Ancient Ubiquitous Protein-1 Mediates Sterol-Induced Ubiquitination of HMG CoA Reductase in Lipid Droplet-Associated Endoplasmic Reticulum Membranes.

From the Howard Hughes Medical Institute and the Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, TX 75390-9046.
Molecular biology of the cell (Impact Factor: 4.47). 12/2012; 24(3). DOI: 10.1091/mbc.E12-07-0564
Source: PubMed


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). Here, 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 proteins-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 connection between ERAD, lipid droplets, and lipid droplet-associated proteins.

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Available from: Youngah Jo, Oct 14, 2014
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    • "Similarly, sequences of 30–40 residues containing hydrophobic amino acids interspersed with helix-breaking residues (serine, proline, cysteine and glycine) were found in other LD-associated proteins. In recent years, their number has increased to include DGAT2 (Kuerschner et al., 2008; McFie et al., 2011), NSDHL (Caldas and Herman, 2003), lipin-1c (Wang et al., 2011b), 17bHSD11 (Horiguchi et al., 2008), GPAT4 (Wilfling et al., 2013), LPCAT1 and LPCAT2 (Moessinger et al., 2011), caveolins (Fujimoto et al., 2001; Ostermeyer et al., 2004; Pol et al., 2001), Rdh10 and LRAT (Jiang and Napoli, 2012; Jiang and Napoli, 2013), AUP1 (Jo et al., 2013; Spandl et al., 2011), UBXD8 (Zehmer et al., 2009), ALDI (Turró et al., 2006), AAM-B and CYB5R3 (Zehmer et al., 2008). For all of these proteins, this segment has been shown to be required for LD localization, and when it was examined, it has been shown to insert as a hairpin in the hydrophobic lipid layer (Caldas and Herman, 2003; Dupree et al., 1993; Moessinger et al., 2011; Spandl et al., 2011; Stevanovic and Thiele, 2013; Zehmer et al., 2008). "
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    • "Through the cascade reactions cooperated with E1, E2 (Ubc7), and E3 (gp78), as well as other cofactors, such as Ufd1 [5], the reductase was modified by Lys-48 linkage ubiquitin chains at the K248 (the dominant ubiquitination site) and K89 [15]. With assistance of at least two proteins associated with gp78, p97/VCP, and Aup1 [4,16], the ubiquitinated reductase was translocated to lipid droplet-associated ER membrane and dislocated from membrane into cytosol for proteasomal degradation [16,17]. This postubiquitination process can be promoted by geranylgeraniol (GG-OH) or its metabolically active geranylgeranyl-pyrophosphate (GG-PP) [15]. "
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    • "The ubiquitination machinery also involves E3 ubiquitin ligases that use adaptor proteins for substrate recognition. Ancient ubiquitous protein 1 (AUP1), a highly conserved protein that localizes to ER and LDs34,38 and is found in LD proteoms,8 acts as an adaptor recruiting different ubiquitin ligases such as E2 conjugase Ube2g2,144 an E3 ligase AMFR/gp78.138,145,146 "
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