proteins. In contrast, the parasitic protozoan Trypanosoma brucei has a single TIM complex consisting of one conserved and five unique subunits. Here, we identify candidates for new subunits of the TIM or the presequence translocase-associated motor (PAM) using a protein-protein interaction network of previously characterized TIM and PAM subunits. This analysis reveals that the trypanosomal TIM complex contains an additional trypanosomatid-specific subunit, designated TbTim15. TbTim15 is associated with the TIM complex, lacks transmembrane domains, and localizes to the intermembrane space. TbTim15 is essential for procyclic and bloodstream forms of trypanosomes. It contains two twin CX9C motifs and mediates import of both presequence-containing and mitochondrial carrier proteins. While the precise function of TbTim15 in mitochondrial protein import is unknown, our results are consistent with the notion that it may function as an import receptor for the non-canonical trypanosomal TIM complex.

Nucleotide Exchange Factors for Hsp70 Molecular Chaperones: GrpE, Hsp110/Grp170, HspBP1/Sil1, and BAG Domain Proteins

Molecular chaperones of the Hsp70 family are key components of the cellular protein-folding machinery. Substrate folding is accomplished by iterative cycles of ATP binding, hydrolysis, and release. The ATPase activity of Hsp70 is regulated by two main classes of cochaperones: J-domain proteins stimulate ATPase hydrolysis by Hsp70, while nucleotide exchange factors (NEFs) facilitate the conversion from the ADP-bound to the ATP-bound state, thus closing the chaperone folding cycle. NEF function can additionally be antagonized by ADP dissociation inhibitors. Beginning with the discovery of the prototypical bacterial NEF, GrpE, a large diversity of nucleotide exchange factors for Hsp70 have been identified, connecting it to a multitude of cellular processes in the eukaryotic cell. Here we review recent advances toward structure and function of nucleotide exchange factors from the Hsp110/Grp170, HspBP1/Sil1, and BAG domain protein families and discuss how these cochaperones connect protein folding with cellular quality control and degradation pathways.

Loss of Function of mtHsp70 Chaperone Variants Leads to Mitochondrial Dysfunction in Congenital Sideroblastic Anemia

Congenital Sideroblastic Anemias (CSA) is a group of rare genetic disorders characterized by the abnormal accumulation of iron in erythrocyte precursors. A common hallmark underlying these pathological conditions is mitochondrial dysfunction due to altered protein homeostasis, heme biosynthesis, and oxidative phosphorylation. A clinical study on congenital sideroblastic anemia has identified mutations in mitochondrial Hsp70 (mtHsp70/Mortalin). Mitochondrial Hsp70 plays a critical role in maintaining mitochondrial function by regulating several pathways, including protein import and folding, and iron-sulfur cluster synthesis. Owing to the structural and functional homology between human and yeast mtHsp70, we have utilized the yeast system to delineate the role of mtHsp70 variants in the etiology of CSA’s. Analogous mutations in yeast mtHsp70 exhibited temperature-sensitive growth phenotypes under non-respiratory and respiratory conditions. In vivo analyses indicate a perturbation in mitochondrial mass and functionality accompanied by an alteration in the organelle network and cellular redox levels. Preliminary in vitro biochemical studies of mtHsp70 mutants suggest impaired import function, altered ATPase activity and substrate interaction. Together, our findings suggest the loss of chaperone activity to be a pivotal factor in the pathophysiology of congenital sideroblastic anemia.

GRPEL2 Knockdown Exerts Redox Regulation in Glioblastoma

Malignant brain tumors are responsible for catastrophic morbidity and mortality globally. Among them, glioblastoma multiforme (GBM) bears the worst prognosis. The GrpE-like 2 homolog (GRPEL2) plays a crucial role in regulating mitochondrial protein import and redox homeostasis. However, the role of GRPEL2 in human glioblastoma has yet to be clarified. In this study, we investigated the function of GRPEL2 in glioma. Based on bioinformatics analyses from the Cancer Gene Atlas (TCGA) and the Chinese Glioma Genome Atlas (CGGA), we inferred that GRPEL2 expression positively correlates with WHO tumor grade (p < 0.001), IDH mutation status (p < 0.001), oligodendroglial differentiation (p < 0.001), and overall survival (p < 0.001) in glioma datasets. Functional validation in LN229 and GBM8401 GBM cells showed that GRPEL2 knockdown efficiently inhibited cellular proliferation. Moreover, GRPEL2 suppression induced cell cycle arrest at the sub-G1 phase. Furthermore, GRPEL2 silencing decreased intracellular reactive oxygen species (ROS) without impending mitochondria membrane potential. The cellular oxidative respiration measured with a Seahorse XFp analyzer exhibited a reduction of the oxygen consumption rate (OCR) in GBM cells by siGRPEL2, which subsequently enhanced autophagy and senescence in glioblastoma cells. Taken together, GRPEL2 is a novel redox regulator of mitochondria bioenergetics and a potential target for treating GBM in the future.

How to get to the other side of the mitochondrial inner membrane – the protein import motor

Biogenesis of mitochondria relies on import of more than 1000 different proteins from the cytosol. Approximately 70% of these proteins follow the presequence pathway – they are synthesized with cleavable N-terminal extensions called presequences and reach the final place of their function within the organelle with the help of the TOM and TIM23 complexes in the outer and inner membranes, respectively. The translocation of proteins along the presequence pathway is powered by the import motor of the TIM23 complex. The import motor of the TIM23 complex is localized at the matrix face of the inner membrane and is likely the most complicated Hsp70-based system identified to date. How it converts the energy of ATP hydrolysis into unidirectional translocation of proteins into mitochondria remains one of the biggest mysteries of this translocation pathway. Here, the knowns and the unknowns of the mitochondrial protein import motor are discussed.

Biochemical Convergence of Mitochondrial Hsp70 System Specialized in Iron-Sulfur Cluster Biogenesis

Mitochondria play a central role in the biogenesis of iron-sulfur cluster(s) (FeS), protein cofactors needed for many cellular activities. After assembly on scaffold protein Isu, the cluster is transferred onto a recipient apo-protein. Transfer requires Isu interaction with an Hsp70 chaperone system that includes a dedicated J-domain protein co-chaperone (Hsc20). Hsc20 stimulates Hsp70's ATPase activity, thus stabilizing the critical Isu-Hsp70 interaction. While most eukaryotes utilize a multifunctional mitochondrial (mt)Hsp70, yeast employ another Hsp70 (Ssq1), a product of mtHsp70 gene duplication. Ssq1 became specialized in FeS biogenesis, recapitulating the process in bacteria, where specialized Hsp70 HscA cooperates exclusively with an ortholog of Hsc20. While it is well established that Ssq1 and HscA converged functionally for FeS transfer, whether these two Hsp70s possess similar biochemical properties was not known. Here, we show that overall HscA and Ssq1 biochemical properties are very similar, despite subtle differences being apparent - the ATPase activity of HscA is stimulated to a somewhat higher levels by Isu and Hsc20, while Ssq1 has a higher affinity for Isu and for Hsc20. HscA/Ssq1 are a unique example of biochemical convergence of distantly related Hsp70s, with practical implications, crossover experimental results can be combined, facilitating understanding of the FeS transfer process.

