title: Conserved SMP domains of the ERMES complex bind phospholipids and mediate tether assembly.
abstract: Membrane contact sites (MCS) between organelles are proposed as nexuses for the exchange of lipids, small molecules, and other signals crucial to cellular function and homeostasis. Various protein complexes, such as the endoplasmic reticulum-mitochondrial encounter structure (ERMES), function as dynamic molecular tethers between organelles. Here, we report the reconstitution and characterization of subcomplexes formed by the cytoplasm-exposed synaptotagmin-like mitochondrial lipid-binding protein (SMP) domains present in three of the five ERMES subunits--the soluble protein Mdm12, the endoplasmic reticulum (ER)-resident membrane protein Mmm1, and the mitochondrial membrane protein Mdm34. SMP domains are conserved lipid-binding domains found exclusively in proteins at MCS. We show that the SMP domains of Mdm12 and Mmm1 associate into a tight heterotetramer with equimolecular stoichiometry. Our 17-Å-resolution EM structure of the complex reveals an elongated crescent-shaped particle in which two Mdm12 subunits occupy symmetric but distal positions at the opposite ends of a central ER-anchored Mmm1 homodimer. Rigid body fitting of homology models of these SMP domains in the density maps reveals a distinctive extended tubular structure likely traversed by a hydrophobic tunnel. Furthermore, these two SMP domains bind phospholipids and display a strong preference for phosphatidylcholines, a class of phospholipids whose exchange between the ER and mitochondria is essential. Last, we show that the three SMP-containing ERMES subunits form a ternary complex in which Mdm12 bridges Mmm1 to Mdm34. Our findings highlight roles for SMP domains in ERMES assembly and phospholipid binding and suggest a structure-based mechanism for the facilitated transport of phospholipids between organelles.
authors.affiliation: Department of Biological Chemistry, David Geffen School of Medicine, University of California, Los Angeles, CA 90095;
authors.forename: Andrew P
authors.initials: AP
authors.lastname: AhYoung
authors.affiliation: Department of Microbiology, Immunology and Molecular Genetics, University of California, Los Angeles, CA 90095; California NanoSystems Institute, University of California, Los Angeles, CA 90095;
authors.forename: Jiansen
authors.initials: J
authors.lastname: Jiang
authors.affiliation: Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095;
authors.forename: Jiang
authors.initials: J
authors.lastname: Zhang
authors.affiliation: Department of Biological Chemistry, David Geffen School of Medicine, University of California, Los Angeles, CA 90095;
authors.forename: Xuan
authors.initials: X
authors.lastname: Khoi Dang
authors.affiliation: Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095; UCLA-DOE Institute for Genomics and Proteomics, University of California, Los Angeles, CA 90095; Molecular Biology Institute, University of California, Los Angeles, CA 90095.
authors.forename: Joseph A
authors.initials: JA
authors.lastname: Loo
authors.affiliation: Department of Microbiology, Immunology and Molecular Genetics, University of California, Los Angeles, CA 90095; California NanoSystems Institute, University of California, Los Angeles, CA 90095; Molecular Biology Institute, University of California, Los Angeles, CA 90095.
authors.forename: Z Hong
authors.initials: ZH
authors.lastname: Zhou
authors.affiliation: Department of Biological Chemistry, David Geffen School of Medicine, University of California, Los Angeles, CA 90095; Molecular Biology Institute, University of California, Los Angeles, CA 90095 pegea@mednet.ucla.edu.
authors.forename: Pascal F
authors.initials: PF
authors.lastname: Egea
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D019281:Dimerization; D008565:Membrane Proteins / Q000737:chemistry / Q000378:metabolism*; D051336:Mitochondrial Membranes / Q000378:metabolism; D008969:Molecular Sequence Data; D010743:Phospholipids / Q000378:metabolism*; D011485:Protein Binding; D050857:Synaptotagmins / Q000378:metabolism*
chemical_list: D008565:Membrane Proteins; D010743:Phospholipids; D050857:Synaptotagmins
keywords: electron microscopy; interorganelle tether; membrane contact sites; membrane protein complex; phospholipid exchange