Mitofusin-mediated contacts between mitochondria and peroxisomes regulate mitochondrial fusion.

Mitofusins are large GTPases that trigger fusion of mitochondrial outer membranes. Similarly to the human mitofusin Mfn2, which also tethers mitochondria to the endoplasmic reticulum (ER), the yeast mitofusin Fzo1 stimulates contacts between Peroxisomes and Mitochondria when overexpressed. Yet, the physiological significance and function of these "PerMit" contacts remain unknown. Here, we demonstrate that Fzo1 naturally localizes to peroxisomes and promotes PerMit contacts in physiological conditions. These contacts are regulated through co-modulation of Fzo1 levels by the ubiquitin-proteasome system (UPS) and by the desaturation status of fatty acids (FAs). Contacts decrease under low FA desaturation but reach a maximum during high FA desaturation. High-throughput genetic screening combined with high-resolution cellular imaging reveal that Fzo1-mediated PerMit contacts favor the transit of peroxisomal citrate into mitochondria. In turn, citrate enters the TCA cycle to stimulate the mitochondrial membrane potential and maintain efficient mitochondrial fusion upon high FA desaturation. These findings thus unravel a mechanism by which inter-organelle contacts safeguard mitochondrial fusion.

The DEAD-Box RNA Helicase Ded1 Is Associated with Translating Ribosomes.

DEAD-box RNA helicases are ATP-dependent RNA binding proteins and RNA-dependent ATPases that possess weak, nonprocessive unwinding activity in vitro, but they can form long-lived complexes on RNAs when the ATPase activity is inhibited. Ded1 is a yeast DEAD-box protein, the functional ortholog of mammalian DDX3, that is considered important for the scanning efficiency of the 48S pre-initiation complex ribosomes to the AUG start codon. We used a modified PAR-CLIP technique, which we call quicktime PAR-CLIP (qtPAR-CLIP), to crosslink Ded1 to 4-thiouridine-incorporated RNAs in vivo using UV light centered at 365 nm. The irradiation conditions are largely benign to the yeast cells and to Ded1, and we are able to obtain a high efficiency of crosslinking under physiological conditions. We find that Ded1 forms crosslinks on the open reading frames of many different mRNAs, but it forms the most extensive interactions on relatively few mRNAs, and particularly on mRNAs encoding certain ribosomal proteins and translation factors. Under glucose-depletion conditions, the crosslinking pattern shifts to mRNAs encoding metabolic and stress-related proteins, which reflects the altered translation. These data are consistent with Ded1 functioning in the regulation of translation elongation, perhaps by pausing or stabilizing the ribosomes through its ATP-dependent binding.

Analysis of ATG8 Family Members Using LC3-Interacting Regions (LIR)-Based Molecular Traps.

The ATG8 family of proteins regulates the autophagy process from the autophagosome maturation and cargo recruitment up to degradation. Autophagy dysfunction is involved in the development of multiple diseases. The LC3 interacting region (LIR)-based molecular traps have been designed to isolate endogenous ATG8 proteins and their interactors in order to facilitate the study of selective autophagy events. Here, we summarize protocols describing LC3 traps and sample preparation as well as adaptations for the analysis of ATG8 proteins in different biological models. This protocol was optimized to prepare affinity columns, reduce background, and improve the protein recovery to be analyzed by immunodetection with antibodies recognizing proteins of interest.

Exploring selective autophagy events in multiple biologic models using LC3-interacting regions (LIR)-based molecular traps.

Autophagy is an essential cellular pathway that ensures degradation of a wide range of substrates including damaged organelles or large protein aggregates. Understanding how this proteolytic pathway is regulated would increase our comprehension on its role in cellular physiology and contribute to identify biomarkers or potential drug targets to develop more specific treatments for disease in which autophagy is dysregulated. Here, we report the development of molecular traps based in the tandem disposition of LC3-interacting regions (LIR). The estimated affinity of LC3-traps for distinct recombinant LC3/GABARAP proteins is in the low nanomolar range and allows the capture of these proteins from distinct mammalian cell lines, S. cerevisiae and C. elegans. LC3-traps show preferences for GABARAP/LGG1 or LC3/LGG2 and pull-down substrates targeted to proteaphagy and mitophagy. Therefore, LC3-traps are versatile tools that can be adapted to multiple applications to monitor selective autophagy events in distinct physiologic and pathologic circumstances.