Regulation of mitochondrial protein import by the nucleotide exchange factors GrpEL1 and GrpEL2 in human cells

Mitochondria are organelles indispensable for maintenance of cellular energy homeostasis. Most mitochondrial proteins are nuclearly encoded and are imported into the matrix compartment where they are properly folded. This process is facilitated by the mitochondrial heat shock protein 70 (mtHsp70), a chaperone contributing to mitochondrial protein quality control. The affinity of mtHsp70 for its protein clients and its chaperone function are regulated by binding of ATP/ADP to mtHsp70's nucleotide-binding domain. Nucleotide exchange factors (NEFs) play a crucial role in exchanging ADP for ATP at mtHsp70's nucleotide-binding domain, thereby modulating mtHsp70's chaperone activity. A single NEF, Mge1, regulates mtHsp70's chaperone activity in lower eukaryotes, but the mammalian orthologs are unknown. Here, we report that two putative NEF orthologs, GrpE-like 1 (GrpEL1) and GrpEL2, modulate mtHsp70's function in human cells. We found that both GrpEL1 and GrpEL2 associate with mtHsp70 as a hetero-oligomeric subcomplex and regulate mtHsp70 function. The formation of this subcomplex was critical for conferring stability to the NEFs, helped fine-tune mitochondrial protein quality control, and regulated crucial mtHsp70 functions, such as import of preproteins and biogenesis of Fe-S clusters. Our results also suggested that GrpEL2 has evolved as a possible stress resistance protein in higher vertebrates to maintain chaperone activity under stress conditions. In conclusion, our findings support the idea that GrpEL1 has a role as a stress modulator in mammalian cells and highlight that multiple NEFs are involved in controlling protein quality in mammalian mitochondria.

Mitochondrial Cochaperone Mge1 Is Involved in Regulating Susceptibility to Fluconazole in Saccharomyces cerevisiae and Candida Species

MGE1 encodes a yeast chaperone involved in Fe-S cluster metabolism and protein import into the mitochondria. In this study, we identified MGE1 as a multicopy suppressor of susceptibility to the antifungal fluconazole in the model yeast Saccharomyces cerevisiae. We demonstrate that this phenomenon is not exclusively dependent on the integrity of the mitochondrial DNA or on the presence of the drug efflux pump Pdr5. Instead, we show that the increased dosage of Mge1 plays a protective role by retaining increased amounts of ergosterol upon fluconazole treatment. Iron metabolism and, more particularly, Fe-S cluster formation are involved in regulating this process, since the responsible Hsp70 chaperone, Ssq1, is required. Additionally, we show the necessity but, by itself, insufficiency of activating the iron regulon in establishing the Mge1-related effect on drug susceptibility. Finally, we confirm a similar role for Mge1 in fluconazole susceptibility in the pathogenic fungi Candida glabrata and Candida albicans.

Although they are mostly neglected compared to bacterial infections, fungal infections pose a serious threat to the human population. While some of them remain relatively harmless, infections that reach the bloodstream often become lethal. Only a few therapies are available, and resistance of the pathogen to these drugs is a frequently encountered problem. It is thus essential that more research is performed on how these pathogens cope with the treatment and cause recurrent infections. Baker’s yeast is often used as a model to study pathogenic fungi. We show here, by using this model, that iron metabolism and the formation of the important iron-sulfur clusters are involved in regulating susceptibility to fluconazole, the most commonly used antifungal drug. We show that the same process likely also occurs in two of the most regularly isolated pathogenic fungi, Candida glabrata and Candida albicans.

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.

The Genomes of Three Uneven Siblings: Footprints of the Lifestyles of Three Trichoderma Species

The genus Trichoderma contains fungi with high relevance for humans, with applications in enzyme production for plant cell wall degradation and use in biocontrol. Here, we provide a broad, comprehensive overview of the genomic content of these species for "hot topic" research aspects, including CAZymes, transport, transcription factors, and development, along with a detailed analysis and annotation of less-studied topics, such as signal transduction, genome integrity, chromatin, photobiology, or lipid, sulfur, and nitrogen metabolism in T. reesei, T. atroviride, and T. virens, and we open up new perspectives to those topics discussed previously. In total, we covered more than 2,000 of the predicted 9,000 to 11,000 genes of each Trichoderma species discussed, which is >20% of the respective gene content. Additionally, we considered available transcriptome data for the annotated genes. Highlights of our analyses include overall carbohydrate cleavage preferences due to the different genomic contents and regulation of the respective genes. We found light regulation of many sulfur metabolic genes. Additionally, a new Golgi 1,2-mannosidase likely involved in N-linked glycosylation was detected, as were indications for the ability of Trichoderma spp. to generate hybrid galactose-containing N-linked glycans. The genomic inventory of effector proteins revealed numerous compounds unique to Trichoderma, and these warrant further investigation. We found interesting expansions in the Trichoderma genus in several signaling pathways, such as G-protein-coupled receptors, RAS GTPases, and casein kinases. A particularly interesting feature absolutely unique to T. atroviride is the duplication of the alternative sulfur amino acid synthesis pathway.

GrpE, Hsp110/Grp170, HspBP1/Sil1 and BAG domain proteins: nucleotide exchange factors for Hsp70 molecular chaperones

Molecular chaperones of the Hsp70 family are key components of the cellular protein folding machinery. Substrate folding is accomplished by iterative cycles of ATP binding, hydrolysis and release. The ATPase activity of Hsp70 is regulated by two main classes of cochaperones: J-domain proteins stimulate ATPase hydrolysis by Hsp70, while nucleotide exchange factors (NEF) facilitate its conversion from the ADP-bound to the ATP-bound state, thus closing the chaperone folding cycle. Beginning with the discovery of the prototypical bacterial NEF GrpE, a large diversity of Hsp70 nucleotide exchange factors has been identified, connecting Hsp70 to a multitude of cellular processes in the eukaryotic cell. Here we review recent advances towards structure and function of nucleotide exchange factors from the Hsp110/Grp170, HspBP1/Sil1 and BAG domain protein families and discuss how these cochaperones connect protein folding with quality control and degradation pathways.

