bims-mitran 2022-11-20 papers

Issue of 2022–11–20
four papers selected by
Andreas Kohler, University of Graz

Cell Signal. 2022 Nov 12. pii: S0898-6568(22)00286-8. [Epub ahead of print] 110524

Phosphorylation of mammalian mitochondrial EF-Tu by Fyn and c-Src kinases.

Emine C Koc, Caroline A Hunter, Hasan Koc.

Src Family Kinases (SFKs) are tyrosine kinases known to regulate glucose and fatty acid metabolism as well as oxidative phosphorylation (OXPHOS) in mammalian mitochondria. We and others discovered the association of the SFK kinases Fyn and c-Src with mitochondrial translation components. This translational system is responsible for the synthesis of 13 mitochondrial (mt)-encoded subunits of the OXPHOS complexes and is, thus, essential for energy generation. Mitochondrial ribosomal proteins and various translation elongation factors including Tu (EF-Tumt) have been identified as possible Fyn and c-Src kinase targets. However, the phosphorylation of specific residues in EF-Tumt by these kinases and their roles in the regulation of protein synthesis are yet to be explored. In this study, we report the association of EF-Tumt with cSrc kinase and mapping of phosphorylated Tyr (pTyr) residues by these kinases. We determined that a specific Tyr residue in EF-Tumt at position 266 (EF-Tumt-Y266), located in a highly conserved c-Src consensus motif is one of the major phosphorylation sites. The potential role of EF-Tumt-Y266 phosphorylation in regulation of mitochondrial translation investigated by site-directed mutagenesis. Its phosphomimetic to Glu residue (EF-Tumt-E266) inhibited ternary complex (EF-Tumt•GTP•aatRNA) formation and translation in vitro. Our findings along with data mining analysis of the c-Src knock out (KO) mice proteome suggest that the SFKs have possible roles for regulation of mitochondrial protein synthesis and oxidative energy metabolism in animals.

Keywords: EF-Tu(mt) (TUFM); Fyn and c-Src kinases; Mitochondrial protein synthesis; SFKs; Translation; Tyr phosphorylation

DOI: https://doi.org/10.1016/j.cellsig.2022.110524

Sci Adv. 2022 Nov 16. 8(46): eabq5234

Nonstop mRNAs generate a ground state of mitochondrial gene expression noise.

Kah Ying Ng, Guleycan Lutfullahoglu Bal, Uwe Richter, Omid Safronov, Lars Paulin, Cory D Dunn, Ville O Paavilainen, Julie Richer, William G Newman, Robert W Taylor, Brendan J Battersby.

A stop codon within the mRNA facilitates coordinated termination of protein synthesis, releasing the nascent polypeptide from the ribosome. This essential step in gene expression is impeded with transcripts lacking a stop codon, generating nonstop ribosome complexes. Here, we use deep sequencing to investigate sources of nonstop mRNAs generated from the human mitochondrial genome. We identify diverse types of nonstop mRNAs on mitochondrial ribosomes that are resistant to translation termination by canonical release factors. Failure to resolve these aberrations by the mitochondrial release factor in rescue (MTRFR) imparts a negative regulatory effect on protein synthesis that is associated with human disease. Our findings reveal a source of underlying noise in mitochondrial gene expression and the importance of responsive ribosome quality control mechanisms for cell fitness and human health.

DOI: https://doi.org/10.1126/sciadv.abq5234

Elife. 2022 11 17. pii: e69916. [Epub ahead of print]11

Unleashing a novel function of endonuclease G in mitochondrial genome instability.

Sumedha Dahal, Humaira Siddiqua, Shivangi Sharma, Ravi K Babu, Diksha Rathore, Sheetal Sharma, Sathees C Raghavan.

Having its genome makes the mitochondrion a unique and semiautonomous organelle within cells. Mammalian mitochondrial DNA (mtDNA) is a double-stranded closed circular molecule of about 16 kb coding for 37 genes. Mutations, including deletions in the mitochondrial genome, can culminate in different human diseases. Mapping the deletion junctions suggests that the breakpoints are generally seen at hotspots. '9-bp deletion' (8271-8281), seen in the intergenic region of cytochrome c oxidase II/tRNA<sup>Lys</sup>, is the most common mitochondrial deletion. While it is associated with several diseases like myopathy, dystonia, and hepatocellular carcinoma, it has also been used as an evolutionary marker. However, the mechanism responsible for its fragility is unclear. In the current study, we show that Endonuclease G, a mitochondrial nuclease responsible for nonspecific cleavage of nuclear DNA during apoptosis, can induce breaks at sequences associated with '9-bp deletion' when it is present on a plasmid or in the mitochondrial genome. Through a series of <i>in vitro</i> and intracellular studies, we show that Endonuclease G binds to G-quadruplex structures formed at the hotspot and induces DNA breaks. Therefore, we uncover a new role for Endonuclease G in generating mtDNA deletions, which depends on the formation of G4 DNA within the mitochondrial genome. In summary, we identify a novel property of Endonuclease G, besides its role in apoptosis and the recently described elimination of paternal mitochondria during fertilisation.

Keywords: E. coli; cell biology; genetics; genomics; human; rat

DOI: https://doi.org/10.7554/eLife.69916

iScience. 2022 Nov 18. 25(11): 105410

Cytosolic and mitochondrial tRNA synthetase inhibitors increase lifespan in a GCN4/atf-4-dependent manner.

Christine E Robbins, Bhumil Patel, Danielle L Sawyer, Barrie Wilkinson, Brian K Kennedy, Mark A McCormick.

Deletion of genes encoding ribosomal proteins extends lifespan in yeast. This increases translation of the functionally conserved transcription factor Gcn4, and lifespan extension in these mutants is GCN4-dependent. Gcn4 is also translationally upregulated by uncharged tRNAs, as are its C aenorhabditis elegans and mammalian functional orthologs. Here, we show that cytosolic tRNA synthetase inhibitors upregulate Gcn4 translation and extend yeast lifespan in a Gcn4-dependent manner. This cytosolic tRNA synthetase inhibitor is also able to extend the lifespan of C. elegans in an atf-4-dependent manner. We show that mitochondrial tRNA synthetase inhibitors greatly extend the lifespan of C. elegans, and this depends on atf-4. This suggests that perturbations of both cytosolic and mitochondrial translation may act in part via the same downstream pathway. These findings establish GCN4 orthologs as conserved longevity factors and, as long-lived mice exhibit elevated ATF4, leave open the possibility that tRNA synthetase inhibitors could also extend lifespan in mammals.

Keywords: Biochemistry; Biological sciences; Molecular biology

DOI: https://doi.org/10.1016/j.isci.2022.105410