title: Carboxyl-terminal Tail-mediated Homodimerizations of Sphingomyelin Synthases Are Responsible for Efficient Export from the Endoplasmic Reticulum.
abstract: Sphingomyelin synthase (SMS) is the key enzyme for cross-talk between bioactive sphingolipids and glycerolipids. In mammals, SMS consists of two isoforms: SMS1 is localized in the Golgi apparatus, whereas SMS2 is localized in both the Golgi and plasma membranes. SMS2 seems to exert cellular functions through protein-protein interactions; however, the existence and functions of quaternary structures of SMS1 and SMS2 remain unclear. Here we demonstrate that both SMS1 and SMS2 form homodimers. The SMSs have six membrane-spanning domains, and the N and C termini of both proteins face the cytosolic side of the Golgi apparatus. Chemical cross-linking and bimolecular fluorescence complementation revealed that the N- and/or C-terminal tails of the SMSs were in close proximity to those of the other SMS in the homodimer. Homodimer formation was significantly decreased by C-terminal truncations, SMS1-ΔC22 and SMS2-ΔC30, indicating that the C-terminal tails of the SMSs are primarily responsible for homodimer formation. Moreover, immunoprecipitation using deletion mutants revealed that the C-terminal tail of SMS2 mainly interacted with the C-terminal tail of its homodimer partner, whereas the C-terminal tail of SMS1 mainly interacted with a site other than the C-terminal tail of its homodimer partner. Interestingly, homodimer formation occurred in the endoplasmic reticulum (ER) membrane before trafficking to the Golgi apparatus. Reduced homodimerization caused by C-terminal truncations of SMSs significantly reduced ER-to-Golgi transport. Our findings suggest that the C-terminal tails of SMSs are involved in homodimer formation, which is required for efficient transport from the ER.
authors.affiliation: From the Faculty of Pharma Sciences, Teikyo University, Kaga 2-11-1, Itabashi-ku, Tokyo 173-8605, Japan and.
authors.forename: Yasuhiro
authors.initials: Y
authors.lastname: Hayashi
authors.affiliation: From the Faculty of Pharma Sciences, Teikyo University, Kaga 2-11-1, Itabashi-ku, Tokyo 173-8605, Japan and.
authors.forename: Yoko
authors.initials: Y
authors.lastname: Nemoto-Sasaki
authors.affiliation: From the Faculty of Pharma Sciences, Teikyo University, Kaga 2-11-1, Itabashi-ku, Tokyo 173-8605, Japan and.
authors.forename: Naoki
authors.initials: N
authors.lastname: Matsumoto
authors.affiliation: From the Faculty of Pharma Sciences, Teikyo University, Kaga 2-11-1, Itabashi-ku, Tokyo 173-8605, Japan and.
authors.forename: Takashi
authors.initials: T
authors.lastname: Tanikawa
authors.affiliation: From the Faculty of Pharma Sciences, Teikyo University, Kaga 2-11-1, Itabashi-ku, Tokyo 173-8605, Japan and.
authors.forename: Saori
authors.initials: S
authors.lastname: Oka
authors.affiliation: From the Faculty of Pharma Sciences, Teikyo University, Kaga 2-11-1, Itabashi-ku, Tokyo 173-8605, Japan and.
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Tanaka
authors.affiliation: the Department of Cell Science, Institute of Biomedical Sciences, Fukushima Medical University School of Medicine, Hikarigaoka-1, Fukushima City, Fukushima 960-1295, Japan.
authors.forename: Seisuke
authors.initials: S
authors.lastname: Arai
authors.affiliation: the Department of Cell Science, Institute of Biomedical Sciences, Fukushima Medical University School of Medicine, Hikarigaoka-1, Fukushima City, Fukushima 960-1295, Japan.
authors.forename: Ikuo
authors.initials: I
authors.lastname: Wada
authors.affiliation: From the Faculty of Pharma Sciences, Teikyo University, Kaga 2-11-1, Itabashi-ku, Tokyo 173-8605, Japan and.
authors.forename: Takayuki
authors.initials: T
authors.lastname: Sugiura
authors.affiliation: From the Faculty of Pharma Sciences, Teikyo University, Kaga 2-11-1, Itabashi-ku, Tokyo 173-8605, Japan and ayamashi@pharm.teikyo-u.ac.jp.
authors.forename: Atsushi
authors.initials: A
authors.lastname: Yamashita
mesh_terms: D000818:Animals; D019556:COS Cells; D002522:Chlorocebus aethiops; D004721:Endoplasmic Reticulum / Q000235:genetics / Q000378:metabolism*; D006056:Golgi Apparatus / Q000235:genetics / Q000378:metabolism*; D057809:HEK293 Cells; D006801:Humans; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D009419:Nerve Tissue Proteins / Q000235:genetics / Q000378:metabolism*; D000072417:Protein Domains; D055503:Protein Multimerization / Q000502:physiology*; D021381:Protein Transport / Q000502:physiology; D017855:Transferases (Other Substituted Phosphate Groups) / Q000235:genetics / Q000378:metabolism*
chemical_list: D008565:Membrane Proteins; D009419:Nerve Tissue Proteins; C455440:SGMS1 protein, human; D017855:Transferases (Other Substituted Phosphate Groups)
keywords: dimerization; glycerophospholipid; protein cross-linking; protein-protein interaction; sphingolipid; sphingomyelin synthase (SMS)