Abstract
Proteasomes mediate the regulated degradation of Insig-1, a membrane protein of the endoplasmic reticulum (ER) that plays a crucial role in lipid metabolism. We showed previously that sterols inhibit this degradation by blocking ubiquitination of Insig-1. Here we show that unsaturated fatty acids stabilize Insig-1 without affecting its ubiquitination. Instead unsaturated fatty acids inhibit extraction of ubiquitinated Insig-1 from membranes, a process known to be mediated by valosin-containing protein and necessary for ER-associated degradation. Valosin-containing protein is recruited to Insig-1 through the action of another protein, Ubxd8. Unsaturated fatty acids block the binding between Ubxd8 and Insig-1, thereby abrogating the membrane extraction of Insig-1. Unsaturated fatty acid-mediated stabilization of Insig-1 enhances the ability of sterols to inhibit proteolytic activation of SREBP-1, which activates transcription of genes involved in fatty acid synthesis. The current study provides a molecular mechanism for regulation of proteasome-mediated ER protein degradation at a postubiquitination step.
Publication types
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Research Support, N.I.H., Extramural
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Research Support, Non-U.S. Gov't
MeSH terms
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Adenosine Triphosphatases / genetics
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Adenosine Triphosphatases / metabolism
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Animals
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CHO Cells
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Carrier Proteins / genetics
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Carrier Proteins / metabolism
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Cell Cycle Proteins / genetics
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Cell Cycle Proteins / metabolism
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Cricetinae
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Cricetulus
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Endoplasmic Reticulum / genetics
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Endoplasmic Reticulum / metabolism*
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Fatty Acids, Unsaturated / biosynthesis*
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Fatty Acids, Unsaturated / genetics
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Humans
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Intracellular Membranes / metabolism
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Intracellular Signaling Peptides and Proteins / genetics
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Intracellular Signaling Peptides and Proteins / metabolism*
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Lipid Metabolism / physiology*
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Membrane Proteins / genetics
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Membrane Proteins / metabolism*
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Proteasome Endopeptidase Complex / genetics
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Proteasome Endopeptidase Complex / metabolism*
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Sterol Regulatory Element Binding Protein 1 / genetics
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Sterol Regulatory Element Binding Protein 1 / metabolism
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Transcription, Genetic / physiology
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Ubiquitination / physiology*
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Valosin Containing Protein
Substances
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Carrier Proteins
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Cell Cycle Proteins
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Fatty Acids, Unsaturated
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INSIG1 protein, human
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Intracellular Signaling Peptides and Proteins
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Membrane Proteins
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SREBF1 protein, human
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Sterol Regulatory Element Binding Protein 1
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Proteasome Endopeptidase Complex
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Adenosine Triphosphatases
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VCP protein, human
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Valosin Containing Protein