RESUMEN
Mitophagy must be carefully regulated to ensure that cells maintain appropriate numbers of functional mitochondria. The SCFFBXL4 ubiquitin ligase complex suppresses mitophagy by controlling the degradation of BNIP3 and NIX mitophagy receptors, and FBXL4 mutations result in mitochondrial disease as a consequence of elevated mitophagy. Here, we reveal that the mitochondrial phosphatase PPTC7 is an essential cofactor for SCFFBXL4-mediated destruction of BNIP3 and NIX, suppressing both steady-state and induced mitophagy. Disruption of the phosphatase activity of PPTC7 does not influence BNIP3 and NIX turnover. Rather, a pool of PPTC7 on the mitochondrial outer membrane acts as an adaptor linking BNIP3 and NIX to FBXL4, facilitating the turnover of these mitophagy receptors. PPTC7 accumulates on the outer mitochondrial membrane in response to mitophagy induction or the absence of FBXL4, suggesting a homoeostatic feedback mechanism that attenuates high levels of mitophagy. We mapped critical residues required for PPTC7-BNIP3/NIX and PPTC7-FBXL4 interactions and their disruption interferes with both BNIP3/NIX degradation and mitophagy suppression. Collectively, these findings delineate a complex regulatory mechanism that restricts BNIP3/NIX-induced mitophagy.
Asunto(s)
Proteínas F-Box , Proteínas de la Membrana , Proteínas Mitocondriales , Mitofagia , Proteolisis , Proteínas Proto-Oncogénicas , Mitofagia/genética , Proteínas de la Membrana/metabolismo , Proteínas de la Membrana/genética , Humanos , Proteínas F-Box/metabolismo , Proteínas F-Box/genética , Proteínas Proto-Oncogénicas/metabolismo , Proteínas Proto-Oncogénicas/genética , Proteínas Mitocondriales/metabolismo , Proteínas Mitocondriales/genética , Mitocondrias/metabolismo , Membranas Mitocondriales/metabolismo , Unión Proteica , Fosfoproteínas Fosfatasas/metabolismo , Fosfoproteínas Fosfatasas/genética , Proteínas Supresoras de Tumor/metabolismo , Proteínas Supresoras de Tumor/genética , Proteínas Ligasas SKP Cullina F-box/metabolismo , Proteínas Ligasas SKP Cullina F-box/genética , Células HeLa , Células HEK293 , Animales , Ubiquitina-Proteína LigasasRESUMEN
Mitophagy is a critical mitochondrial quality control process that selectively removes dysfunctional or excess mitochondria through the autophagy-lysosome system. The process is tightly controlled to ensure cellular and physiological homeostasis. Insufficient mitophagy can result in failure to remove damaged mitochondria and consequent cellular degeneration, but it is equally important to appropriately restrain mitophagy to prevent excessive mitochondrial depletion. Here, we discuss our recent discovery that the SKP1-CUL1-F-box (SCF)-FBXL4 (F-box and leucine-rich repeat protein 4) E3 ubiquitin ligase localizes to the mitochondrial outer membrane, where it constitutively mediates the ubiquitination and degradation of BNIP3L/NIX and BNIP3 mitophagy receptors to suppress mitophagy. The post-translational regulation of BNIP3L and BNIP3 is disrupted in mitochondrial DNA depletion syndrome 13 (MTDPS13), a multi-systemic disorder caused by mutations in the FBXL4 gene and characterized by elevated mitophagy and mitochondrial DNA/mtDNA depletion in patient fibroblasts. Our results demonstrate that mitophagy is not solely stimulated in response to specific conditions but is instead also actively suppressed through the continuous degradation of BNIP3L and BNIP3 mediated by the SCF-FBXL4 ubiquitin ligase. Thus, cellular conditions or signaling events that prevent the FBXL4-mediated turnover of BNIP3L and BNIP3 on specific mitochondria are expected to facilitate their selective removal.