Mitochondrial heat shock protein (Hsp) 70 and Hsp10 cooperate in the formation of Hsp60 complexes

Mitochondrial Hsp70 (mtHsp70) mediates essential functions for mitochondrial biogenesis, like import and folding of proteins. In these processes, the chaperone cooperates with cochaperones, the presequence translocase, and other chaperone systems. The chaperonin Hsp60, together with its cofactor Hsp10, catalyzes folding of a subset of mtHsp70 client proteins. Hsp60 forms heptameric ring structures that provide a cavity for protein folding. How the Hsp60 rings are assembled is poorly understood. In a comprehensive interaction study, we found that mtHsp70 associates with Hsp60 and Hsp10. Surprisingly, mtHsp70 interacts with Hsp10 independently of Hsp60. The mtHsp70-Hsp10 complex binds to the unassembled Hsp60 precursor to promote its assembly into mature Hsp60 complexes. We conclude that coupling to Hsp10 recruits mtHsp70 to mediate the biogenesis of the heptameric Hsp60 rings.

The evolution and function of co-chaperones in mitochondria

Mitochondrial chaperones mediate and affect critical organellar processes, essential for cellular function. These chaperone systems have both prokaryotic and eukaryotic features. While some of the mitochondrial co-chaperones have clear homologues in prokaryotes, some are unique to eukaryotes and have no homologues in the chaperone machinery of other cellular compartments. The mitochondrial co-chaperones are required for protein import into the organelle and in enforcing the structure of the main chaperones. In addition to novel types of interaction with their senior partners, unexpected and essential interactions between the co-chaperones themselves have recently been described.

Methionine sulfoxide reductase 2 reversibly regulates Mge1, a cochaperone of mitochondrial Hsp70, during oxidative stress

Methionine sulfoxide reductases are important regulators of oxidative stress, as they reduce oxidized methionine in proteins. Mge1, a cochaperone of mtHsp70, is a physiological substrate of Mxr2 and regulates reversibly to maintain mitochondrial protein homeostasis and oxidative stress.

Peptide methionine sulfoxide reductases are conserved enzymes that reduce oxidized methionines in protein(s). Although these reductases have been implicated in several human diseases, there is a dearth of information on the identity of their physiological substrates. By using Saccharomyces cerevisiae as a model, we show that of the two methionine sulfoxide reductases (MXR1, MXR2), deletion of mitochondrial MXR2 renders yeast cells more sensitive to oxidative stress than the cytosolic MXR1. Our earlier studies showed that Mge1, an evolutionarily conserved nucleotide exchange factor of Hsp70, acts as an oxidative sensor to regulate mitochondrial Hsp70. In the present study, we show that Mxr2 regulates Mge1 by selectively reducing MetO at position 155 and restores the activity of Mge1 both in vitro and in vivo. Mge1 M155L mutant rescues the slow-growth phenotype and aggregation of proteins of mxr2Δ strain during oxidative stress. By identifying the first mitochondrial substrate for Mxrs, we add a new paradigm to the regulation of the oxidative stress response pathway.

9-O-butyl-13-(4-isopropylbenzyl)berberine, KR-72, is a potent antifungal agent that inhibits the growth of Cryptococcus neoformans by regulating gene expression

In this study we explored the mode of action of KR-72, a 9-O-butyl-13-(4-isopropylbenzyl)berberine derivative previously shown to exhibit potent antifungal activity against a variety of human fungal pathogens. The DNA microarray data revealed that KR-72 treatment significantly changed the transcription profiles of C. neoformans, affecting the expression of more than 2,000 genes. Genes involved in translation and transcription were mostly upregulated, whereas those involved in the cytoskeleton, intracellular trafficking, and lipid metabolism were downregulated. KR-72 also exhibited a strong synergistic effect with the antifungal agent FK506. KR-72 treatment regulated the expression of several essential genes, including ECM16, NOP14, HSP10 and MGE1, which are required for C. neoformans growth. The KR-72-mediated induction of MGE1 also likely reduced the viability of C. neoformans by impairing cell cycle or the DNA repair system. In conclusion, KR-72 showed antifungal activity by modulating diverse biological processes through a mode of action distinct from those of clinically available antifungal drugs such as polyene and azole drugs.

HSPA8/HSC70 chaperone protein: structure, function, and chemical targeting

HSPA8/HSC70 protein is a fascinating chaperone protein. It represents a constitutively expressed, cognate protein of the HSP70 family, which is central in many cellular processes. In particular, its regulatory role in autophagy is decisive. We focused this review on HSC70 structure-function considerations and based on this, we put a particular emphasis on HSC70 targeting by small molecules and peptides in order to develop intervention strategies that deviate some of HSC70 properties for therapeutic purposes. Generating active biomolecules regulating autophagy via its effect on HSC70 can effectively be designed only if we understand the fine relationships between HSC70 structure and functions.

Mge1, a nucleotide exchange factor of Hsp70, acts as an oxidative sensor to regulate mitochondrial Hsp70 function

Yeast Mge1, the cochaperone of mitochondrial heat shock protein 70 (mHsp70), is essential for exchanging ATP for ADP on mHsp70 and thus for recycling of mHsp70 for mitochondrial protein import and folding. Mge1 acts as an oxidative sensor to regulate mHsp70 function.

Despite the growing evidence of the role of oxidative stress in disease, its molecular mechanism of action remains poorly understood. The yeast Saccharomyces cerevisiae provides a valuable model system in which to elucidate the effects of oxidative stress on mitochondria in higher eukaryotes. Dimeric yeast Mge1, the cochaperone of heat shock protein 70 (Hsp70), is essential for exchanging ATP for ADP on Hsp70 and thus for recycling of Hsp70 for mitochondrial protein import and folding. Here we show an oxidative stress–dependent decrease in Mge1 dimer formation accompanied by a concomitant decrease in Mge1–Hsp70 complex formation in vitro. The Mge1-M155L substitution mutant stabilizes both Mge1 dimer and Mge1–Hsp70 complex formation. Most important, the Mge1-M155L mutant rescues the slow-growth phenomenon associated with the wild-type Mge1 strain in the presence of H₂O₂ in vivo, stimulation of the ATPase activity of Hsp70, and the protein import defect during oxidative stress in vitro. Furthermore, cross-linking studies reveal that Mge1–Hsp70 complex formation in mitochondria isolated from wild-type Mge1 cells is more susceptible to reactive oxygen species compared with mitochondria from Mge1-M155L cells. This novel oxidative sensor capability of yeast Mge1 might represent an evolutionarily conserved function, given that human recombinant dimeric Mge1 is also sensitive to H₂O₂.

Comparative analysis of hemolymph proteome maps in diapausing and non-diapausing larvae of Sesamia nonagrioides

Background

Sesamia nonagrioides is a noctuid that feeds on maize, sugar cane and sorghum in North Africa and Southern Europe. Larvae reared under long day conditions pupate after 5 or 6 larval instars, whereas larvae reared under short day conditions enter diapause and undergo up to 12 molts before dying or pupating. To better understand the mechanism of larval development and diapause, we identified proteins with different expressions in the sixth instar of diapausing and non-diapausing larvae.

