title: Ancient ubiquitous protein-1 mediates sterol-induced ubiquitination of 3-hydroxy-3-methylglutaryl CoA reductase in lipid droplet-associated endoplasmic reticulum membranes.
abstract: Sterol-induced binding to Insigs in endoplasmic reticulum (ER) membranes triggers ubiquitination of the cholesterol biosynthetic enzyme 3-hydroxy-3-methylglutaryl CoA reductase. This ubiquitination, which is mediated by Insig-associated ubiquitin ligases gp78 and Trc8, is obligatory for extraction of reductase from lipid droplet-associated ER membranes into the cytosol for proteasome-mediated, ER-associated degradation (ERAD). In this study, we identify lipid droplet-associated, ancient, ubiquitous protein-1 (Aup1) as one of several proteins that copurify with gp78. RNA interference (RNAi) studies show that Aup1 recruits the ubiquitin-conjugating enzyme Ubc7 to lipid droplets and facilitates its binding to both gp78 and Trc8. The functional significance of these interactions is revealed by the observation that RNAi-mediated knockdown of Aup1 blunts sterol-accelerated ubiquitination of reductase, which appears to occur in lipid droplet-associated membranes and subsequent ERAD of the enzyme. In addition, Aup1 knockdown inhibits ERAD of Insig-1, another substrate for gp78, as well as that of membrane-bound precursor forms of sterol-regulatory, element-binding protein-1 and -2, transcription factors that modulate expression of genes encoding enzymes required for cholesterol synthesis. Considered together, these findings not only implicate a role for Aup1 in maintenance of intracellular cholesterol homeostasis, but they also highlight the close connections among ERAD, lipid droplets, and lipid droplet-associated proteins.
authors.affiliation: Howard Hughes Medical Institute and Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, TX 75390-9046, USA.
authors.forename: Youngah
authors.initials: Y
authors.lastname: Jo
authors.forename: Isamu Z
authors.initials: IZ
authors.lastname: Hartman
authors.forename: Russell A
authors.initials: RA
authors.lastname: DeBose-Boyd
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D016466:CHO Cells; D002352:Carrier Proteins / Q000737:chemistry / Q000378:metabolism*; D017124:Conserved Sequence; D006224:Cricetinae; D004721:Endoplasmic Reticulum / Q000201:enzymology; D060746:Endoplasmic Reticulum-Associated Degradation; D055785:Gene Knockdown Techniques; D006801:Humans; D057927:Hydrophobic and Hydrophilic Interactions; D006903:Hydroxymethylglutaryl CoA Reductases / Q000378:metabolism*; D007425:Intracellular Membranes / Q000201:enzymology*; D008055:Lipids / Q000502:physiology*; D008565:Membrane Proteins; D008969:Molecular Sequence Data; D021381:Protein Transport; D034622:RNA Interference; D060285:Receptors, Autocrine Motility Factor / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D011956:Receptors, Cell Surface / Q000378:metabolism; D044763:Ubiquitin-Conjugating Enzymes / Q000378:metabolism; D054875:Ubiquitination
chemical_list: C464942:AUP1 protein, human; D002352:Carrier Proteins; D008055:Lipids; D008565:Membrane Proteins; C114025:RNF139 protein, human; D011956:Receptors, Cell Surface; D006903:Hydroxymethylglutaryl CoA Reductases; C471617:UBE2G2 protein, human; D044763:Ubiquitin-Conjugating Enzymes; C555000:AMFR protein, human; D060285:Receptors, Autocrine Motility Factor