title: Gp78, a membrane-anchored ubiquitin ligase, associates with Insig-1 and couples sterol-regulated ubiquitination to degradation of HMG CoA reductase.
abstract: Sterol-regulated ubiquitination is an obligatory step in ER-associated degradation (ERAD) of HMG CoA reductase, a rate-limiting enzyme in cholesterol synthesis. Accelerated degradation of reductase, one of several strategies animal cells use to limit production of cholesterol, requires sterol-induced binding of the enzyme to ER membrane proteins called Insigs. Once formed, the reductase-Insig complex is recognized by a putative membrane-associated ubiquitin ligase (E3) that mediates the reductase ubiquitination reaction. Here, we show that gp78, a membrane bound E3, binds to Insig-1 and is required for sterol-regulated ubiquitination of reductase. In addition, gp78 couples regulated ubiquitination to degradation of reductase by binding to VCP, an ATPase that plays a key role in recognition and degradation of ERAD substrates. The current results identify gp78 as the E3 that initiates sterol-accelerated degradation of reductase, and Insig-1 as a bridge between gp78/VCP and the reductase substrate.
authors.affiliation: Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, Texas 75390, USA.
authors.forename: Bao-Liang
authors.initials: BL
authors.lastname: Song
authors.forename: Navdar
authors.initials: N
authors.lastname: Sever
authors.forename: Russell A
authors.initials: RA
authors.lastname: DeBose-Boyd
mesh_terms: D000251:Adenosine Triphosphatases; D000818:Animals; D018797:Cell Cycle Proteins / Q000235:genetics / Q000378:metabolism; D002460:Cell Line; D006224:Cricetinae; D006801:Humans; D006903:Hydroxymethylglutaryl CoA Reductases / Q000378:metabolism*; D047908:Intracellular Signaling Peptides and Proteins; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D008954:Models, Biological; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D060285:Receptors, Autocrine Motility Factor; D018121:Receptors, Cytokine / Q000235:genetics / Q000378:metabolism*; D013261:Sterols / Q000378:metabolism*; D025801:Ubiquitin / Q000378:metabolism*; D044767:Ubiquitin-Protein Ligases / Q000235:genetics / Q000378:metabolism*; D000074405:Valosin Containing Protein
chemical_list: D018797:Cell Cycle Proteins; C108626:INSIG1 protein, human; D047908:Intracellular Signaling Peptides and Proteins; D008565:Membrane Proteins; D018121:Receptors, Cytokine; D013261:Sterols; D025801:Ubiquitin; D006903:Hydroxymethylglutaryl CoA Reductases; C555000:AMFR protein, human; D060285:Receptors, Autocrine Motility Factor; D044767:Ubiquitin-Protein Ligases; D000251:Adenosine Triphosphatases; C000614178:VCP protein, human; D000074405:Valosin Containing Protein