Results

A total of 52 differentially regulated proteins were detected in the hemolymph of the diapausing or non-diapausing larvae at the beginning or end of the sixth instar. From these proteins, 11 were identified by mass spectrometry (MALDI-TOF MS or MALDI-TOF/TOF MS/MS): 5 were upregulated in the hemolymph of non-diapausing larvae and 6 in the hemolymph of the diapausing larvae. Interestingly, some proteins were expressed only in non-diapausing larvae but none was expressed only in the hemolymph of diapausing larvae. The possible functions of some of these proteins related to diapause maintenance or to larval-pupal metamorphosis are discussed.

Conclusions

The 2-DE proteomic map of S. nonagrioides hemolymph shows differential protein expression in diapausing and non-diapausing larvae. Some proteins that showed higher expression in the diapausing larvae at the end of the sixth instar could be involved in JH level maintenance thus in the diapause status maintenance. On the contrary, other proteins that showed the highest expression or that were expressed only in the non-diapausing larvae could be involved in larval-pupal metamorphosis.

Recent gene duplication and subfunctionalization produced a mitochondrial GrpE, the nucleotide exchange factor of the Hsp70 complex, specialized in thermotolerance to chronic heat stress in Arabidopsis

The duplication and divergence of heat stress (HS) response genes might help plants adapt to varied HS conditions, but little is known on the topic. Here, we examined the evolution and function of Arabidopsis (Arabidopsis thaliana) mitochondrial GrpE (Mge) proteins. GrpE acts as a nucleotide-exchange factor in the Hsp70/DnaK chaperone machinery. Genomic data show that AtMge1 and AtMge2 arose from a recent whole-genome duplication event. Phylogenetic analysis indicated that duplication and preservation of Mges occurred independently in many plant species, which suggests a common tendency in the evolution of the genes. Intron retention contributed to the divergence of the protein structure of Mge paralogs in higher plants. In both Arabidopsis and tomato (Solanum lycopersicum), Mge1 is induced by ultraviolet B light and Mge2 is induced by heat, which suggests regulatory divergence of the genes. Consistently, AtMge2 but not AtMge1 is under the control of HsfA1, the master regulator of the HS response. Heterologous expression of AtMge2 but not AtMge1 in the temperature-sensitive Escherichia coli grpE mutant restored its growth at 43°C. Arabidopsis T-DNA knockout lines under different HS regimes revealed that Mge2 is specifically required for tolerating prolonged exposure to moderately high temperature, as compared with the need of the heat shock protein 101 and the HS-associated 32-kD protein for short-term extreme heat. Therefore, with duplication and subfunctionalization, one copy of the Arabidopsis Mge genes became specialized in a distinct type of HS. We provide direct evidence supporting the connection between gene duplication and adaptation to environmental stress.

The HSP70 chaperone machinery: J proteins as drivers of functional specificity

Heat shock 70 kDa proteins (HSP70s) are ubiquitous molecular chaperones that function in a myriad of biological processes, modulating polypeptide folding, degradation and translocation across membranes, and protein-protein interactions. This multitude of roles is not easily reconciled with the universality of the activity of HSP70s in ATP-dependent client protein-binding and release cycles. Much of the functional diversity of the HSP70s is driven by a diverse class of cofactors: J proteins. Often, multiple J proteins function with a single HSP70. Some target HSP70 activity to clients at precise locations in cells and others bind client proteins directly, thereby delivering specific clients to HSP70 and directly determining their fate.

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.

Maintenance and expression of the S. cerevisiae mitochondrial genome--from genetics to evolution and systems biology

As a legacy of their endosymbiotic eubacterial origin, mitochondria possess a residual genome, encoding only a few proteins and dependent on a variety of factors encoded by the nuclear genome for its maintenance and expression. As a facultative anaerobe with well understood genetics and molecular biology, Saccharomyces cerevisiae is the model system of choice for studying nucleo-mitochondrial genetic interactions. Maintenance of the mitochondrial genome is controlled by a set of nuclear-coded factors forming intricately interconnected circuits responsible for replication, recombination, repair and transmission to buds. Expression of the yeast mitochondrial genome is regulated mostly at the post-transcriptional level, and involves many general and gene-specific factors regulating splicing, RNA processing and stability and translation. A very interesting aspect of the yeast mitochondrial system is the relationship between genome maintenance and gene expression. Deletions of genes involved in many different aspects of mitochondrial gene expression, notably translation, result in an irreversible loss of functional mtDNA. The mitochondrial genetic system viewed from the systems biology perspective is therefore very fragile and lacks robustness compared to the remaining systems of the cell. This lack of robustness could be a legacy of the reductive evolution of the mitochondrial genome, but explanations involving selective advantages of increased evolvability have also been postulated.

Cytoprotective role of nitric oxide associated with Hsp70 expression in neonatal obstructive nephropathy

Nitric oxide (NO) has emerged as an important endogenous inhibitor of apoptosis. In this study, we postulated that the mechanism of apoptosis inhibition by NO would include stimulation of heat shock protein 70 (Hsp70) expression. Rats were subjected to unilateral ureteral obstruction (UUO) or sham operation, and kidneys were harvested 5 and 14 days after obstruction. After 14 days of obstruction, decreased endogenous NO and lower inducible nitric oxide synthase (iNOS) expression at mRNA and protein levels associated with downregulation of Hsp70 protein expression were shown in apoptosis induction, regulated by mitochondrial signal pathway, through the increased pro-apoptotic ratio Bax/BcL(2) and consequently caspase 3 activity. Conversely, 5 days after kidney obstruction, increased Hsp70 expression linked to increase NO and iNOS expression at transcriptional and post-transcriptional levels with absence of apoptotic response, were demonstrated. In obstructed neonatal rats, in vivo administration of l-Arginine induced heat shock protein 70 (Hsp70) expression, which was associated with cytoprotection from apoptosis and transiently decreased nicotinamide adenine dinucleotide phosphate reduced form (NADPH) oxidase activity. Opposite effects were obtained after nitro L-Arginine methyl ester (L-NAME) treatment. The interaction between B-cell lymphoma 2 anti-apoptotic members (BcL(2)) and Hsp70 in the presence of L-Arginine and L-NAME, was determined by coimmunoprecipitation. Binding of BcL(2) and Hsp70 increased after L-Arginine administration. These findings suggest that NO can produce resistance to obstruction-induced cell death by mitochondrial apoptotic pathway, through the induction of Hsp70 expression, in neonatal unilateral ureteral obstruction.

