title: Dissecting seipin function: the localized accumulation of phosphatidic acid at ER/LD junctions in the absence of seipin is suppressed by Sei1p(ΔNterm) only in combination with Ldb16p.
abstract: BACKGROUND
  Seipin is required for the correct assembly of cytoplasmic lipid droplets. In the absence of the yeast seipin homolog Sei1p (formerly Fld1p), droplets are slow to bud from the endoplasmic reticulum, lack the normal component of proteins on their surface, are highly heterogeneous in size and shape, often bud into the nucleus, and promote local proliferation of the endoplasmic reticulum in which they become tangled. But the mechanism by which seipin catalyzes lipid droplet formation is still uncertain.
  
  
  RESULTS
  Seipin prevents a localized accumulation of phosphatidic acid (PA puncta) at ER-droplet junctions. PA puncta were detected with three different probes: Opi1p, Spo20p(51-91) and Pah1p. A system of droplet induction was used to show that PA puncta were not present until droplets were formed; the puncta appeared regardless of whether droplets consisted of triacylglycerol or steryl ester. Deletion strains were used to demonstrate that a single phosphatidic acid-producing enzyme is not responsible for the generation of the puncta, and the puncta remain resistant to overexpression of enzymes that metabolize phosphatidic acid, suggesting that this lipid is trapped in a latent compartment. Suppression of PA puncta requires the first 14 amino acids of Sei1p (Nterm), a domain that is also important for initiation of droplet assembly. Consistent with recent evidence that Ldb16p and Sei1p form a functional unit, the PA puncta phenotype in the ldb16Δ sei1Δ strain was rescued by human seipin. Moreover, PA puncta in the sei1Δ strain expressing Sei1p(ΔNterm) was suppressed by overexpression of Ldb16p, suggesting a functional interaction of Nterm with this protein. Overexpression of both Sei1p and Ldb16p, but not Sei1p alone, is sufficient to cause a large increase in droplet number. However, Ldb16p alone increases triacylglycerol accumulation in the ldb16Δ sei1Δ background.
  
  
  CONCLUSIONS
  We hypothesize that seipin prevents formation of membranes with extreme curvature at endoplasmic reticulum/droplet junctions that would attract phosphatidic acid. While Ldb16p alone can affect triacylglycerol accumulation, proper droplet formation requires the collaboration of Sei1p and Ldb16.
authors.affiliation: Department of Pharmacology, University of Texas Southwestern Medical School, Dallas, TX, 75390-9041, USA.
authors.forename: Sungwon
authors.initials: S
authors.lastname: Han
authors.affiliation: Department of Pharmacology, University of Texas Southwestern Medical School, Dallas, TX, 75390-9041, USA.
authors.forename: Derk D
authors.initials: DD
authors.lastname: Binns
authors.affiliation: Department of Molecular Biology and Genetics, Cornell University, Ithaca, NY, 14853-2703, USA.
authors.forename: Yu-Fang
authors.initials: YF
authors.lastname: Chang
authors.affiliation: Department of Pharmacology, University of Texas Southwestern Medical School, Dallas, TX, 75390-9041, USA. Joel.Goodman@UTSouthwestern.edu.
authors.forename: Joel M
authors.initials: JM
authors.lastname: Goodman
mesh_terms: D001692:Biological Transport; D004721:Endoplasmic Reticulum / Q000235:genetics / Q000378:metabolism*; D044388:GTP-Binding Protein gamma Subunits / Q000235:genetics / Q000378:metabolism; D006801:Humans; D066292:Lipid Droplets / Q000378:metabolism*; D008565:Membrane Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D024101:Mitochondrial Proteins / Q000235:genetics / Q000378:metabolism*; D010712:Phosphatidic Acids / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D012441:Saccharomyces cerevisiae / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D014280:Triglycerides / Q000378:metabolism*
chemical_list: C432679:BSCL2 protein, human; C527368:Fld1 protein, S cerevisiae; D044388:GTP-Binding Protein gamma Subunits; C000594798:LDB16 protein, S cerevisiae; D008565:Membrane Proteins; D024101:Mitochondrial Proteins; D010712:Phosphatidic Acids; D029701:Saccharomyces cerevisiae Proteins; D014280:Triglycerides