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1. Sestrin2 Phosphorylation by ULK1 Induces Autophagic Degradation of Mitochondria Damaged by Copper-Induced Oxidative Stress

   https://doi.org/10.3390/ijms21176130  

   Kim, Heejeong ; Jeon, Byeong Tak ; Kim, Isaac M. ; Bennett, Sydney J. ; Lorch, Carolyn M. ; Viana, Martonio Ponte ; Myers, Jacob F. ; Trupp, Caroline J. ; Whipps, Zachary T. ; Kundu, Mondira ; et al ( September 2020 , International Journal of Molecular Sciences)

   null (Ed.)

   Selective autolysosomal degradation of damaged mitochondria, also called mitophagy, is an indispensable process for maintaining integrity and homeostasis of mitochondria. One well-established mechanism mediating selective removal of mitochondria under relatively mild mitochondria-depolarizing stress is PINK1-Parkin-mediated or ubiquitin-dependent mitophagy. However, additional mechanisms such as LC3-mediated or ubiquitin-independent mitophagy induction by heavy environmental stress exist and remain poorly understood. The present study unravels a novel role of stress-inducible protein Sestrin2 in degradation of mitochondria damaged by transition metal stress. By utilizing proteomic methods and studies in cell culture and rodent models, we identify autophagy kinase ULK1-mediated phosphorylation sites of Sestrin2 and demonstrate Sestrin2 association with mitochondria adaptor proteins in HEK293 cells. We show that Ser-73 and Ser-254 residues of Sestrin2 are phosphorylated by ULK1, and a pool of Sestrin2 is strongly associated with mitochondrial ATP5A in response to Cu-induced oxidative stress. Subsequently, this interaction promotes association with LC3-coated autolysosomes to induce degradation of mitochondria damaged by Cu-induced ROS. Treatment of cells with antioxidants or a Cu chelator significantly reduces Sestrin2 association with mitochondria. These results highlight the ULK1-Sestrin2 pathway as a novel stress-sensing mechanism that can rapidly induce autophagic degradation of mitochondria under severe heavy metal stress. 

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2. A VDAC1-mediated NEET protein chain transfers [2Fe-2S] clusters between the mitochondria and the cytosol and impacts mitochondrial dynamics

   https://doi.org/10.1073/pnas.2121491119  

   Karmi, Ola ; Marjault, Henri-Baptiste ; Bai, Fang ; Roy, Susmita ; Sohn, Yang-Sung ; Darash Yahana, Merav ; Morcos, Faruck ; Ioannidis, Konstantinos ; Nahmias, Yaakov ; Jennings, Patricia A. ; et al ( February 2022 , Proceedings of the National Academy of Sciences)

   Mitochondrial inner NEET (MiNT) and the outer mitochondrial membrane (OMM) mitoNEET (mNT) proteins belong to the NEET protein family. This family plays a key role in mitochondrial labile iron and reactive oxygen species (ROS) homeostasis. NEET proteins contain labile [2Fe-2S] clusters which can be transferred to apo-acceptor proteins. In eukaryotes, the biogenesis of [2Fe-2S] clusters occurs within the mitochondria by the iron–sulfur cluster (ISC) system; the clusters are then transferred to [2Fe-2S] proteins within the mitochondria or exported to cytosolic proteins and the cytosolic iron–sulfur cluster assembly (CIA) system. The last step of export of the [2Fe-2S] is not yet fully characterized. Here we show that MiNT interacts with voltage-dependent anion channel 1 (VDAC1), a major OMM protein that connects the intermembrane space with the cytosol and participates in regulating the levels of different ions including mitochondrial labile iron (mLI). We further show that VDAC1 is mediating the interaction between MiNT and mNT, in which MiNT transfers its [2Fe-2S] clusters from inside the mitochondria to mNT that is facing the cytosol. This MiNT–VDAC1–mNT interaction is shown both experimentally and by computational calculations. Additionally, we show that modifying MiNT expression in breast cancer cells affects the dynamics of mitochondrial structure and morphology, mitochondrial function, and breast cancer tumor growth. Our findings reveal a pathway for the transfer of [2Fe-2S] clusters, which are assembled inside the mitochondria, to the cytosol. 