Thermal adaptation of the yeast mitochondrial Hsp70 system is regulated by the reversible unfolding of its nucleotide exchange factor

The Hsp70 protein switches during its functional cycle from an ADP-bound state with a high affinity for substrates to a low-affinity, ATP-bound state, with concomitant release of the client protein. The rate of the chaperone cycle is regulated by co-chaperones such as nucleotide exchange factors that significantly accelerate the ADP/ATP exchange. Mge1p, a mitochondrial matrix protein with homology to bacterial GrpE, serves as the nucleotide exchange factor of mitochondrial Hsp70. Here, we analyze the influence of temperature on the structure and functional properties of Mge1p from the yeast Saccharomyces cerevisiae. Mge1p is a dimer in solution that undergoes a reversible thermal transition at heat-shock temperatures, i.e. above 37 degrees C, that involves protein unfolding and dimer dissociation. The thermally denatured protein is unable to interact stably with mitochondrial Hsp70, and therefore is unable to regulate its ATPase and chaperone cycle. Crosslinking of wild-type mitochondria reveals that Mge1p undergoes the same dimer to monomer temperature-dependent shift, and that the nucleotide exchange factor does not associate with its Hsp70 partner at stress temperatures (i.e. > or =45 degrees C). Once the stress conditions disappear, Mge1p refolds and recovers both structure and functional properties. Therefore, Mge1p can act as a thermosensor for the mitochondrial Hsp70 system, regulating the nucleotide exchange rates under heat shock, as has been described for two bacterial GrpE proteins. The thermosensor activity is conserved in the GrpE-like nucleotide exchange factors although, as discussed here, it is achieved through a different structural mechanism.

The putative helical lid of the Hsp70 peptide-binding domain is required for efficient preprotein translocation into mitochondria

The mitochondrial Hsp70 (Ssc1) is an essential component of the preprotein import machinery, responsible for the unfolding and movement of polypeptide chains through the mitochondrial membranes into the matrix. Here, we have analyzed the role of the carboxy-terminal variable domain during the protein translocation reaction. This segment is thought to form an alpha-helical lid over the substrate binding site. Truncated Ssc1 molecules lacking parts or all of the lid region showed reduced binding to substrate proteins but were able to interact with the co-chaperone Mge1 and the inner membrane anchor Tim44. Deletions of the complete lid resulted in a lethal phenotype in vivo, caused by the inability to sustain a productive preprotein import function. The translocation defect in vitro was not overcome by artificial unfolding of the preprotein prior to the import reaction. Despite a reduced substrate affinity, the presence of a minimal lid segment in Ssc1 was sufficient to support preprotein import. However, at low reaction temperatures or low matrix ATP levels, protein import rates were significantly reduced due to an unproductive interaction with the preprotein in transit. We conclude that the carboxy-terminal domain performs a crucial role in the import process by enhancing the import motor function of Ssc1 during polypeptide translocation.

Sti1 is a novel activator of the Ssa proteins

The molecular chaperones Hsp70 and Hsp90 are involved in the folding and maturation of key regulatory proteins in eukaryotes. Of specific importance in this context is a ternary multichaperone complex in which Hsp70 and Hsp90 are connected by Hop. In Saccharomyces cerevisiae two components of the complex, yeast Hsp90 (yHsp90) and Sti1, the yeast homologue of Hop, had already been identified, but it remained to be shown which of the 14 different yeast Hsp70s are part of the Sti1 complex and what were the functional consequences resulting from this interaction. With a two-hybrid approach and co-immunoprecipitations, we show here that Sti1 specifically interacts with the Ssa group of the cytosolic yeast Hsp70 proteins. Using purified components, we reconstituted the dimeric Ssa1-Sti1 complex and the ternary Ssa1-Sti1-yHsp90 complex in vitro. The dissociation constant between Sti1 and Ssa1 was determined to be 2 orders of magnitude weaker than the affinity of Sti1 for yHsp90. Surprisingly, binding of Sti1 activates the ATPase of Ssa1 by a factor of about 200, which is in contrast to the behavior of Hop in the mammalian Hsp70 system. Analysis of the underlying activation mechanism revealed that ATP hydrolysis is rate-limiting in the Ssa1 ATPase cycle and that this step is accelerated by Sti1. Thus, Sti1 is a potent novel effector for the Hsp70 ATPase.

GrpE, a nucleotide exchange factor for DnaK

The cochaperone GrpE functions as a nucleotide exchange factor to promote dissociation of adenosine 5'-diphosphate (ADP) from the nucleotide-binding cleft of DnaK. GrpE and the DnaJ cochaperone act in concert to control the flux of unfolded polypeptides into and out of the substrate-binding domain of DnaK by regulating the nucleotide-bound state of DnaK. DnaJ stimulates nucleotide hydrolysis, and GrpE promotes the exchange of ADP for adenosine triphosphate (ATP) and also augments peptide release from the DnaK substrate-binding domain in an ATP-independent manner. The eukaryotic cytosol does not contain GrpE per se because GrpE-like function is provided by the BAG1 protein, which acts as a nucleotide exchange factor for cytosolic Hsp70s. GrpE, which plays a prominent role in mitochondria, chloroplasts, and bacterial cytoplasms, is a fascinating molecule with an unusual quaternary structure. The long alpha-helices of GrpE have been hypothesized to act as a thermosensor and to be involved in the decrease in GrpE-dependent nucleotide exchange that is observed in vitro at temperatures relevant to heat shock. This review describes the molecular biology of GrpE and focuses on the structural and kinetic aspects of nucleotide exchange, peptide release, and the thermosensor hypothesis.

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.

The two mitochondrial heat shock proteins 70, Ssc1 and Ssq1, compete for the cochaperone Mge1

Two members of the heat shock protein 70 kDa (Hsp70) family, Ssc1 and Ssq1, perform important functions in the mitochondrial matrix. The essential Ssc1 is an abundant ATP-binding protein required for both import and folding of mitochondrial proteins. The function of Ssc1 is supported by an interaction with the preprotein translocase subunit Tim44, the cochaperone Mdj1, and the nucleotide exchange factor Mge1. In contrast, only limited information is available on Ssq1. So far, a basic characterization of Ssq1 has demonstrated its involvement in the maintenance of mitochondrial DNA, the maturation of the yeast frataxin (Yfh1) after import, and assembly of the mitochondrial Fe/S cluster. Here, we analyzed the biochemical properties and the interaction partners of Ssq1 in detail. Ssq1 showed typical chaperone properties by binding to unfolded substrate proteins in an ATP-regulated manner. Ssq1 was able to form a specific complex with the nucleotide exchange factor Mge1. In particular, complex formation in organello was enhanced significantly when Ssc1 was inactivated selectively. However, even under these conditions, no interaction of Ssq1 with the two other mitochondrial Hsp70-cochaperones, Tim44 and Mdj1, was observed. The Ssq1-Mge1 interaction showed a lower overall stability but the same characteristic nucleotide-dependence as the Ssc1-Mge1 interaction. A quantitative analysis of the interaction properties indicated a competition of Ssq1 with Ssc1 for binding to Mge1. Perturbation of Mge1 function or amounts resulted in direct effects on Ssq1 activity in intact mitochondria. We conclude that mitochondria represent the unique case where two Hsp70s compete for the interaction with one nucleotide exchange factor.

