Ubiquitin-Proteasome-dependent Degradation of a Mitofusin, a Critical Regulator of Mitochondrial Fusion

Laboratory of Protein Dynamics and Signaling, National Cancer Institute, Frederick, MD 21702, USA.
Molecular biology of the cell (Impact Factor: 4.47). 07/2008; 19(6):2457-64. DOI: 10.1091/mbc.E08-02-0227
Source: PubMed


The mitochondrion is a dynamic membranous network whose morphology is conditioned by the equilibrium between ongoing fusion and fission of mitochondrial membranes. In the budding yeast, Saccharomyces cerevisiae, the transmembrane GTPase Fzo1p controls fusion of mitochondrial outer membranes. Deletion or overexpression of Fzo1p have both been shown to alter the mitochondrial fusion process indicating that maintenance of steady-state levels of Fzo1p are required for efficient mitochondrial fusion. Cellular levels of Fzo1p are regulated through degradation of Fzo1p by the F-box protein Mdm30p. How Mdm30p promotes degradation of Fzo1p is currently unknown. We have now determined that during vegetative growth Mdm30p mediates ubiquitylation of Fzo1p and that degradation of Fzo1p is an ubiquitin-proteasome-dependent process. In vivo, Mdm30p associates through its F-box motif with other core components of Skp1-Cullin-F-box (SCF) ubiquitin ligases. We show that the resulting SCF(Mdm30p) ligase promotes ubiquitylation of Fzo1p at mitochondria and its subsequent degradation by the 26S proteasome. These results provide the first demonstration that a cytosolic ubiquitin ligase targets a critical regulatory molecule at the mitochondrial outer membrane. This study provides a framework for developing an understanding of the function of Mdm30p-mediated Fzo1p degradation in the multistep process of mitochondrial fusion.

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Available from: Mickael M Cohen, May 30, 2014
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    • "UBP9/13 and CG5794 are not orthologous , but the underlying principle may be similar. Known targets of ubiquitylation in mitochondria include proteins involved in mitochondrial dynamics, such as mitofusins (Yonashiro et al, 2006; Cohen et al, 2008). The closest homologues of CG5794 in yeast and human perform diverse tasks related to peroxisomal protein import (Debelyy et al, 2011), cell-cycle regulation (Bozza & Zhuang, 2011), methylmercury resistance (Hwang et al, 2012), nuclear DNA repair (Sy et al, 2013), and signaling (Lui et al, 2011; Poalas et al, 2013). "
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    • "Samples were taken at the indicated time points, lysed, and loaded on native gel for activity assay. In order to detect Fzo1 levels, CHX chase was performed as described previously (Cohen et al., 2008, 2011). "
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    • "In fact, the recently described mitochondria-associated degradation (MAD) pathway involves the ubiquitin-selective chaperone Cdc48/p97 in retrotranslocation of substrates from mitochondria for degradation by the 26S proteasome (Cohen et al., 2008; Heo et al., 2010; Karbowski and Youle, 2011; Taylor and Rutter, 2011). Upon high oxidative stress conditions, Cdc48 and UPS components including the E3 ligase HUWE1 are recruited to the mitochondrial outer membrane (Cohen et al., 2008; Heo et al., 2010; Taylor and Rutter, 2011). This network of different mitochondrial quality control systems is transcriptionally controlled by the UPR mt (Haynes and Ron, 2010; Pellegrino et al., 2013); however, the coordination between mitochondrial and cellular proteostasis pathways has not been addressed so far. "
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    ABSTRACT: Mitochondria maintain cellular homeostasis by coordinating ATP synthesis with metabolic activity, redox signaling, and apoptosis. Excessive levels of mitochondria-derived reactive oxygen species (ROS) promote mitochondrial dysfunction, triggering numerous metabolic disorders. However, the molecular basis for the harmful effects of excessive ROS formation is largely unknown. Here, we identify a link between mitochondrial stress and ubiquitin-dependent proteolysis, which supports cellular surveillance both in Caenorhabditis elegans and humans. Worms defective in respiration with elevated ROS levels are limited in turnover of a GFP-based substrate protein, demonstrating that mitochondrial stress affects the ubiquitin/proteasome system (UPS). Intriguingly, we observed similar proteolytic defects for disease-causing IVD and COX1 mutations associated with mitochondrial failure in humans. Together, these results identify a conserved link between mitochondrial metabolism and ubiquitin-dependent proteostasis. Reduced UPS activity during pathological conditions might potentiate disease progression and thus provides a valuable target for therapeutic intervention.
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