title: Sterol-regulated transport of SREBPs from endoplasmic reticulum to Golgi: Insig renders sorting signal in Scap inaccessible to COPII proteins.
abstract: Two classes of sterols, cholesterol and oxysterols, block export of sterol regulatory element-binding proteins (SREBPs) from the endoplasmic reticulum (ER) to the Golgi by preventing the binding of COPII-coated proteins to a hexapeptide sorting signal (MELADL) in Scap, the SREBP-escort protein. Here, we show that anti-MELADL blocks COPII binding in vitro, and microinjection of Fab anti-MELADL blocks Scap.SREBP movement in cells. Cholesterol and oxysterols block COPII binding to MELADL by binding to different intracellular receptors, cholesterol to Scap and oxysterols to Insig. Cysteine labeling shows that both binding events produce a conformational change near the MELADL sequence, abrogating COPII binding but not anti-MELADL binding. Mutagenesis experiments raise the possibility that the distance of MELADL from the ER membrane is crucial for COPII binding, and we speculate that sterols and Insig block SREBP transport by altering the location of MELADL with respect to the membrane, rendering it inaccessible to COPII proteins.
authors.affiliation: Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA.
authors.forename: Li-Ping
authors.initials: LP
authors.lastname: Sun
authors.forename: Joachim
authors.initials: J
authors.lastname: Seemann
authors.forename: Joseph L
authors.initials: JL
authors.lastname: Goldstein
authors.forename: Michael S
authors.initials: MS
authors.lastname: Brown
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D019138:Antibodies, Blocking / Q000494:pharmacology; D016466:CHO Cells; D022181:COP-Coated Vesicles / Q000378:metabolism*; D002999:Clone Cells; D006224:Cricetinae; D003412:Cricetulus; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D006056:Golgi Apparatus / Q000378:metabolism*; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins / Q000037:antagonists & inhibitors / Q000172:deficiency / Q000235:genetics / Q000276:immunology / Q000378:metabolism / Q000502:physiology*; D008565:Membrane Proteins / Q000037:antagonists & inhibitors* / Q000276:immunology / Q000378:metabolism; D008969:Molecular Sequence Data; D021382:Protein Sorting Signals / Q000502:physiology*; D021381:Protein Transport / Q000502:physiology; D051779:Sterol Regulatory Element Binding Proteins / Q000378:metabolism*; D033921:Vesicular Transport Proteins / Q000037:antagonists & inhibitors* / Q000378:metabolism
chemical_list: D019138:Antibodies, Blocking; D047908:Intracellular Signaling Peptides and Proteins; D008565:Membrane Proteins; D021382:Protein Sorting Signals; C512739:SEC23A protein, human; C102196:SREBP cleavage-activating protein; D051779:Sterol Regulatory Element Binding Proteins; D033921:Vesicular Transport Proteins