No abstract

Mitochondria are dynamic organelles with essential roles in signaling and metabolism. We recently identified a cellular structure called the mitochondrial-derived compartment (MDC) that is generated from mitochondria in response to amino acid overabundance stress. How cells form MDCs is unclear. Here, we show that MDCs are dynamic structures that form and stably persist at sites of contact between the ER and mitochondria. MDC biogenesis requires the ER–mitochondria encounter structure (ERMES) and the conserved GTPase Gem1, factors previously implicated in lipid exchange and membrane tethering at ER–mitochondria contacts. Interestingly, common genetic suppressors of abnormalities displayed by ERMES mutants exhibit distinct abilities to rescue MDC formation in ERMES-depleted strains and are incapable of rescuing MDC formation in cells lacking Gem1. Thus, the function of ERMES and Gem1 in MDC biogenesis may extend beyond their conventional role in maintaining mitochondrial phospholipid homeostasis. Overall, this study identifies an important function for ER–mitochondria contacts in the biogenesis of MDCs.

48Amino acids are essential building blocks of life. However, increasing evidence 49 suggests that elevated amino acids cause cellular toxicity associated with numerous 50 metabolic disorders. How cells cope with elevated amino acids remains poorly 51 understood. Here, we show that a previously identified cellular structure, the 52 mitochondrial-derived compartment (MDC), is a dynamic, lumen-containing organelle 53 that functions to protect cells from amino acid stress. In response to amino acid 54 elevation, MDCs are generated from mitochondria, where they selectively sequester 55 and remove Tom70, a surface receptor required for import of nutrient carriers of the 56 SLC25 family. MDC formation is regulated by levels of mitochondrial carriers, and its 57 activation by amino acids occurs simultaneously with removal of plasma membrane-58 localized transporters via the multi-vesicular body (MVB) pathway. Combined loss of 59 MDC and MVB formation renders cells sensitive to elevated amino acids, suggesting 60 these pathways operate as a coordinated network to protect cells from amino acid 61 toxicity. 62 63 KEYWORDS 64Mitochondria, vacuole, amino acid, MDC, branched-chain amino acids, nutrient 65 transporter, lysosome 66 67 68 69 70 nutrient transporters via the multi-vesicular body (MVB) pathway also serves as a key 94 mechanism to control cellular nutrient uptake, and protects cells from amino acid toxicity 95 (Katzmann et al., 2002;Risinger et al., 2006;Rubio-Texeira and Kaiser, 2006; Ruiz et 96 al., 2017). Beyond lysosomes and MVBs, additional mechanisms cells utilize to protect 97 themselves from amino acid toxicity remain unclear. 98While investigating the impact of lysosome failure on mitochondrial health, we 99 identified a new cellular structure that forms from mitochondria when lysosomal 100 acidification is impaired, called the mitochondrial-derived compartment (MDC) (Hughes 101 et al., 2016). Upon formation, MDCs selectively incorporate a number of mitochondrial 102 proteins including Tom70, an outer membrane (OM) import receptor for mitochondrial 103 nutrient transporters (Sollner et al., 1990). By contrast, MDCs exclude most other 104 mitochondrial proteins, including those in the mitochondrial matrix, the intermembrane 105 space (IMS), and the majority of inner membrane (IM) proteins. After formation, MDCs 106 are released from mitochondria via mitochondrial fission and are degraded by 107 autophagy (Hughes et al., 2016). Currently, we know that MDCs are Tom70-enriched 108 foci that associate with mitochondria when cells lose lysosome acidification. Beyond 109 that, we understand little about the dynamics and regulation of MDC formation, as well 110 as the function of this new cellular compartment. 111Here, we show that MDCs are dynamic mitochondrial-associated compartments 112 that are generated in response to perturbations in intracellular amino acid homeostasis. 113Specifically, we find that high levels of branched-chain amino acids (BCAAs) and their 114 breakdown products promote MDC formation. ...

The integrated stress response (ISR) is a conserved mechanism by which eukaryotic cells remodel gene expression to adapt to intrinsic and extrinsic stressors rapidly and reversibly. The ISR is initiated when stress-activated protein kinases phosphorylate the major translation initiation factor eukaryotic translation initiation factor 2ɑ (eIF2ɑ), which globally suppresses translation initiation activity and permits the selective translation of stress-induced genes including important transcription factors such as activating transcription factor 4 (ATF4). Translationally repressed messenger RNAs (mRNAs) and noncoding RNAs assemble into cytoplasmic RNA-protein granules and polyadenylated RNAs are concomitantly stabilized. Thus, regulated changes in mRNA translation, stability, and localization to RNA-protein granules contribute to the reprogramming of gene expression that defines the ISR. We discuss fundamental mechanisms of RNA regulation during the ISR and provide an overview of a growing class of genetic disorders associated with mutant alleles of key translation factors in the ISR pathway.

SUMMARYWe recently identified a new cellular structure in yeast, called the Mitochondrial-Derived Compartment (MDC), that forms on mitochondria in response to amino acid excess. While emerging evidence supports an important function for MDCs in protecting cells from metabolic stress, whether this system exists beyond yeast remains unclear. Here, we show that MDCs are conserved in mammals, and like their yeast counterparts, are responsive to the intracellular amino acid content. Specifically, we find that inhibition of protein translation stimulates formation of dynamic, micron-sized compartments that associate with the mitochondrial network. These compartments are enriched for the carrier receptor Tomm70A and other select mitochondrial outer and inner membrane cargo, associate with the ER membrane, and require the conserved GTPase Miro1 for formation. Mammalian MDCs are responsive to changes in amino acid levels during translation inhibition, and are not activated by other common cellular stressors. Thus, MDCs represent an evolutionarily conserved nutrient-responsive mitochondrial remodeling system.