The regulation of mitochondrial homeostasis by the ubiquitin proteasome system.

From mitochondrial quality control pathways to the regulation of specific functions, the Ubiquitin Proteasome System (UPS) could be compared to a Swiss knife without which mitochondria could not maintain its integrity in the cell. Here, we review the mechanisms that the UPS employs to regulate mitochondrial function and efficiency. For this purpose, we depict how Ubiquitin and the Proteasome participate in diverse quality control pathways that safeguard entry into the mitochondrial compartment. A focus is then achieved on the UPS-mediated control of the yeast mitofusin Fzo1 which provides insights into the complex regulation of this particular protein in mitochondrial fusion. We ultimately dissect the mechanisms by which the UPS controls the degradation of mitochondria by autophagy in both mammalian and yeast systems. This organization should offer a useful overview of this abundant but fascinating literature on the crosstalks between mitochondria and the UPS.

An ubiquitin-dependent balance between mitofusin turnover and fatty acids desaturation regulates mitochondrial fusion.

Mitochondrial integrity relies on homotypic fusion between adjacent outer membranes, which is mediated by large GTPases called mitofusins. The regulation of this process remains nonetheless elusive. Here, we report a crosstalk between the ubiquitin protease Ubp2 and the ubiquitin ligases Mdm30 and Rsp5 that modulates mitochondrial fusion. Ubp2 is an antagonist of Rsp5, which promotes synthesis of the fatty acids desaturase Ole1. We show that Ubp2 also counteracts Mdm30-mediated turnover of the yeast mitofusin Fzo1 and that Mdm30 targets Ubp2 for degradation thereby inducing Rsp5-mediated desaturation of fatty acids. Exogenous desaturated fatty acids inhibit Ubp2 degradation resulting in higher levels of Fzo1 and maintenance of efficient mitochondrial fusion. Our results demonstrate that the Mdm30-Ubp2-Rsp5 crosstalk regulates mitochondrial fusion by coordinating an intricate balance between Fzo1 turnover and the status of fatty acids saturation. This pathway may link outer membrane fusion to lipids homeostasis.

Ubiquitination of ERMES components by the E3 ligase Rsp5 is involved in mitophagy.

Mitochondria are dynamic organelles that undergo permanent fission and fusion events. These processes play an essential role in maintaining normal cellular function. In the yeast Saccharomyces cerevisiae, the endoplasmic reticulum-mitochondrial encounter structure (ERMES) is a marker of sites of mitochondrial division, but it is also involved in a plethora of other mitochondrial functions. However, it remains unclear how these different functions are regulated. We show here that Mdm34 and Mdm12, 2 components of ERMES, are ubiquitinated by the E3 ligase Rsp5. This ubiquitination is not involved in mitochondrial dynamics or in the distribution and turnover of ERMES. Nevertheless, the ubiquitination of Mdm34 and Mdm12 was required for efficient mitophagy. We thus report here the first identification of ubiquitinated substrates participating in yeast mitophagy.

The DEAD-box helicase Ded1 from yeast is an mRNP cap-associated protein that shuttles between the cytoplasm and nucleus.

The DEAD-box helicase Ded1 is an essential yeast protein that is closely related to mammalian DDX3 and to other DEAD-box proteins involved in developmental and cell cycle regulation. Ded1 is considered to be a translation-initiation factor that helps the 40S ribosome scan the mRNA from the 5' 7-methylguanosine cap to the AUG start codon. We used IgG pull-down experiments, mass spectrometry analyses, genetic experiments, sucrose gradients, in situ localizations and enzymatic assays to show that Ded1 is a cap-associated protein that actively shuttles between the cytoplasm and the nucleus. NanoLC-MS/MS analyses of purified complexes show that Ded1 is present in both nuclear and cytoplasmic mRNPs. Ded1 physically interacts with purified components of the nuclear CBC and the cytoplasmic eIF4F complexes, and its enzymatic activity is stimulated by these factors. In addition, we show that Ded1 is genetically linked to these factors. Ded1 comigrates with these proteins on sucrose gradients, but treatment with rapamycin does not appreciably alter the distribution of Ded1; thus, most of the Ded1 is in stable mRNP complexes. We conclude that Ded1 is an mRNP cofactor of the cap complex that may function to remodel the different mRNPs and thereby regulate the expression of the mRNAs.

Gex1 is a yeast glutathione exchanger that interferes with pH and redox homeostasis.

