Plant mitochondrial protein import: the ins and outs

The majority of the mitochondrial proteome, required to fulfil its diverse range of functions, is cytosolically synthesised and translocated via specialised machinery. The dedicated translocases, receptors, and associated proteins have been characterised in great detail in yeast over the last several decades, yet many of the mechanisms that regulate these processes in higher eukaryotes are still unknown. In this review, we highlight the current knowledge of mitochondrial protein import in plants. Despite the fact that the mechanisms of mitochondrial protein import have remained conserved across species, many unique features have arisen in plants to encompass the developmental, tissue-specific, and stress-responsive regulation in planta. An understanding of unique features and mechanisms in plants provides us with a unique insight into the regulation of mitochondrial biogenesis in higher eukaryotes.

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Mutant huntingtin disrupts mitochondrial proteostasis by interacting with TIM23

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1904101116 ◽

2019 ◽

Vol 116 (33) ◽

pp. 16593-16602 ◽

Cited By ~ 12

Author(s):

Svitlana Yablonska ◽

Vinitha Ganesan ◽

Lisa M. Ferrando ◽

JinHo Kim ◽

Anna Pyzel ◽

...

Keyword(s):

Cell Lines ◽

Protein Import ◽

Mitochondrial Protein ◽

Biological Significance ◽

Intermembrane Space ◽

Mutant Huntingtin ◽

Mitochondrial Protein Import ◽

Inner Membrane ◽

Mitochondrial Proteome ◽

Brain Tissues

Mutant huntingtin (mHTT), the causative protein in Huntington’s disease (HD), associates with the translocase of mitochondrial inner membrane 23 (TIM23) complex, resulting in inhibition of synaptic mitochondrial protein import first detected in presymptomatic HD mice. The early timing of this event suggests that it is a relevant and direct pathophysiologic consequence of mHTT expression. We show that, of the 4 TIM23 complex proteins, mHTT specifically binds to the TIM23 subunit and that full-length wild-type huntingtin (wtHTT) and mHTT reside in the mitochondrial intermembrane space. We investigated differences in mitochondrial proteome between wtHTT and mHTT cells and found numerous proteomic disparities between mHTT and wtHTT mitochondria. We validated these data by quantitative immunoblotting in striatal cell lines and human HD brain tissue. The level of soluble matrix mitochondrial proteins imported through the TIM23 complex is lower in mHTT-expressing cell lines and brain tissues of HD patients compared with controls. In mHTT-expressing cell lines, membrane-bound TIM23-imported proteins have lower intramitochondrial levels, whereas inner membrane multispan proteins that are imported via the TIM22 pathway and proteins integrated into the outer membrane generally remain unchanged. In summary, we show that, in mitochondria, huntingtin is located in the intermembrane space, that mHTT binds with high-affinity to TIM23, and that mitochondria from mHTT-expressing cells and brain tissues of HD patients have reduced levels of nuclearly encoded proteins imported through TIM23. These data demonstrate the mechanism and biological significance of mHTT-mediated inhibition of mitochondrial protein import, a mechanism likely broadly relevant to other neurodegenerative diseases.

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Plant mitochondrial protein import: mechanisms and control

Functional Plant Biology ◽

10.1071/pp99064 ◽

1999 ◽

Vol 26 (8) ◽

pp. 725 ◽

Cited By ~ 1

Author(s):

James Whelan

Keyword(s):

Protein Import ◽

Current Knowledge ◽

Mitochondrial Protein ◽

Mitochondrial Proteins ◽

Mitochondrial Protein Import ◽

Future Directions ◽

Receptor Structure ◽

Processing Peptidase ◽

Targeting Signals ◽

And Control

The characterisation of components of the plant mitochondrial import apparatus along with the availability of over one hundred nuclear-encoded mitochondrial proteins allows the study of plant mitochondrial protein import in homologous systems. From these studies it has emerged that although similarities in the import process exist with other organisms, significance differences exist, such as receptor structure, location of processing peptidase and targeting signals. These differences mean that previous studies carried out in heterologous systems must be re-evaluated. Further studies into protein import in plants need to be directed at understanding the mechanism of import and how this process may be controlled. In this review the latter points will be dealt with in terms of summarising our current knowledge and possible future directions.

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The peptidases involved in plant mitochondrial protein import

Journal of Experimental Botany ◽

10.1093/jxb/erz365 ◽

2019 ◽

Vol 70 (21) ◽

pp. 6005-6018 ◽

Cited By ~ 8

Author(s):

Abi S Ghifari ◽

Shaobai Huang ◽

Monika W Murcha

Keyword(s):

Mitochondrial Biogenesis ◽

Protein Import ◽

Mitochondrial Protein ◽

Mitochondrial Protein Import ◽

Mitochondrial Proteome ◽

Encoded Proteins

Mitochondrial biogenesis requires correct targeting and import of nuclear-encoded proteins to ensure the mitochondrial proteome responds to meet the plant’s energetic demands. Protein-degrading machineries also play key roles in protein import and mitochondrial biogenesis.

