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FEBS Letters Jun 2023Mitochondria are organelles indispensable for the correct functioning of eukaryotic cells. Their significance for cellular homeostasis is manifested by the existence of... (Review)
Review
Mitochondria are organelles indispensable for the correct functioning of eukaryotic cells. Their significance for cellular homeostasis is manifested by the existence of complex quality control pathways that monitor organellar fitness. Mitochondrial biogenesis relies on the efficient import of mitochondrial precursor proteins, a large majority of which are encoded by nuclear DNA and synthesized in the cytosol. This creates a demand for highly specialized import routes that comprise cytosolic factors and organellar translocases. The passage of newly encoded mitochondrial precursor proteins through the cytosol to the translocase of the outer mitochondrial membrane (TOM) is under tight surveillance. As a result of mitochondrial import defects, mitochondrial precursor proteins accumulate in the cytosol or clog the TOM complex, which in turn stimulates cellular stress responses to minimize the consequences of these challenges. These responses are critical for maintaining protein homeostasis under conditions of mitochondrial stress. The present review summarizes recent advances in the field of mitochondrial protein import quality control and discusses the role of this quality control within the network of cellular mechanisms that maintain the cellular homeostasis of proteins.
Topics: Proteostasis; Mitochondria; Mitochondrial Membranes; Mitochondrial Proteins; Carrier Proteins; Protein Transport; Homeostasis
PubMed: 37276075
DOI: 10.1002/1873-3468.14677 -
Cells Dec 2021Mitochondria play a critical role in providing energy, maintaining cellular metabolism, and regulating cell survival and death. To carry out these crucial functions,... (Review)
Review
Mitochondria play a critical role in providing energy, maintaining cellular metabolism, and regulating cell survival and death. To carry out these crucial functions, mitochondria employ more than 1500 proteins, distributed between two membranes and two aqueous compartments. An extensive network of dedicated proteins is engaged in importing and sorting these nuclear-encoded proteins into their designated mitochondrial compartments. Defects in this fundamental system are related to a variety of pathologies, particularly engaging the most energy-demanding tissues. In this review, we summarize the state-of-the-art knowledge about the mitochondrial protein import machinery and describe the known interrelation of its failure with age-related neurodegenerative and cardiovascular diseases.
Topics: Aging; Animals; Cardiovascular Diseases; Humans; Mitochondrial Membranes; Mitochondrial Proteins; Neurodegenerative Diseases; Protein Transport
PubMed: 34944035
DOI: 10.3390/cells10123528 -
Molecular Cell Sep 2022Peroxisomes are ubiquitous organelles whose dysfunction causes fatal human diseases. Most peroxisomal enzymes are imported from the cytosol by the receptor PEX5, which...
Peroxisomes are ubiquitous organelles whose dysfunction causes fatal human diseases. Most peroxisomal enzymes are imported from the cytosol by the receptor PEX5, which interacts with a docking complex in the peroxisomal membrane and then returns to the cytosol after monoubiquitination by a membrane-embedded ubiquitin ligase. The mechanism by which PEX5 shuttles between cytosol and peroxisomes and releases cargo inside the lumen is unclear. Here, we use Xenopus egg extract to demonstrate that PEX5 accompanies cargo completely into the lumen, utilizing WxxxF/Y motifs near its N terminus that bind a lumenal domain of the docking complex. PEX5 recycling is initiated by an amphipathic helix that binds to the lumenal side of the ubiquitin ligase. The N terminus then emerges in the cytosol for monoubiquitination. Finally, PEX5 is extracted from the lumen, resulting in the unfolding of the receptor and cargo release. Our results reveal the unique mechanism by which PEX5 ferries proteins into peroxisomes.
Topics: Carrier Proteins; Humans; Ligases; Peroxisome-Targeting Signal 1 Receptor; Peroxisomes; Protein Transport; Receptors, Cytoplasmic and Nuclear; Ubiquitin
PubMed: 35931083
DOI: 10.1016/j.molcel.2022.07.004 -
Current Opinion in Structural Biology Aug 2021The bacterial outer membrane forms an impermeable barrier to the environment, but a wide variety of substances must cross it without compromising the membrane. Perhaps,... (Review)
Review
The bacterial outer membrane forms an impermeable barrier to the environment, but a wide variety of substances must cross it without compromising the membrane. Perhaps, the most fascinating transport phenomenon is the import and export of very large protein toxins using relatively small β-barrel proteins residing in the outer membrane. Progress has been made on three systems in recent years that shed light on this process. In this review, we summarize bacteriocin (toxin) import using TonB-dependent transporters and protein secretion by autotransporters and two partner secretion systems.