In the yeast Saccharomyces cerevisiae, glutathione plays a major role in heavy metal detoxification and protection of cells against oxidative stress. We show that Gex1 is a new glutathione exchanger. Gex1 and its paralogue Gex2 belong to the major facilitator superfamily of transporters and display similarities to the Aft1-regulon family of siderophore transporters. Gex1 was found mostly at the vacuolar membrane and, to a lesser extent, at the plasma membrane. Gex1 expression was induced under conditions of iron depletion and was principally dependent on the iron-responsive transcription factor Aft2. However, a gex1Δ gex2Δ strain displayed no defect in known siderophore uptake. The deletion mutant accumulated intracellular glutathione, and cells overproducing Gex1 had low intracellular glutathione contents, with glutathione excreted into the extracellular medium. Furthermore, the strain overproducing Gex1 induced acidification of the cytosol, confirming the involvement of Gex1 in proton transport as a probable glutathione/proton antiporter. Finally, the imbalance of pH and glutathione homeostasis in the gex1Δ gex2Δ and Gex1-overproducing strains led to modulations of the cAMP/protein kinase A and protein kinase C1 mitogen-activated protein kinase signaling pathways.

Expressing the Erwinia amylovora type III effector DspA/E in the yeast Saccharomyces cerevisiae strongly alters cellular trafficking.

Erwinia amylovora is responsible for fire blight, a necrotic disease of apples and pears. E. amylovora relies on a type III secretion system (T3SS) to induce disease on host plants. DspA/E belongs to the AvrE family of type III effector. Effectors of the AvrE family are injected via the T3SS in plant cell and are important to promote bacterial growth following infection and to suppress plant defense responses. Their mode of action in the plant cells is unknown. Here we study the physiological effects induced by dspA/E expression in the yeast Saccharomyces cerevisiae. Expression of dspA/E in the yeast inhibits cell growth. This growth inhibition is associated with perturbations of the actin cytoskeleton and endocytosis.

Versatile role of the yeast ubiquitin ligase Rsp5p in intracellular trafficking.

The ubiquitin ligase (E3) Rsp5p is the only member of the Nedd (neural-precursor-cell-expressed, developmentally down-regulated) 4 family of E3s present in yeast. Rsp5p has several proteasome-independent functions in membrane protein trafficking, including a role in the ubiquitination of most plasma membrane proteins, leading to their endocytosis. Rsp5p is also required for the ubiquitination of endosomal proteins, leading to their sorting to the internal vesicles of MVBs (multivesicular bodies). Rsp5p catalyses the attachment of non-conventional ubiquitin chains, linked through ubiquitin Lys-63, to some endocytic and MVB cargoes. This modification appears to be required for efficient sorting, possibly because these chains have a greater affinity for the ubiquitin-binding domains present within endocytic or MVB sorting complexes. The mechanisms involved in the recognition of plasma membrane and MVB substrates by Rsp5p remain unclear. A subset of Rsp5/Nedd4 substrates have a 'PY motif' and are recognized directly by the WW (Trp-Trp) domains of Rsp5p. Most Rsp5p substrates do not carry PY motifs, but some may depend on PY-containing proteins for their ubiquitination by Rsp5p, consistent with the latter's acting as specificity factors or adaptors. As in other ubiquitin-conjugating systems, these adaptors are also Rsp5p substrates and undergo ubiquitin-dependent trafficking. In the present review, we discuss recent examples illustrating the role of Rsp5p in membrane protein trafficking and providing new insights into the regulation of this E3 by adaptor proteins.

Substrate- and ubiquitin-dependent trafficking of the yeast siderophore transporter Sit1.

Eukaryotic plasma membrane transporters are subjected to a tightly regulated intracellular trafficking. The yeast siderophore iron transporter1 (Sit1) displays substrate-regulated trafficking. It is targeted to the plasma membrane or to a vacuolar degradative pathway when synthesized in the presence or absence of external substrate, respectively. Sorting of Sit1 to the vacuolar pathway is dependent on the clathrin adaptor Gga2, and more specifically on its C-GAT subdomain. Plasma membrane undergoes substrate-induced ubiquitylation dependent on the Rsp5 ubiquitin protein ligase. Sit1 is also ubiquitylated in an Rsp5-dependent manner in internal compartments when expressed in the absence of substrate. In several rsp5 mutants including cells deleted for RSP5, Sit1 expressed in the absence of substrate is correctly targeted to the endosomal pathway but its sorting to multivesicular bodies (MVBs) is impaired. Consequently, it displays endosome to plasma membrane targeting, with kinetics similar to those observed in vps mutants defective for MVB sorting. Plasma membrane Sit1 is modified by Lys63-linked ubiquitin chains. We also show for the first time in yeast that modification by this latter type of ubiquitin chains is required directly or indirectly for efficient MVB sorting, as it is for efficient internalization at the plasma membrane.