Transport of proteins into mitochondria

Most mitochondrial proteins are nuclear-encoded and synthesised as preproteins on polysomes in the cytosol. They must be targeted to and translocated into mitochondria. Newly synthesised preproteins interact with cytosolic factors until their recognition by receptors on the surface of mitochondria. Import into or across the outer membrane is mediated by a dynamic protein complex coined the translocase of the outer membrane (TOM). Preproteins that are imported into the matrix or inner membrane of mitochondria require the action of one of two translocation complexes of the inner membrane (TIMs). The import pathway of preproteins is predetermined by their intrinsic targeting and sorting signals. Energy input in the form of ATP and the electrical gradient across the inner membrane is required for protein translocation into mitochondria. Newly imported proteins may require molecular chaperones for their correct folding.

Import of proteins into mitochondria: a novel pathomechanism for progressive neurodegeneration

The vast majority of mitochondrial proteins are encoded as precursors by the nuclear genome. A major aspect of mitochondrial biogenesis is therefore the transfer of nuclear-encoded, cytosplasmically synthesized precursor proteins across and into the mitochondrial membranes. During the past years the use of simple model organisms such as the yeasts S. cerevisiae and N. crassa has helped considerably to identify and unravel the structure and function of a substantial number of components involved in targeting of nuclear-encoded preproteins to mitochondria. Several pathways and a number of components were characterized that are involved in guiding mitochondrial preproteins to their specific sites of function. In particular, import of nuclear-encoded precursor proteins into and across the mitochondrial inner membrane is mediated by two distinct translocases, the TIM23 complex and the TIM22 complex. Both TIM complexes cooperate with the general preprotein translocase of the outer membrane, TOM complex. The TIM complexes differ in the their substrate specificity. While the TIM23 complex mediates import of preproteins with a positively charged matrix targeting signal, the TIM22 complex facilitates the insertion of a class of hydrophobic proteins with internal targeting signals into the inner membrane. Most recently the rapid progress of research has allowed elucidation of a new mitochondrial disease on the molecular level. This rare X-linked progressive neurodegenerative disorder, named Mohr-Tranebjaerg (MT syndrome), is caused by mutations in the DDP1 gene and includes sensorineural deafness, blindness, mental retardation and a complex movement disorder. The analysis of the novel pathomechanism is based on the homology of the affected DDP1 protein to a family of conserved yeast components acting along the TIM22 pathway. This contribution briefly summarizes the current knowledge of the pathways of protein import and proposes a mechanism to explain how defective import leads to neurodegeneration.

The mitochondrial proteins Ssq1 and Jac1 are required for the assembly of iron sulfur clusters in mitochondria

Mitochondria of the yeast Saccharomyces cerevisiae contain three different Hsp70 chaperones, Ssc1, Ecm10 and Ssq1. Ssc1 is an essential protein that mediates the import of nuclear-encoded proteins into the organelle and their subsequent folding. The nucleotide state of Ssc1 is thereby regulated by the nucleotide exchange factor Mge1. Here, we show that Mge1 interacts with Ssq1 in an ATP-dependent manner, suggesting that Mge1 also regulates Ssq1 function. In contrast to Ssc1, Ssq1 does not associate with the Tim44 subunit of the protein translocating complex, indicating a different function of both chaperones. Mutants in Ssq1 were reported to have low levels of iron sulfur (FeS) cluster-containing enzymes. Employing an assay that allowed us to monitor the conversion of the apoform of mitochondrial ferredoxin into its FeS-containing holoform, Ssq1 was demonstrated to be required for the FeS cluster assembly in mitochondria. The mitochondrial DnaJ homolog Jac1 is crucial for this process, whereas Mdj1 function is dispensable. Furthermore, the presence of frataxin is necessary for FeS cluster assembly into ferredoxin suggesting a role for frataxin at the level of the formation of holo-ferredoxin.

The mitochondrial protein import motor

Mitochondrial proteins are synthesized as precursor proteins in the cytosol and are posttranslationally imported into the organelle. A complex system of translocation machineries recognizes and transports the precursor polypeptide across the mitochondrial membranes. Energy for the translocation process is mainly supplied by the mitochondrial membrane potential (deltapsi) and the hydrolysis of ATP. Mitochondrial Hsp70 (mtHsp70) has been identified as the major ATPase driving the membrane transport of the precursor polypeptides into the mitochondrial matrix. Together with the partner proteins Tim44 and Mge1, mtHsp70 forms an import motor complex interacting with the incoming preproteins at the inner face of the inner membrane. This import motor complex drives the movement of the polypeptides in the translocation channel and the unfolding of carboxy-terminal parts of the preproteins on the outside of the outer membrane. Two models of the molecular mechanism of mtHsp70 during polypeptide translocation are discussed. In the 'trapping' model, precursor movement is generated by Brownian movement of the polypeptide chain in the translocation pore. This random movement is made vectorial by the interaction with mtHsp70 in the matrix. The detailed characterization of conditional mutants of the import motor complex provides the basis for an extended model. In this 'pulling' model, the attachment of mtHsp70 at the inner membrane via Tim44 and a conformational change induced by ATP results in the generation of an inward-directed force on the bound precursor polypeptide. This active role of the import motor complex is necessary for the translocation of proteins containing tightly folded domains. We suggest that both mechanisms complement each other to reach a high efficiency of preprotein import.

Maintenance and integrity of the mitochondrial genome: a plethora of nuclear genes in the budding yeast

Instability of the mitochondrial genome (mtDNA) is a general problem from yeasts to humans. However, its genetic control is not well documented except in the yeast Saccharomyces cerevisiae. From the discovery, 50 years ago, of the petite mutants by Ephrussi and his coworkers, it has been shown that more than 100 nuclear genes directly or indirectly influence the fate of the rho(+) mtDNA. It is not surprising that mutations in genes involved in mtDNA metabolism (replication, repair, and recombination) can cause a complete loss of mtDNA (rho(0) petites) and/or lead to truncated forms (rho(-)) of this genome. However, most loss-of-function mutations which increase yeast mtDNA instability act indirectly: they lie in genes controlling functions as diverse as mitochondrial translation, ATP synthase, iron homeostasis, fatty acid metabolism, mitochondrial morphology, and so on. In a few cases it has been shown that gene overexpression increases the levels of petite mutants. Mutations in other genes are lethal in the absence of a functional mtDNA and thus convert this petite-positive yeast into a petite-negative form: petite cells cannot be recovered in these genetic contexts. Most of the data are explained if one assumes that the maintenance of the rho(+) genome depends on a centromere-like structure dispensable for the maintenance of rho(-) mtDNA and/or the function of mitochondrially encoded ATP synthase subunits, especially ATP6. In fact, the real challenge for the next 50 years will be to assemble the pieces of this puzzle by using yeast and to use complementary models, especially in strict aerobes.