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Mitochondrial diseases caused by dysfunctional mitochondrial protein import

Biochemical Society Transactions ◽

10.1042/bst20180239 ◽

2018 ◽

Vol 46 (5) ◽

pp. 1225-1238 ◽

Cited By ~ 8

Author(s):

Thomas Daniel Jackson ◽

Catherine Sarah Palmer ◽

Diana Stojanovski

Keyword(s):

Mitochondrial Disease ◽

Genetic Disease ◽

Molecular Basis ◽

Protein Import ◽

Mitochondrial Protein ◽

Mitochondrial Diseases ◽

Mitochondrial Proteins ◽

Eukaryotic Cells ◽

Mitochondrial Protein Import ◽

Mitochondrial Proteome

Mitochondria are essential organelles which perform complex and varied functions within eukaryotic cells. Maintenance of mitochondrial health and functionality is thus a key cellular priority and relies on the organelle's extensive proteome. The mitochondrial proteome is largely encoded by nuclear genes, and mitochondrial proteins must be sorted to the correct mitochondrial sub-compartment post-translationally. This essential process is carried out by multimeric and dynamic translocation and sorting machineries, which can be found in all four mitochondrial compartments. Interestingly, advances in the diagnosis of genetic disease have revealed that mutations in various components of the human import machinery can cause mitochondrial disease, a heterogenous and often severe collection of disorders associated with energy generation defects and a multisystem presentation often affecting the cardiovascular and nervous systems. Here, we review our current understanding of mitochondrial protein import systems in human cells and the molecular basis of mitochondrial diseases caused by defects in these pathways.

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Mitochondrial Protein Quality Control Mechanisms

Genes ◽

10.3390/genes11050563 ◽

2020 ◽

Vol 11 (5) ◽

pp. 563 ◽

Cited By ~ 4

Author(s):

Pooja Jadiya ◽

Dhanendra Tomar

Keyword(s):

Quality Control ◽

Protein Import ◽

Protein Quality ◽

Current Knowledge ◽

Mitochondrial Protein ◽

Protein Quality Control ◽

Redox Balance ◽

Ubiquitin Proteasome System ◽

Control Mechanisms ◽

Mitochondrial Proteome

Mitochondria serve as a hub for many cellular processes, including bioenergetics, metabolism, cellular signaling, redox balance, calcium homeostasis, and cell death. The mitochondrial proteome includes over a thousand proteins, encoded by both the mitochondrial and nuclear genomes. The majority (~99%) of proteins are nuclear encoded that are synthesized in the cytosol and subsequently imported into the mitochondria. Within the mitochondria, polypeptides fold and assemble into their native functional form. Mitochondria health and integrity depend on correct protein import, folding, and regulated turnover termed as mitochondrial protein quality control (MPQC). Failure to maintain these processes can cause mitochondrial dysfunction that leads to various pathophysiological outcomes and the commencement of diseases. Here, we summarize the current knowledge about the role of different MPQC regulatory systems such as mitochondrial chaperones, proteases, the ubiquitin-proteasome system, mitochondrial unfolded protein response, mitophagy, and mitochondria-derived vesicles in the maintenance of mitochondrial proteome and health. The proper understanding of mitochondrial protein quality control mechanisms will provide relevant insights to treat multiple human diseases.

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De novo mutations in TOMM70, a receptor of the mitochondrial import translocase, cause neurological impairment

Human Molecular Genetics ◽

10.1093/hmg/ddaa081 ◽

2020 ◽

Vol 29 (9) ◽

pp. 1568-1579 ◽

Cited By ~ 1

Author(s):

Debdeep Dutta ◽

Lauren C Briere ◽

Oguz Kanca ◽

Paul C Marcogliese ◽

Melissa A Walker ◽

...

Keyword(s):

White Matter ◽

Protein Import ◽

De Novo ◽

Mitochondrial Protein ◽

Neurological Impairment ◽

Loss Of Function ◽

De Novo Mutations ◽

Coding Region ◽

Mitochondrial Protein Import ◽

Mitochondrial Proteome

Abstract The translocase of outer mitochondrial membrane (TOMM) complex is the entry gate for virtually all mitochondrial proteins and is essential to build the mitochondrial proteome. TOMM70 is a receptor that assists mainly in mitochondrial protein import. Here, we report two individuals with de novo variants in the C-terminal region of TOMM70. While both individuals exhibited shared symptoms including hypotonia, hyper-reflexia, ataxia, dystonia and significant white matter abnormalities, there were differences between the two individuals, most prominently the age of symptom onset. Both individuals were undiagnosed despite extensive genetics workups. Individual 1 was found to have a p.Thr607Ile variant while Individual 2 was found to have a p.Ile554Phe variant in TOMM70. To functionally assess both TOMM70 variants, we replaced the Drosophila Tom70 coding region with a Kozak-mini-GAL4 transgene using CRISPR-Cas9. Homozygous mutant animals die as pupae, but lethality is rescued by the mini-GAL4-driven expression of human UAS-TOMM70 cDNA. Both modeled variants lead to significantly less rescue indicating that they are loss-of-function alleles. Similarly, RNAi-mediated knockdown of Tom70 in the developing eye causes roughening and synaptic transmission defect, common findings in neurodegenerative and mitochondrial disorders. These phenotypes were rescued by the reference, but not the variants, of TOMM70. Altogether, our data indicate that de novo loss-of-function variants in TOMM70 result in variable white matter disease and neurological phenotypes in affected individuals.

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