Trafficking of siderophore transporters in Saccharomyces cerevisiae and intracellular fate of ferrioxamine B conjugates.

We have studied the intracellular trafficking of Sit1 [ferrioxamine B (FOB) transporter] and Enb1 (enterobactin transporter) in Saccharomyces cerevisiae using green fluorescent protein (GFP) fusion proteins. Enb1 was constitutively targeted to the plasma membrane. Sit1 was essentially targeted to the vacuolar degradation pathway when synthesized in the absence of substrate. Massive plasma membrane sorting of Sit1 was induced by various siderophore substrates of Sit1, and by coprogen, which is not a substrate of Sit1. Thus, different siderophore transporters use different regulated trafficking processes. We also studied the fate of Sit1-mediated internalized siderophores. Ferrioxamine B was recovered in isolated vacuolar fractions, where it could be detected spectrophotometrically. Ferrioxamine B coupled to an inhibitor of mitochondrial protoporphyrinogen oxidase (acifluorfen) could not reach its target unless the cells were disrupted, confirming the tight compartmentalization of siderophores within cells. Ferrioxamine B coupled to a fluorescent moiety, FOB-nitrobenz-2-oxa-1,3-diazole, used as a Sit1-dependent iron source, accumulated in the vacuolar lumen even in mutants displaying a steady-state accumulation of Sit1 at the plasma membrane or in endosomal compartments. Thus, the fates of siderophore transporters and siderophores diverge early in the trafficking process.

Interactions between conserved domains within homodimers in the BIG1, BIG2, and GBF1 Arf guanine nucleotide exchange factors.

Guanine nucleotide exchange factors carrying a Sec7 domain (ArfGEFs) activate the small GTP-binding protein Arf, a major regulator of membrane remodeling and protein trafficking in eukaryotic cells. Only two of the seven subfamilies of ArfGEFs (GBF and BIG) are found in all eukaryotes. In addition to the Sec7 domain, which catalyzes GDP/GTP exchange on Arf, the GBF and BIG ArfGEFs have five common homology domains. Very little is known about the functions of these noncatalytic domains, but it is likely that they serve to integrate upstream signals that define the conditions of Arf activation. Here we describe interactions between two conserved domains upstream of the Sec7 domain (DCB and HUS) that determine the architecture of the N-terminal regions of the GBF and BIG ArfGEFs using a combination of biochemical, yeast two-hybrid, and cellular assays. Our data demonstrate a strong interaction between DCB domains within GBF1, BIG1, and BIG2 to maintain homodimers and an interaction between DCB and HUS domains within each homodimer. The DCB/HUS interaction is mediated by the HUS box, the most conserved motif in large ArfGEFs after the Sec7 domain. In support of the in vitro data, we show that both the DCB and the HUS domains are necessary for GBF1 dimerization in mammalian cells and that the DCB domain is essential for yeast viability. We propose that the dimeric DCB-HUS structural unit exists in all members of the GBF and BIG ArfGEF groups and in the related Mon2p family and probably serves an important regulatory role in Arf activation.

Heterologous expression of a plant uracil transporter in yeast: improvement of plasma membrane targeting in mutants of the Rsp5p ubiquitin protein ligase.

Plasma membrane proteins involved in transport processes play a crucial role in cell physiology. On account of these properties, these molecules are ideal targets for development of new therapeutic and agronomic agents. However, these proteins are of low abundance, which limits their study. Although yeast seems ideal for expressing heterologous transporters, plasma membrane proteins are often retained in intracellular compartments. We tried to find yeast mutants potentially able to improve functional expression of a whole set of heterologous transporters. We focused on Arabidopsis thaliana ureide transporter 1 (AtUPS1), previously cloned by functional complementation in yeast. Tagged versions of AtUPS1 remain mostly trapped in the endoplasmic reticulum and were able to reach slowly the plasma membrane. In contrast, untagged AtUPS1 is rapidly delivered to plasma membrane, where it remains in stable form. Tagged and untagged versions of AtUPS1 were expressed in cells deficient in the ubiquitin ligase Rsp5p, involved in various stages of the intracellular trafficking of membrane-bound proteins. rsp5 mutants displayed improved steady state amounts of untagged and tagged versions of AtUPS1. rsp5 cells are thus powerful tools to solve the many problems inherent to heterologous expression of membrane proteins in yeast, including ER retention.

