Date Published: April 8, 2019
Publisher: Public Library of Science
Author(s): Aya Mizuike, Shingo Kobayashi, Takashi Rikukawa, Akinori Ohta, Hiroyuki Horiuchi, Ryouichi Fukuda, Alvaro Galli.
Interorganelle phospholipid transfer is critical for eukaryotic membrane biogenesis. In the yeast Saccharomyces cerevisiae, phosphatidylserine (PS) synthesized by PS synthase, Pss1, in the endoplasmic reticulum (ER) is decarboxylated to phosphatidylethanolamine (PE) by PS decarboxylase, Psd1, in the ER and mitochondria or by Psd2 in the endosome, Golgi, and/or vacuole, but the mechanism of interorganelle PS transport remains to be elucidated. Here we report that Sfh1, a member of Sec14 family proteins of S. cerevisiae, possesses the ability to enhance PE production by Psd2. Overexpression of SFH1 in the strain defective in Psd1 restored its growth on non-fermentable carbon sources and increased the intracellular and mitochondrial PE levels. Sfh1 was found to bind various phospholipids, including PS, in vivo. Bacterially expressed and purified Sfh1 was suggested to have the ability to transport fluorescently labeled PS between liposomes by fluorescence dequenching assay in vitro. Biochemical subcellular fractionation suggested that a fraction of Sfh1 localizes to the endosome, Golgi, and/or vacuole. We propose a model that Sfh1 promotes PE production by Psd2 by transferring phospholipids between the ER and endosome.
In eukaryotic cells, enzymes involved in phospholipid biosynthesis localize to defined organelles, and therefore interorganelle lipid transport is critical for the biogenesis of membranes. Organellar membranes exhibit distinct lipid compositions, which influence the structures and functions of those membranes [1, 2]. The unequal distribution of phospholipids among the organelles and the plasma membrane are generated and maintained by the local synthesis and metabolism of each phospholipid species [3, 4] and the directional transport of phospholipids between membranes. Therefore, interorganellar lipid transport plays an important role in the maintenance of membrane homeostasis. In the budding yeast Saccharomyces cerevisiae, interorganelle lipid transport is involved in phospholipid synthesis by the CDP-diacylglycerol (CDP-DAG) pathway (Fig 1). In this pathway, phosphatidylserine (PS) is synthesized from CDP-DAG and serine by PS synthase, Pss1, in the endoplasmic reticulum (ER) membrane [5, 6]. PS is then decarboxylated to phosphatidylethanolamine (PE) by PS decarboxylase, Psd1, in the mitochondria or the ER, or by Psd2 in the endosome, Golgi, and/or vacuole [7–11]. A portion of PE is methylated to phosphatidylcholine (PC) by the PE methyltransferases, Pem1 and Pem2 . PE and PC are also synthesized through the Kennedy pathway from ethanolamine (Etn), phosphoethanolamine, choline (Cho), or phosphocholine in the ER and the Golgi apparatus [13–17].
In this study, we identified SFH1 as a multi-copy suppressor of the growth defect of psd1Δ on non-fermentable carbon sources and characterized its physiological function.