Topics: Bacterial Outer Membrane; Bacterial Outer Membrane Proteins; Bacterial Proteins; Biological Transport; Protein Transport
PubMed: 33901701
DOI: 10.1016/j.sbi.2021.03.007 -
ELife May 2023Mitochondrial biogenesis requires the import of >1,000 mitochondrial preproteins from the cytosol. Most studies on mitochondrial protein import are focused on the core...
Mitochondrial biogenesis requires the import of >1,000 mitochondrial preproteins from the cytosol. Most studies on mitochondrial protein import are focused on the core import machinery. Whether and how the biophysical properties of substrate preproteins affect overall import efficiency is underexplored. Here, we show that protein traffic into mitochondria can be disrupted by amino acid substitutions in a single substrate preprotein. Pathogenic missense mutations in ADP/ATP translocase 1 (ANT1), and its yeast homolog ADP/ATP carrier 2 (Aac2), cause the protein to accumulate along the protein import pathway, thereby obstructing general protein translocation into mitochondria. This impairs mitochondrial respiration, cytosolic proteostasis, and cell viability independent of ANT1's nucleotide transport activity. The mutations act synergistically, as double mutant Aac2/ANT1 causes severe clogging primarily at the translocase of the outer membrane (TOM) complex. This confers extreme toxicity in yeast. In mice, expression of a super-clogger ANT1 variant led to neurodegeneration and an age-dependent dominant myopathy that phenocopy ANT1-induced human disease, suggesting clogging as a mechanism of disease. More broadly, this work implies the existence of uncharacterized amino acid requirements for mitochondrial carrier proteins to avoid clogging and subsequent disease.
Topics: Animals; Humans; Mice; Saccharomyces cerevisiae; Saccharomyces cerevisiae Proteins; Mitochondria; Mitochondrial ADP, ATP Translocases; Carrier Proteins; Protein Transport; Mitochondrial Proteins; Mitochondrial Membrane Transport Proteins
PubMed: 37129366
DOI: 10.7554/eLife.84330 -
Proceedings of the National Academy of... Feb 2021Mitochondrial protein import requires outer membrane receptors that evolved independently in different lineages. Here we used quantitative proteomics and in vitro...
Mitochondrial protein import requires outer membrane receptors that evolved independently in different lineages. Here we used quantitative proteomics and in vitro binding assays to investigate the substrate preferences of ATOM46 and ATOM69, the two mitochondrial import receptors of The results show that ATOM46 prefers presequence-containing, hydrophilic proteins that lack transmembrane domains (TMDs), whereas ATOM69 prefers presequence-lacking, hydrophobic substrates that have TMDs. Thus, the ATOM46/yeast Tom20 and the ATOM69/yeast Tom70 pairs have similar substrate preferences. However, ATOM46 mainly uses electrostatic, and Tom20 hydrophobic, interactions for substrate binding. In vivo replacement of ATOM46 by yeast Tom20 did not restore import. However, replacement of ATOM69 by the recently discovered Tom36 receptor of hydrogenosomes, while not allowing for growth, restored import of a large subset of trypanosomal proteins that lack TMDs. Thus, even though ATOM69 and Tom36 share the same domain structure and topology, they have different substrate preferences. The study establishes complementation experiments, combined with quantitative proteomics, as a highly versatile and sensitive method to compare in vivo preferences of protein import receptors. Moreover, it illustrates the role determinism and contingencies played in the evolution of mitochondrial protein import receptors.
Topics: Animals; Carrier Proteins; Evolution, Molecular; Mitochondria; Mitochondrial Membrane Transport Proteins; Mitochondrial Precursor Protein Import Complex Proteins; Mitochondrial Proteins; Protein Binding; Protein Precursors; Protein Transport; Saccharomyces cerevisiae; Saccharomyces cerevisiae Proteins; Trypanosoma brucei brucei
PubMed: 33526678
DOI: 10.1073/pnas.2017774118 -
Current Genetics Aug 2019In this report, we summarize recent findings about a role of the outer membrane metabolite channel VDAC/porin in protein import into mitochondria. Mitochondria fulfill... (Review)
Review
In this report, we summarize recent findings about a role of the outer membrane metabolite channel VDAC/porin in protein import into mitochondria. Mitochondria fulfill key functions for cellular energy metabolism. Their biogenesis involves the import of about 1000 different proteins that are produced as precursors on cytosolic ribosomes. The translocase of the outer membrane (TOM complex) forms the entry gate for mitochondrial precursor proteins. Dedicated protein translocases sort the preproteins into the different mitochondrial subcompartments. While protein transport pathways are analyzed to some detail, only little is known about regulatory mechanisms that fine-tune protein import upon metabolic signaling. Recently, a dual role of the voltage-dependent anion channel (VDAC), also termed porin, in mitochondrial protein biogenesis was reported. First, VDAC/porin promotes as a coupling factor import of carrier proteins into the inner membrane. Second, VDAC/porin regulates the formation of the TOM complex. Thus, the major metabolite channel in the outer membrane VDAC/porin connects protein import to mitochondrial metabolism.