Yeast functional analysis: identification of two essential genes involved in ER to Golgi trafficking.

We screened for genes potentially involved in the secretory and vacuolar pathways a collection of 61 yeast strains, each bearing an essential orphan gene regulated by the tetO7-CYC1 promoter that can be down-regulated by doxycycline. After down-regulating the expression of these genes, we performed systematic Western blot analysis for markers of the secretory and vacuolar pathways that undergo post-translational modifications in their intracellular trafficking. Accumulation of protein precursors, revealed by Western immunoblot analysis, indicates defects in the secretory pathway or in associated biochemical modifications. After screening the whole collection, we identified two genes involved in ER to Golgi trafficking: RER2, a cis-prenyl transferase, and USE1, the function of which was unknown. We demonstrated that repression of USE1 also leads to BiP secretion, and therefore likely affects retrograde, in addition to anterograde, ER to Golgi trafficking. The collection also includes two essential genes involved in intracellular trafficking that were conveniently repressed without resulting growth or trafficking defects.

Yeast Vps55p, a functional homolog of human obesity receptor gene-related protein, is involved in late endosome to vacuole trafficking.

The Saccharomyces cerevisiae VPS55 (YJR044c) gene encodes a small protein of 140 amino acids with four potential transmembrane domains. VPS55 belongs to a family of genes of unknown function, including the human gene encoding the obesity receptor gene-related protein (OB-RGRP). Yeast cells with a disrupted VPS55 present normal vacuolar morphology, but exhibit an abnormal secretion of the Golgi form of the soluble vacuolar carboxypeptidase Y. However, trafficking of the membrane-bound vacuolar alkaline phosphatase remains normal. The endocytosis of uracil permease, used as an endocytic marker, is normal in vps55Delta cells, but its degradation is delayed and this marker transiently accumulates in late endosomal compartments. We also found that Vps55p is mainly localized in the late endosomes. Collectively, these results indicate that Vps55p is involved in late endosome to vacuole trafficking. Finally, we show that human OB-RGRP displays the same distribution as Vps55p and corrects the phenotypic defects of the vps55Delta strain. Therefore, the function of Vps55p has been conserved throughout evolution. This study highlights the importance of the multispanning Vps55p and OB-RGRP in membrane trafficking to the vacuole/lysosome of eukaryotic cells.

Mutants defective in secretory/vacuolar pathways in the EUROFAN collection of yeast disruptants.

We have screened the EUROFAN (European Functional Analysis Network) deletion strain collection for yeast mutants defective in secretory/vacuolar pathways and/or associated biochemical modifications. We used systematic Western immunoblotting to analyse the electrophoretic pattern of several markers of the secretory/vacuolar pathways, the soluble alpha-factor, the periplasmic glycoprotein invertase, the plasma membrane GPI-anchored protein Gas1p, and two vacuolar proteins, the soluble carboxypeptidase Y and the membrane-bound alkaline phosphatase, which are targeted to the vacuole by different pathways. We also used colony immunoblotting to monitor the secretion of carboxypeptidase Y into the medium, to identify disruptants impaired in vacuolar targeting. We identified 25 mutants among the 631 deletion strains. Nine of these mutants were disrupted in genes identified in recent years on the basis of their involvement in trafficking (VPS53, VAC7, VAM6, APM3, SYS1), or glycosylation (ALG12, ALG9, OST4, ROT2). Three of these genes were identified on the basis of trafficking defects by ourselves and others within the EUROFAN project (TLG2, RCY1, MON2). The deletion of ERV29, which encodes a COPII vesicle protein, impaired carboxypeptidase Y trafficking from the endoplasmic reticulum to the Golgi apparatus. We also identified eight unknown ORFs, the deletion of which reduced Golgi glycosylation or impaired the Golgi to vacuole trafficking of carboxypeptidase Y. YJR044c, which we identified as a new VPS gene, encodes a protein with numerous homologues of unknown function in sequence databases.