Protein translocation into mitochondria: the role of TIM complexes

Import of nuclear-encoded mitochondrial preproteins is mediated by a general translocase in the outer membrane, the TOM complex, and by two distinct translocases in the mitochondrial inner membrane, the TIM23 complex and the TIM22 complex. Both TIM complexes cooperate with the TOM complex but facilitate import of different classes of precursor proteins. Precursors with an N-terminal presequence are imported via the TIM23 complex, whereas mitochondrial carrier proteins require the TIM22 complex for insertion into the inner membrane. This review discusses recent advances in understanding the structure and function of the translocases of the inner membrane and the possible role of Tim proteins in the development of the Mohr-Tranebjaerg syndrome, a mitochondrial disorder leading to neurodegeneration.

Mechanisms of protein translocation into mitochondria

Mitochondrial biogenesis utilizes a complex proteinaceous machinery for the import of cytosolically synthesized preproteins. At least three large multisubunit protein complexes, one in the outer membrane and two in the inner membrane, have been identified. These translocase complexes cooperate with soluble proteins from the cytosol, the intermembrane space and the matrix. The translocation of presequence-containing preproteins through the outer membrane channel includes successive electrostatic interactions of the charged mitochondrial targeting sequence with a chain of import components. Translocation across the inner mitochondrial membrane utilizes the energy of the proton motive force of the inner membrane and the hydrolysis of ATP. The matrix chaperone system of the mitochondrial heat shock protein 70 forms an ATP-dependent import motor by interaction with the polypeptide chain in transit and components of the inner membrane translocase. The precursors of integral inner membrane proteins of the metabolite carrier family interact with newly identified import components of the intermembrane space and are inserted into the inner membrane by a second translocase complex. A comparison of the full set of import components between the yeast Sacccharomyces cerevisiae and the nematode Caenorhabditis elegans demonstrates an evolutionary conservation of most components of the mitochondrial import machinery with a possible greater divergence for the import pathway of the inner membrane carrier proteins.

Intragenic suppressors of Hsp70 mutants: interplay between the ATPase- and peptide-binding domains

ATP hydrolysis and polypeptide binding, the two key activities of Hsp70 molecular chaperones, are inherent properties of different domains of the protein. The coupling of these two activities is critical because the bound nucleotide determines, in part, the affinity of Hsp70s for protein substrate. In addition, cochaperones of the Hsp40 (DnaJ) class, which stimulate Hsp70 ATPase activity, have been proposed to play an important role in promoting efficient Hsp70 substrate binding. Because little is understood about this functional interaction between domains of Hsp70s, we investigated mutations in the region encoding the ATPase domain that acted as intragenic suppressors of a lethal mutation (I485N) mapping to the peptide-binding domain of the mitochondrial Hsp70 Ssc1. Analogous amino acid substitution in the ATPase domain of the Escherichia coli Hsp70 DnaK had a similar intragenic suppressive effect on the corresponding I462T temperature-sensitive peptide-binding domain mutation. I462T protein had a normal basal ATPase activity and was capable of nucleotide-dependent conformation changes. However, the reduced affinity of I462T for substrate peptide (and DnaJ) is likely responsible for the inability of I462T to function in vivo. The suppressor mutation (D79A) appears to partly alleviate the defect in DnaJ ATPase stimulation caused by I462T, suggesting that alteration in the interaction with DnaJ may alter the chaperone cycle to allow productive interaction with polypeptide substrates. Preservation of the intragenic suppression phenotypes between eukaryotic mitochondrial and bacterial Hsp70s suggests that the phenomenon studied here is a fundamental aspect of the function of Hsp70:Hsp40 chaperone machines.

The J-related segment of tim44 is essential for cell viability: a mutant Tim44 remains in the mitochondrial import site, but inefficiently recruits mtHsp70 and impairs protein translocation

Tim44 is a protein of the mitochondrial inner membrane and serves as an adaptor protein for mtHsp70 that drives the import of preproteins in an ATP-dependent manner. In this study we have modified the interaction of Tim44 with mtHsp70 and characterized the consequences for protein translocation. By deletion of an 18-residue segment of Tim44 with limited similarity to J-proteins, the binding of Tim44 to mtHsp70 was weakened. We found that in the yeast Saccharomyces cerevisiae the deletion of this segment is lethal. To investigate the role of the 18-residue segment, we expressed Tim44Delta18 in addition to the endogenous wild-type Tim44. Tim44Delta18 is correctly targeted to mitochondria and assembles in the inner membrane import site. The coexpression of Tim44Delta18 together with wild-type Tim44, however, does not stimulate protein import, but reduces its efficiency. In particular, the promotion of unfolding of preproteins during translocation is inhibited. mtHsp70 is still able to bind to Tim44Delta18 in an ATP-regulated manner, but the efficiency of interaction is reduced. These results suggest that the J-related segment of Tim44 is needed for productive interaction with mtHsp70. The efficient cooperation of mtHsp70 with Tim44 facilitates the translocation of loosely folded preproteins and plays a crucial role in the import of preproteins which contain a tightly folded domain.

The preprotein translocase of the mitochondrial inner membrane: function and evolution

Growing mitochondria acquire most of their proteins by the uptake of mitochondrial preproteins from the cytosol. To mediate this protein import, both mitochondrial membranes contain independent protein transport systems: the Tom machinery in the outer membrane and the Tim machinery in the inner membrane. Transport of proteins across the inner membrane and sorting to the different inner mitochondrial compartments is mediated by several protein complexes which have been identified in the past years. A complex containing the integral membrane proteins Tim17 and Tim23 constitutes the import channel for preproteins containing amino-terminal hydrophilic presequences. This complex is associated with Tim44 which serves as an adaptor protein for the binding of mtHsp70 to the membrane. mtHsp70, a 70 kDa heat shock protein of the mitochondrial matrix, drives the ATP-dependent import reaction of the processed preprotein after cleavage of the presequence. Preproteins containing internal targeting information are imported by a separate import machinery, which consists of the intermembrane-space proteins Tim9, Tim10, and Tim12, and the inner membrane proteins Tim22 and Tim54. The proteins Tim17, Tim22, and Tim23 have in common a similar topology in the membrane and a homologous amino acid sequence. Moreover, they show a sequence similarity to OEP16, a channel-forming amino acid transporter in the outer envelope of chloroplasts, and to LivH, a component of a prokaryotic amino acid permease, defining a new PRAT-family of preprotein and amino acid transporters.

Separation of structural and dynamic functions of the mitochondrial translocase: Tim44 is crucial for the inner membrane import sites in translocation of tightly folded domains, but not of loosely folded preproteins

The essential gene TIM44 encodes a subunit of the inner mitochondrial membrane preprotein translocase that forms a complex with the matrix heat-shock protein Hsp70. The specific role of Tim44 in protein import has not yet been defined because of the lack of means to block its function. Here we report on a Saccharomyces cerevisiae mutant allele of TIM44 that allows selective and efficient inactivation of Tim44 in organello. Surprisingly, the mutant mitochondria are still able to import preproteins. The import rate is only reduced by approximately 30% compared with wild-type as long as the preproteins do not carry stably folded domains. Moreover, the number of import sites is not reduced. However, the mutant mitochondria are strongly impaired in pulling folded domains of preproteins close to the outer membrane and in promoting their unfolding. Our results demonstrate that Tim44 is not an essential structural component of the import channel, but is crucial for import of folded domains. We suggest that the concerted action of Tim44 and mtHsp70 drives unfolding of preproteins and accelerates translocation of loosely folded preproteins. While mtHsp70 is essential for import of both tightly and loosly folded preproteins, Tim44 plays a more specialized role in translocation of tightly folded domains.

