bims-mitpro 2024-08-04 papers

bims-mitpro

Biomed News

on Mitochondrial Proteostasis

Issue of 2024‒08‒04
nine papers selected by
Andreas Kohler, Umeå University

The ATP-driven extractor ATAD1/Msp1 proof-reads protein translocation into mitochondria.
Cytoplasmic ribosomes on mitochondria alter the local membrane environment for protein import.
The TOM complex from an evolutionary perspective and the functions of TOMM70.
Structure of the intact Tom20 receptor in the human translocase of the outer membrane complex.
Mitochondria-derived vesicles: potential nano-batteries to recharge the cellular powerhouse.
Inhibition of K63 ubiquitination by G-Protein pathway suppressor 2 (GPS2) regulates mitochondria-associated translation.
LRPPRC and SLIRP synergize to maintain sufficient and orderly mammalian mitochondrial translation.
Huntingtin contains an ubiquitin-binding domain and regulates lysosomal targeting of mitochondrial and RNA-binding proteins.
Opa1 processing is dispensable in mouse development but is protective in mitochondrial cardiomyopathy.

Trends Cell Biol. 2024 Jul 31. pii: S0962-8924(24)00145-4. [Epub ahead of print]

The ATP-driven extractor ATAD1/Msp1 proof-reads protein translocation into mitochondria.

Maria Bohnert, Christos Gatsogiannis, Johannes M Herrmann.

  The accumulation of translocation intermediates in the mitochondrial import machinery threatens cellular fitness and is associated with cancer and neurodegeneration. A recent study by Weidberg and colleagues identifies ATAD1 as an ATP-driven extraction machine on the mitochondrial surface that pulls precursors into the cytosol to prevent clogging of mitochondrial import pores.

Keywords:  AAA protein; cancer; integrated stress response (ISR); mitochondria; protein import; quality control

DOI:  https://doi.org/10.1016/j.tcb.2024.07.007
bioRxiv. 2024 Jul 19. pii: 2024.07.17.604013. [Epub ahead of print]

Cytoplasmic ribosomes on mitochondria alter the local membrane environment for protein import.

Ya-Ting Chang, Benjamin A Barad, Hamidreza Rahmani, Brian M Zid, Danielle A Grotjahn.

Most of the mitochondria proteome is nuclear-encoded, synthesized by cytoplasmic ribosomes, and targeted to mitochondria post-translationally. However, a subset of mitochondrial-targeted proteins is imported co-translationally, although the molecular mechanisms governing this process remain unclear. We employ cellular cryo-electron tomography to visualize interactions between cytoplasmic ribosomes and mitochondria in Saccharomyces cerevisiae. We use surface morphometrics tools to identify a subset of ribosomes optimally oriented on mitochondrial membranes for protein import. This allows us to establish the first subtomogram average structure of a cytoplasmic ribosome on the surface of the mitochondria in the native cellular context, which showed three distinct connections with the outer mitochondrial membrane surrounding the peptide exit tunnel. Further, this analysis demonstrated that cytoplasmic ribosomes primed for mitochondrial protein import cluster on the outer mitochondrial membrane at sites of local constrictions of the outer and inner mitochondrial membrane. Overall, our study reveals the architecture and the spatial organization of cytoplasmic ribosomes at the mitochondrial surface, providing a native cellular context to define the mechanisms that mediate efficient mitochondrial co-translational protein import.

DOI: https://doi.org/10.1101/2024.07.17.604013
Biol Chem. 2024 Aug 02.

The TOM complex from an evolutionary perspective and the functions of TOMM70.

Metin Özdemir, Sven Dennerlein.