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3. Are granulins copper sequestering proteins?

   https://doi.org/10.1002/prot.26031  

   Bhopatkar, Anukool A. ; Rangachari, Vijayaraghavan ( December 2020 , Proteins: Structure, Function, and Bioinformatics)

   Granulins (GRN 1‐7) are short (~6 kDa), cysteine‐rich proteins that are generated upon the proteolytic processing of progranulin (PGRN). These peptides, along with their precursor, have been implicated in multiple pathophysiological roles, especially in neurodegenerative diseases. Previously we showed that GRN‐3 and GRN‐5 are fully disordered in the reduced form implicating redox sensitive attributes to the proteins. Redox‐based modulations are often carried out by metalloproteins in mitigating oxidative stress and maintaining metal‐homeostasis within cells. To probe whether GRNs play a role in metal sequestration, we tested the metal binding propensity of the reduced forms of GRNs −3 and − 5 under neutral and acidic pH mimicking cytosolic and lysosomal conditions, respectively. We found, at neutral pH, both GRNs selectively bind Cu and no other divalent metal cations, with a greater specificity for Cu(I). Binding of Cu did not result in a disorder‐to‐order structural transition but partly triggered the multimerization of GRNs via uncoordinated cystines at both pH conditions. Overall, the results indicate that GRNs −3 and − 5 have surprisingly strong affinity for Cu in the pM range, comparable to other known copper sequestering proteins. The results also hint at a potential of GRNs to reduce Cu(II) to Cu(I), a process that has significance in mitigating Cu‐induced ROS cytotoxicity in cells. Together, this report uncovers metal‐coordinating property of GRNs for the first time, which may have profound significance in their structure and pathophysiological functions.

    

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4. UbiB proteins regulate cellular CoQ distribution in Saccharomyces cerevisiae

   https://doi.org/10.1038/s41467-021-25084-7  

   Kemmerer, Zachary A. ; Robinson, Kyle P. ; Schmitz, Jonathan M. ; Manicki, Mateusz ; Paulson, Brett R. ; Jochem, Adam ; Hutchins, Paul D. ; Coon, Joshua J. ; Pagliarini, David J. ( August 2021 , Nature Communications)

   Beyond its role in mitochondrial bioenergetics, Coenzyme Q (CoQ, ubiquinone) serves as a key membrane-embedded antioxidant throughout the cell. However, how CoQ is mobilized from its site of synthesis on the inner mitochondrial membrane to other sites of action remains a longstanding mystery. Here, using a combination ofSaccharomyces cerevisiaegenetics, biochemical fractionation, and lipid profiling, we identify two highly conserved but poorly characterized mitochondrial proteins, Ypl109c (Cqd1) and Ylr253w (Cqd2), that reciprocally affect this process. Loss of Cqd1 skews cellular CoQ distribution away from mitochondria, resulting in markedly enhanced resistance to oxidative stress caused by exogenous polyunsaturated fatty acids, whereas loss of Cqd2 promotes the opposite effects. The activities of both proteins rely on their atypical kinase/ATPase domains, which they share with Coq8—an essential auxiliary protein for CoQ biosynthesis. Overall, our results reveal protein machinery central to CoQ trafficking in yeast and lend insights into the broader interplay between mitochondria and the rest of the cell.

    

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5. ATF4‐dependent increase in mitochondrial‐endoplasmic reticulum tethering following OPA1 deletion in skeletal muscle

   https://doi.org/10.1002/jcp.31204  

   Hinton, Jr., Antentor ; Katti, Prasanna ; Mungai, Margaret ; Hall, Duane D. ; Koval, Olha ; Shao, Jianqiang ; Vue, Zer ; Lopez, Edgar Garza ; Rostami, Rahmati ; Neikirk, Kit ; et al ( February 2024 , Journal of Cellular Physiology)

   Mitochondria and endoplasmic reticulum (ER) contact sites (MERCs) are protein‐ and lipid‐enriched hubs that mediate interorganellar communication by contributing to the dynamic transfer of Ca²⁺, lipid, and other metabolites between these organelles. Defective MERCs are associated with cellular oxidative stress, neurodegenerative disease, and cardiac and skeletal muscle pathology via mechanisms that are poorly understood. We previously demonstrated that skeletal muscle‐specific knockdown (KD) of the mitochondrial fusion mediator optic atrophy 1 (OPA1) induced ER stress and correlated with an induction of Mitofusin‐2, a known MERC protein. In the present study, we tested the hypothesis thatOpa1downregulation in skeletal muscle cells alters MERC formation by evaluating multiple myocyte systems, including from mice andDrosophila, and in primary myotubes. Our results revealed that OPA1 deficiency induced tighter and more frequent MERCs in concert with a greater abundance of MERC proteins involved in calcium exchange. Additionally, loss of OPA1 increased the expression of activating transcription factor 4 (ATF4), an integrated stress response (ISR) pathway effector. ReducingAtf4expression prevented the OPA1‐loss‐induced tightening of MERC structures. OPA1 reduction was associated with decreased mitochondrial and sarcoplasmic reticulum, a specialized form of ER, calcium, which was reversed following ATF4 repression. These data suggest that mitochondrial stress, induced by OPA1 deficiency, regulates skeletal muscle MERC formation in an ATF4‐dependent manner.

    

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