Topics: Carrier Proteins; Cytosol; Energy Metabolism; Mitochondria; Mitochondrial Membranes; Mitochondrial Precursor Protein Import Complex Proteins; Mitochondrial Proteins; Organelle Biogenesis; Protein Transport; Ribosomes; Saccharomyces cerevisiae; Voltage-Dependent Anion Channels
PubMed: 30944955
DOI: 10.1007/s00294-019-00965-z -
Biological Chemistry Jul 2023Most mitochondrial proteins are nuclear-encoded and imported by the protein import machinery based on specific targeting signals. The proteins that carry an...
Most mitochondrial proteins are nuclear-encoded and imported by the protein import machinery based on specific targeting signals. The proteins that carry an amino-terminal targeting signal (presequence) are imported via the presequence import pathway that involves the translocases of the outer and inner membranes - TOM and TIM23 complexes. In this article, we discuss how mitochondrial matrix and inner membrane precursor proteins are imported along the presequence pathway in with a focus on the dynamics of the TIM23 complex, and further update with some of the key findings that advanced the field in the last few years.
Topics: Protein Transport; Saccharomyces cerevisiae; Mitochondria; Mitochondrial Proteins; Mitochondrial Precursor Protein Import Complex Proteins
PubMed: 37155927
DOI: 10.1515/hsz-2023-0133 -
IUBMB Life Oct 2023The abundance of each cellular protein is dynamically adjusted to the prevailing metabolic and stress conditions by modulation of their synthesis and degradation rates.... (Review)
Review
The abundance of each cellular protein is dynamically adjusted to the prevailing metabolic and stress conditions by modulation of their synthesis and degradation rates. The proteasome represents the major machinery for the degradation of proteins in eukaryotic cells. How the ubiquitin-proteasome system (UPS) controls protein levels and removes superfluous and damaged proteins from the cytosol and the nucleus is well characterized. However, recent studies showed that the proteasome also plays a crucial role in mitochondrial protein quality control. This mitochondria-associated degradation (MAD) thereby acts on two layers: first, the proteasome removes mature, functionally compromised or mis-localized proteins from the mitochondrial surface; and second, the proteasome cleanses the mitochondrial import pore of import intermediates of nascent proteins that are stalled during translocation. In this review, we provide an overview about the components and their specific functions that facilitate proteasomal degradation of mitochondrial proteins in the yeast Saccharomyces cerevisiae. Thereby we explain how the proteasome, in conjunction with a set of intramitochondrial proteases, maintains mitochondrial protein homeostasis and dynamically adapts the levels of mitochondrial proteins to specific conditions.
Topics: Proteasome Endopeptidase Complex; Mitochondrial Proteins; Mitochondria; Ubiquitin; Saccharomyces cerevisiae; Saccharomyces cerevisiae Proteins
PubMed: 37178401
DOI: 10.1002/iub.2734 -
Cell Reports Mar 2024Resting CD4 T cells resist productive HIV-1 infection. The HIV-2/simian immunodeficiency virus protein viral accessory protein X (Vpx) renders these cells permissive to...
Resting CD4 T cells resist productive HIV-1 infection. The HIV-2/simian immunodeficiency virus protein viral accessory protein X (Vpx) renders these cells permissive to infection, presumably by alleviating blocks at cytoplasmic reverse transcription and subsequent nuclear import of reverse-transcription/pre-integration complexes (RTC/PICs). Here, spatial analyses using quantitative virus imaging techniques reveal that HIV-1 capsids containing RTC/PICs are readily imported into the nucleus, recruit the host dependency factor CPSF6, and translocate to nuclear speckles in resting CD4 T cells. Reverse transcription, however, remains incomplete, impeding proviral integration and viral gene expression. Vpx or pharmacological inhibition of the deoxynucleotide triphosphohydrolase (dNTPase) activity of the restriction factor SAM domain and HD domain-containing protein 1 (SAMHD1) increases levels of nuclear reverse-transcribed cDNA and facilitates HIV-1 integration. Nuclear import and intranuclear transport of viral complexes therefore do not pose important blocks to HIV-1 in resting CD4 T cells, and the limitation to reverse transcription by SAMHD1's dNTPase activity constitutes the main pre-integration block to infection.
Topics: Animals; Humans; HIV-1; CD4-Positive T-Lymphocytes; SAM Domain and HD Domain-Containing Protein 1; HIV Infections; HIV-2; HIV Seropositivity; Viral Regulatory and Accessory Proteins; Monomeric GTP-Binding Proteins; HEK293 Cells
PubMed: 38478523
DOI: 10.1016/j.celrep.2024.113941