  In humans, up to 1,500 mitochondrial precursor proteins are synthesized at cytosolic ribosomes and must be imported into the organelle. This is not only essential for mitochondrial but also for many cytosolic functions. The majority of mitochondrial precursor proteins are imported over the translocase of the outer membrane (TOM). In recent years, high-resolution structure analyses from different organisms shed light on the composition and arrangement of the TOM complex. Although significant similarities have been found, differences were also observed, which have been favored during evolution and could reflect the manifold functions of TOM with cellular signaling and its response to altered metabolic situations. A key component within these regulatory mechanisms is TOMM70, which is involved in protein import, forms contacts to the ER and the nucleus, but is also involved in cellular defense mechanisms during infections.

Keywords:  TOM complex; mitochondria; mitochondrial import

DOI:  https://doi.org/10.1515/hsz-2024-0043
PNAS Nexus. 2024 Jul;3(7): pgae269

Structure of the intact Tom20 receptor in the human translocase of the outer membrane complex.

Jiayue Su, Xuyang Tian, Ziyi Wang, Jiawen Yang, Shan Sun, Sen-Fang Sui.

  The translocase of the outer membrane (TOM) complex serves as the main gate for preproteins entering mitochondria and thus plays a pivotal role in sustaining mitochondrial stability. Precursor proteins, featuring amino-terminal targeting signals (presequences) or internal targeting signals, are recognized by the TOM complex receptors Tom20, Tom22, and Tom70, and then translocated into mitochondria through Tom40. By using chemical cross-linking to stabilize Tom20 in the TOM complex, this study unveils the structure of the human TOM holo complex, encompassing the intact Tom20 component, at a resolution of approximately 6 Å by cryo-electron microscopy. Our structure shows the TOM holo complex containing only one Tom20 subunit, which is located right at the center of the complex and stabilized by extensive interactions with Tom22, Tom40, and Tom6. Based on the structure, we proposed a possible translocation mode of TOM complex, by which different receptors could work simultaneously to ensure that the preproteins recognized by them are all efficiently translocated into the mitochondria.

Keywords:  Tom20; cryo-electron microscopy; mitochondria; translocase of the outer membrane complex

DOI:  https://doi.org/10.1093/pnasnexus/pgae269
Extracell Vesicles Circ Nucl Acids. 2024 Jun;5(2): 271-275

Mitochondria-derived vesicles: potential nano-batteries to recharge the cellular powerhouse.

Shalini Mishra, Gagan Deep.

  Mitochondria dysfunction is increasingly recognized as a critical factor in various pathogenic processes. The mechanism governing mitochondrial quality control serves as an adaptive response, ensuring the preservation of mitochondrial morphology, quantity, and overall function, crucial for cell survival. The generation of mitochondria-derived vesicles (MDVs) is one of the processes of mitochondrial quality control. Recent literature has suggested MDV heterogeneity; however, the detailed characteristics of various MDV subtypes still need to be studied better. Recent studies have shown that MDVs also play a role in inter-organelle communication for mitochondria besides quality control. For instance, Hazan et al. demonstrated that functional mitochondria from Saccharomyces cerevisiae release vesicles independent of the fission machinery. These vesicles, falling within the typical size range of MDVs, were selectively loaded with mitochondrial proteins, especially with functional ATP synthase subunits. Intriguingly, these MDVs maintained membrane potential and could generate ATP. Moreover, MDVs could fuse with naïve mitochondria, transferring their ATP generation machinery. Lastly, this study revealed a potential delivery mechanism of ATP-producing vesicles, presenting a promising avenue to rejuvenate ATP-deficient mitochondria. Overall, this study unveils a novel mechanism for inter-organelle communication by vesicles, which is crucial for maintaining cellular homeostasis and could also be important in pathological conditions.

Keywords:  F1-F0 ATP synthase; Mitochondria; inter-organelle communication; mitochondria-derived vesicle

DOI:  https://doi.org/10.20517/evcna.2023.71
Pharmacol Res. 2024 Jul 31. pii: S1043-6618(24)00281-0. [Epub ahead of print] 107336

Inhibition of K63 ubiquitination by G-Protein pathway suppressor 2 (GPS2) regulates mitochondria-associated translation.

