Eukaryotic cells store lipids in cytosolic organelles known as lipid droplets (LDs). Lipid droplet bud from the endoplasmic reticulum (ER), and may be harvested by the vacuole for energy during prolonged periods of starvation. How cells spatially coordinate LD production is poorly understood. Here, we demonstrate that yeast ER-vacuole contact sites (NVJs) physically expand in response to metabolic stress, and serve as sites for LD production. NVJ tether Mdm1 demarcates sites of LD budding, and interacts with fatty acyl-CoA synthases at the NVJ periphery. Artificially expanding the NVJ through over-expressing Mdm1 is sufficient to drive NVJ-associated LD production, whereas ablating the NVJ induces defects in fatty acid-to-triglyceride production. Collectively, our data suggest a tight metabolic link between nutritional stress and LD biogenesis that is spatially coordinated at ER-vacuole contact sites.

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Lipins are evolutionarily conserved proteins found from yeasts to humans. Mammalian and yeast lipin proteins have been shown to control gene expression and to enzymatically convert phosphatidate to diacylglycerol, an essential precursor in triacylglcerol (TAG) and phospholipid synthesis. Loss of lipin 1 in the mouse, but not in humans, leads to lipodystrophy and fatty liver disease. Here we show that the single lipin orthologue of Drosophila melanogaster (dLipin) is essential for normal adipose tissue (fat body) development and TAG storage. dLipin mutants are characterized by reductions in larval fat body mass, whole-animal TAG content, and lipid droplet size. Individual cells of the underdeveloped fat body are characterized by increased size and ultrastructural defects affecting cell nuclei, mitochondria, and autophagosomes. Under starvation conditions, dLipin is transcriptionally upregulated and functions to promote survival. Together, these data show that dLipin is a central player in lipid and energy metabolism, and they establish Drosophila as a genetic model for further studies of conserved functions of the lipin family of metabolic regulators.Neutral lipids, or triacylglycerols (TAG), are principal energy stores of the eukaryotic cell. Metazoans have evolved specialized tissues that store TAG and make free fatty acids or other derivatives of TAG available to other tissues. Besides having storage functions, these specialized adipose tissues participate in the control of energy homeostasis by producing and releasing hormones and other signaling molecules (21, 26).Severe underdevelopment of the adipose tissue is observed in mice carrying the fatty liver dystrophy (fld) mutation. Lack of fat tissue is associated with transient postnatal accumulation of TAG in the liver, defects in the peripheral nervous system, and insulin resistance (15,16,29). Cloning of the fld gene (renamed lipin 1 [22]) revealed that it encodes a member of an evolutionarily old family of proteins found in a wide variety of eukaryotic organisms, including fungi, plants, and protozoans (22). Both yeast and mammalian lipin proteins act as type 1 phosphatidate phosphatases (PAP1), converting phosphatidate to diacylglycerol (DAG), and as transcriptional coregulators (4, 5, 11, 32). The protein domains responsible for these activities are conserved in lipin proteins of other species, indicating that this functional dichotomy is both evolutionarily old and central to lipin function. DAG produced by mammalian lipin 1 serves as a direct biosynthetic precursor of TAG and phospholipids, whereas the transcriptional coregulator function contributes to the control of genes involved in hepatic ␤-oxidation of fatty acids, the tricarboxylic acid (TCA) cycle, and mitochondrial oxidative phosphorylation (5). While these processes appear to be upregulated by lipin 1, enzymes involved in fatty acid and TAG synthesis are downregulated. In yeast, lipin suppresses genes involved in phospholipid synthesis (32). Loss of lipin in yeast leads to the overgrowth of i...

Lipid droplets (LDs) serve as cytoplasmic reservoirs for energy-rich fatty acids (FAs) stored in the form of triacylglycerides (TAGs). During nutrient stress, yeast LDs cluster adjacent to the vacuole/lysosome, but how this LD accumulation is coordinated remains poorly understood. The ER protein Mdm1 is a molecular tether that plays a role in clustering LDs during nutrient depletion, but its mechanism of function remains unknown. Here, we show that Mdm1 associates with LDs through its hydrophobic N-terminal region, which is sufficient to demarcate sites for LD budding. Mdm1 binds FAs via its Phox-associated domain and coenriches with fatty acyl–coenzyme A ligase Faa1 at LD bud sites. Consistent with this, loss of MDM1 perturbs free FA activation and Dga1-dependent synthesis of TAGs, elevating the cellular FA level, which perturbs ER morphology and sensitizes yeast to FA-induced lipotoxicity. We propose that Mdm1 coordinates FA activation adjacent to the vacuole to promote LD production in response to stress, thus maintaining ER homeostasis.

Circulating carbohydrates are an essential energy source, perturbations in which are pathognomonic of various diseases, diabetes being the most prevalent. Yet many of the genes underlying diabetes and its characteristic hyperglycaemia remain elusive. Here we use physiological and genetic interrogations in D. melanogaster to uncover the ‘glucome', the complete set of genes involved in glucose regulation in flies. Partial genomic screens of ∼1,000 genes yield ∼160 hyperglycaemia ‘flyabetes' candidates that we classify using fat body- and muscle-specific knockdown and biochemical assays. The results highlight the minor glucose fraction as a physiological indicator of metabolism in Drosophila. The hits uncovered in our screen may have conserved functions in mammalian glucose homeostasis, as heterozygous and homozygous mutants of Ck1alpha in the murine adipose lineage, develop diabetes. Our findings demonstrate that glucose has a role in fly biology and that genetic screenings carried out in flies may increase our understanding of mammalian pathophysiology.