RESUMEN
Lipid droplets (LDs) are the major fat storage organelles in eukaryotic cells, but how their size is regulated is unknown. Using genetic screens in C. elegans for LD morphology defects in intestinal cells, we found that mutations in atlastin, a GTPase required for homotypic fusion of endoplasmic reticulum (ER) membranes, cause not only ER morphology defects, but also a reduction in LD size. Similar results were obtained after depletion of atlastin or expression of a dominant-negative mutant, whereas overexpression of atlastin had the opposite effect. Atlastin depletion in Drosophila fat bodies also reduced LD size and decreased triglycerides in whole animals, sensitizing them to starvation. In mammalian cells, co-overexpression of atlastin-1 and REEP1, a paralog of the ER tubule-shaping protein DP1/REEP5, generates large LDs. The effect of atlastin-1 on LD size correlates with its activity to promote membrane fusion in vitro. Our results indicate that atlastin-mediated fusion of ER membranes is important for LD size regulation.
Asunto(s)
Proteínas de Caenorhabditis elegans/metabolismo , Vesículas Citoplasmáticas/química , GTP Fosfohidrolasas/metabolismo , Proteínas de Unión al GTP/metabolismo , Proteínas de la Membrana/metabolismo , Animales , Células COS , Caenorhabditis elegans/metabolismo , Proteínas de Caenorhabditis elegans/antagonistas & inhibidores , Proteínas de Caenorhabditis elegans/genética , Chlorocebus aethiops , Vesículas Citoplasmáticas/metabolismo , Drosophila/metabolismo , Retículo Endoplásmico/metabolismo , GTP Fosfohidrolasas/antagonistas & inhibidores , GTP Fosfohidrolasas/genética , Proteínas de Unión al GTP/genética , Humanos , Proteínas de la Membrana/genética , Proteínas de Transporte de Membrana/genética , Proteínas de Transporte de Membrana/metabolismo , Mutación , Interferencia de ARN , ARN Interferente Pequeño/metabolismoRESUMEN
At the subcellular level, fat storage is confined to the evolutionarily conserved compartments termed lipid droplets (LDs), which are closely associated with the endoplasmic reticulum (ER). However, the molecular mechanisms that enable ER-LD interaction and facilitate neutral lipid loading into LDs are poorly understood. In this paper, we present evidence that FATP1/acyl-CoA synthetase and DGAT2/diacylglycerol acyltransferase are components of a triglyceride synthesis complex that facilitates LD expansion. A loss of FATP1 or DGAT2 function blocked LD expansion in Caenorhabditis elegans. FATP1 preferentially associated with DGAT2, and they acted synergistically to promote LD expansion in mammalian cells. Live imaging indicated that FATP1 and DGAT2 are ER and LD resident proteins, respectively, and electron microscopy revealed FATP1 and DGAT2 foci close to the LD surface. Furthermore, DGAT2 that was retained in the ER failed to support LD expansion. We propose that the evolutionarily conserved FATP1-DGAT2 complex acts at the ER-LD interface and couples the synthesis and deposition of triglycerides into LDs both physically and functionally.