Strong precursor-pore interactions constrain models for mitochondrial protein import

Mitochondrial precursor proteins are imported from the cytosol into the matrix compartment through a proteinaceous translocation pore. Import is driven by mitochondrial Hsp70 (mHsp70), a matrix-localized ATPase. There are currently two postulated mechanisms for this function of mHsp70: 1) The "Brownian ratchet" model proposes that the precursor chain diffuses within the pore, and that binding of mHsp70 to the lumenal portion of the chain biases this diffusion. 2) The "power stroke" model proposes that mHsp70 undergoes a conformational change that actively pulls the precursor chain through the pore. Here we formulate these two models quantitatively, and compare their performance in light of recent experimental evidence that precursor chains interact strongly with the walls of the translocation pore. Under these conditions the simulated Brownian ratchet is inefficient, whereas the power stroke mechanism seems to be a plausible description of the import process.

Aminotransferase variants as probes for the role of the N-terminal region of a mature protein in mitochondrial precursor import and processing

Of the two homologous isozymes of aspartate aminotransferase that are also nearly identical in their folded structures, only the mitochondrial form (mAAT) is synthesized as a precursor (pmAAT). After its in vitro synthesis in rabbit reticulocyte lysate, it can also be efficiently imported into isolated rat liver mitochondria, where it is processed to its native form by removal of the N-terminal presequence. The homologous cytosolic isoenzyme (cAAT) is not imported into mitochondria, even after fusion of the mitochondrial presequence from pmAAT to its N-terminal end. Substitution of the 30-residue N-terminal peptide of the mature portion of pmAAT with the corresponding sequence from the homologous, import-incompetent cytosolic isozyme (pcmAAT) does not prevent import but reduces substantially its processing in the matrix. A detectable amount of the pcmAAT chimera is found associated with the inner mitochondrial membrane. Single and double substitution mutants of Trp-5 and Trp-6 at the N-terminal end of the mature protein are imported into mitochondria with efficiency similar to that of wild type. However, replacement of Trp-5 with proline, or of both tryptophans with either alanine (W5A/W6A mutant) or valine and alanine (W5V/W6A mutant), allows import but interferes with the correct processing of the imported protein despite the presence of an intact cleavage site for the processing peptidase. Similar cleavage results were obtained using newly synthesized proteins and mitochondrial matrix extracts. These results indicate that translocation and processing for a precursor are independent events and that sequences C-terminal to the cleavage site are indeed important for the correct maturation of pmAAT in the matrix, probably because of their contribution to the conformation and flexibility of the peptide region surrounding the cleavage site required for efficient processing. The same region from the mature component of the protein may play a role in the commitment of the passenger protein to complete its translocation into the matrix.

Mitochondrial preprotein translocase

Mitochondria import most of their proteins from the cytosol. Dynamic protein complexes in the mitochondrial outer and inner membranes are responsible for the specific recognition and membrane translocation of preproteins. The preprotein translocase of the outer mitochondrial membrane contains several import receptors and a general import pore. The preprotein translocase of the inner membrane consists of a channel interacting with preproteins in transit and an import motor that includes the matrix heat shock protein Hsp70. Acidic patches of import components are thought to guide the import of positively charged signal sequences (acid chain hypothesis). Energy input is derived from the inner membrane potential and ATP. Proteins in the mitochondrial matrix are required for proteolytic processing and folding of imported proteins. The dynamic nature of the membrane translocase permits sorting of preproteins at distinct stages of the import pathway.

A mutant form of mitochondrial GrpE suppresses the sorting defect caused by an alteration in the presequence of cytochrome b2

Transport of preproteins across the inner mitochondrial membrane requires the action of the matrix heat shock protein Hsp70. Together with its co-chaperone mitochondrial GrpE (Mge1), mtHsp70 transiently binds to the inner membrane translocase subunit Tim44 in a nucleotide-regulated manner, forming an ATP-dependent import driving machinery. We report that a mutant form of Mge1 (Mge1-100) is completely absent in mtHsp70-Tim44 complexes, although its ability to interact with soluble mtHsp70 is only partially reduced. While this mge1-100 mutation only partially retards preprotein translocation into the matrix, it exerts a selective effect on sorting of cytochrome b2 to the intermembrane space. A cytochrome b2 with an altered sorting signal, which is only processed to the intermediate stage and mistargeted to the matrix of wild-type mitochondria, is processed to the mature form and correctly targeted to the intermembrane space of mge1-100 mitochondria. These results suggest that (1) Mge1-100 discriminates between soluble and membrane-bound mtHsp70 and (2) the membrane-bound mtHsp70-Mge1 driving system competes with the sorting machinery for translocation of preproteins like cytochrome b2.

Role of mitochondrial GrpE and phosphate in the ATPase cycle of matrix Hsp70

The yeast mitochondrial GrpE homologue, Mge1, assists matrix Hsp70 in both protein translocation across the mitochondrial membranes and subsequent protein folding. We expressed mtHsp70 and Mge1 in Escherichia coli and analyzed their function in the ATP hydrolysis cycle. Mge1 stimulates ATP hydrolysis by mtHsp70 about twofold. Addition of inorganic phosphate inhibits ATP hydrolysis by preventing ADP release from mtHsp70. Mge1 has no direct effect on gamma-phosphate release from mtHsp70, yet indirectly relieves the phosphate inhibition by stimulating ADP release. We conclude that Mge1 promotes the ATPase cycle of mtHsp70 by increasing the rate of ADP release. ATP then rapidly binds to mtHsp70 such that the total amount of mtHsp70-bound nucleotide is not changed by Mge1.

Crystal structure of the nucleotide exchange factor GrpE bound to the ATPase domain of the molecular chaperone DnaK

The crystal structure of the adenine nucleotide exchange factor GrpE in complex with the adenosine triphosphatase (ATPase) domain of Escherichia coli DnaK [heat shock protein 70 (Hsp70)] was determined at 2.8 angstrom resolution. A dimer of GrpE binds asymmetrically to a single molecule of DnaK. The structure of the nucleotide-free ATPase domain in complex with GrpE resembles closely that of the nucleotide-bound mammalian Hsp70 homolog, except for an outward rotation of one of the subdomains of the protein. This conformational change is not consistent with tight nucleotide binding. Two long alpha helices extend away from the GrpE dimer and suggest a role for GrpE in peptide release from DnaK.