Yuan Gao, Julian Kwan, Joseph Orofino, Giulia Burrone, Sahana Mitra, Ting-Yu Fan, Justin English, Ryan Hekman, Andrew Emili, Shawn M Lyons, Maria Dafne Cardamone, Valentina Perissi.

  G-Protein Pathway Suppressor 2 (GPS2) is an inhibitor of non-proteolytic K63 ubiquitination mediated by the E2 ubiquitin-conjugating enzyme Ubc13. Previous studies have associated GPS2-mediated restriction of ubiquitination with the regulation of insulin signaling, inflammatory responses and mitochondria-nuclear communication across different tissues and cell types. However, a detailed understanding of the targets of GPS2/Ubc13 activity is lacking. Here, we have dissected the GPS2-regulated K63 ubiquitome in mouse embryonic fibroblasts and human breast cancer cells, unexpectedly finding an enrichment for proteins involved in RNA binding and translation on the outer mitochondrial membrane. Validation of selected targets of GPS2-mediated regulation, including the RNA-binding protein PABPC1 and translation factors RPS1, RACK1 and eIF3M, revealed a mitochondrial-specific strategy for regulating the translation of nuclear-encoded mitochondrial proteins via non-proteolytic ubiquitination. Removal of GPS2-mediated inhibition, either via genetic deletion or stress-induced nuclear translocation, promotes the import-coupled translation of selected mRNAs leading to the increased expression of an adaptive antioxidant program. In light of GPS2 role in nuclear-mitochondria communication, these findings reveal an exquisite regulatory network for modulating mitochondrial gene expression through spatially coordinated transcription and translation.

Keywords:  FCCP; GPS2; Homo-harringtonine; NSC697923; PABPC1; Puromycin; Tamoxifen; Ubiquitin; mitochondria; mitochondria stress response (MSR); translation

DOI:  https://doi.org/10.1016/j.phrs.2024.107336
Nucleic Acids Res. 2024 Aug 01. pii: gkae662. [Epub ahead of print]

LRPPRC and SLIRP synergize to maintain sufficient and orderly mammalian mitochondrial translation.

Diana Rubalcava-Gracia, Kristina Bubb, Fredrik Levander, Stephen P Burr, Amelie V August, Patrick F Chinnery, Camilla Koolmeister, Nils-Göran Larsson.

In mammals, the leucine-rich pentatricopeptide repeat protein (LRPPRC) and the stem-loop interacting RNA-binding protein (SLIRP) form a complex in the mitochondrial matrix that is required throughout the life cycle of most mitochondrial mRNAs. Although pathogenic mutations in the LRPPRC and SLIRP genes cause devastating human mitochondrial diseases, the in vivo function of the corresponding proteins is incompletely understood. We show here that loss of SLIRP in mice causes a decrease of complex I levels whereas other OXPHOS complexes are unaffected. We generated knock-in mice to study the in vivo interdependency of SLIRP and LRPPRC by mutating specific amino acids necessary for protein complex formation. When protein complex formation is disrupted, LRPPRC is partially degraded and SLIRP disappears. Livers from Lrpprc knock-in mice had impaired mitochondrial translation except for a marked increase in the synthesis of ATP8. Furthermore, the introduction of a heteroplasmic pathogenic mtDNA mutation (m.C5024T of the tRNAAla gene) into Slirp knockout mice causes an additive effect on mitochondrial translation leading to embryonic lethality and reduced growth of mouse embryonic fibroblasts. To summarize, we report that the LRPPRC/SLIRP protein complex is critical for maintaining normal complex I levels and that it also coordinates mitochondrial translation in a tissue-specific manner.

DOI: https://doi.org/10.1093/nar/gkae662
Proc Natl Acad Sci U S A. 2024 Aug 06. 121(32): e2319091121

Huntingtin contains an ubiquitin-binding domain and regulates lysosomal targeting of mitochondrial and RNA-binding proteins.

Gianna M Fote, Vinay V Eapen, Ryan G Lim, Clinton Yu, Lisa Salazar, Nicolette R McClure, Jharrayne McKnight, Thai B Nguyen, Marie C Heath, Alice L Lau, Mark A Villamil, Ricardo Miramontes, Ian H Kratter, Steven Finkbeiner, Jack C Reidling, Joao A Paulo, Peter Kaiser, Lan Huang, David E Housman, Leslie M Thompson, Joan S Steffan.

  Understanding the normal function of the Huntingtin (HTT) protein is of significance in the design and implementation of therapeutic strategies for Huntington's disease (HD). Expansion of the CAG repeat in the HTT gene, encoding an expanded polyglutamine (polyQ) repeat within the HTT protein, causes HD and may compromise HTT's normal activity contributing to HD pathology. Here, we investigated the previously defined role of HTT in autophagy specifically through studying HTT's association with ubiquitin. We find that HTT interacts directly with ubiquitin in vitro. Tandem affinity purification was used to identify ubiquitinated and ubiquitin-associated proteins that copurify with a HTT N-terminal fragment under basal conditions. Copurification is enhanced by HTT polyQ expansion and reduced by mimicking HTT serine 421 phosphorylation. The identified HTT-interacting proteins include RNA-binding proteins (RBPs) involved in mRNA translation, proteins enriched in stress granules, the nuclear proteome, the defective ribosomal products (DRiPs) proteome and the brain-derived autophagosomal proteome. To determine whether the proteins interacting with HTT are autophagic targets, HTT knockout (KO) cells and immunoprecipitation of lysosomes were used to investigate autophagy in the absence of HTT. HTT KO was associated with reduced abundance of mitochondrial proteins in the lysosome, indicating a potential compromise in basal mitophagy, and increased lysosomal abundance of RBPs which may result from compensatory up-regulation of starvation-induced macroautophagy. We suggest HTT is critical for appropriate basal clearance of mitochondrial proteins and RBPs, hence reduced HTT proteostatic function with mutation may contribute to the neuropathology of HD.

Keywords:  Huntingtin; RNA-binding proteins; autophagy; ubiquitin; ubiquitin-binding domain

DOI:  https://doi.org/10.1073/pnas.2319091121
Sci Adv. 2024 Aug 02. 10(31): eadp0443

Opa1 processing is dispensable in mouse development but is protective in mitochondrial cardiomyopathy.

Sofia Ahola, Lilli A Pazurek, Fiona Mayer, Philipp Lampe, Steffen Hermans, Lore Becker, Oana V Amarie, Helmut Fuchs, Valerie Gailus-Durner, Martin Hrabe de Angelis, Dietmar Riedel, Hendrik Nolte, Thomas Langer.

Mitochondrial fusion and fission accompany adaptive responses to stress and altered metabolic demands. Inner membrane fusion and cristae morphogenesis depends on optic atrophy 1 (Opa1), which is expressed in different isoforms and is cleaved from a membrane-bound, long to a soluble, short form. Here, we have analyzed the physiological role of Opa1 isoforms and Opa1 processing by generating mouse lines expressing only one cleavable Opa1 isoform or a non-cleavable variant thereof. Our results show that expression of a single cleavable or non-cleavable Opa1 isoform preserves embryonic development and the health of adult mice. Opa1 processing is dispensable under metabolic and thermal stress but prolongs life span and protects against mitochondrial cardiomyopathy in OXPHOS-deficient Cox10-/- mice. Mechanistically, loss of Opa1 processing disturbs the balance between mitochondrial biogenesis and mitophagy, suppressing cardiac hypertrophic growth in Cox10-/- hearts. Our results highlight the critical regulatory role of Opa1 processing, mitochondrial dynamics, and metabolism for cardiac hypertrophy.

DOI: https://doi.org/10.1126/sciadv.adp0443