Multiprotein complexes that link dislocation, ubiquitination, and extraction of misfolded proteins from the endoplasmic reticulum membrane

Abstract

Polypeptides that fail to pass quality control in the endoplasmic reticulum (ER) are dislocated from the ER membrane to the cytosol where they are degraded by the proteasome. Derlin-1, a member of a family of proteins that bears homology to yeast Der1p, was identified as a factor that is required for the human cytomegalovirus US11-mediated dislocation of class I MHC heavy chains from the ER membrane to the cytosol. Derlin-1 acts in concert with the AAA ATPase p97 to remove dislocation substrate proteins from the ER membrane, but it is unknown whether other factors aid Derlin-1 in its function. Mammalian genomes encode two additional, related proteins (Derlin-2 and Derlin-3). The similarity of the mammalian Derlin-2 and Derlin-3 proteins to yeast Der1p suggested that these as-yet-uncharacterized Derlins also may play a role in ER protein degradation. We demonstrate here that Derlin-2 is an ER-resident protein that, similar to Derlin-1, participates in the degradation of proteins from the ER. Furthermore, we show that Derlin-2 forms a robust multiprotein complex with the p97 AAA ATPase as well as the mammalian orthologs of the yeast Hrd1p͞ Hrd3p ubiquitin-ligase complex. The data presented here define a set of interactions between proteins involved in dislocation of misfolded polypeptides from the ER.Derlin ͉ HRD1 ͉ SEL1L ͉ p97 ͉ VIMP

Sel1L is indispensable for mammalian endoplasmic reticulum-associated degradation, endoplasmic reticulum homeostasis, and survival

“…Cellular Sel1L is five-to sevenfold more abundant than Hrd1 (58). As Sel1L and Hrd1 exist in a 1:1 stoichiometric complex in ERAD (14,18), Sel1L may exist in an Hrd1-independent complex. Indeed, a recent study showed that Sel1L may form a Sel1L-LC3-I complex to deliver certain proteins involved in ERAD, such as OS9 for lysosomal degradation, thereby controlling the efficacy of ERAD (29).…”

“…The Hrd1-Hrd3 complex was first discovered in yeast, by the Hampton group, to be responsible for the degradation of 3-hydroxy-3-methylglutaryl-CoA reductase (13,14) and in C. elegens by the Greenwald group through genetic interactions with Notch (20,21). Recent studies from several groups have elegantly demonstrated that Sel1L is an integral part of the mammalian Hrd1 ERAD complex and is necessary for the ERAD process for a subset of model substrates (15)(16)(17)(18)(19) and endogenous substrates, including luminal hedgehog (22), transmembrane CD147 (23), and ATF6 (24). However, although Hrd3p determines the stability of Hrd1p in yeast (4,14), knockdown of Sel1L seems to have negligible effect on the steady-state level of Hrd1 protein in cultured mammalian cells (15,25,26).…”

“…Among several key E3 ligases that have been identified so far, hydroxymethylglutaryl reductase degradation protein 1 (Hrd1) is a principle ER-resident E3 ligase and forms a complex with an ER-resident single-transmembrane protein Hrd3 in yeast or Suppressor/Enhancer of Lin-12-like (Sel1L) in mammals, responsible for the degradation of a subset of misfolded proteins in the ER (13)(14)(15)(16)(17)(18)(19). The Hrd1-Hrd3 complex was first discovered in yeast, by the Hampton group, to be responsible for the degradation of 3-hydroxy-3-methylglutaryl-CoA reductase (13,14) and in C. elegens by the Greenwald group through genetic interactions with Notch (20,21).…”

The Endoplasmic Reticulum (ER)-associated Degradation System Regulates Aggregation and Degradation of Mutant Neuroserpin

“…This mechanism is essentially the same as that involved in the quality control of other misfolded and aberrantly synthesized proteins in the ER (10,11). Because VCP/p97 is an essential regulator in ERAD that facilitates the retrotranslocation of ubiquitinated ER proteins to the cytosol for degradation (12), forms a complex with the relevant E3s and functions in classical ERAD (17,18), the activity of VCP/p97 ** ** ** ** ** * * FIGURE 6. Effects of endogenous Hrd1 and gp78 on mutant neuroserpin degradation.…”

“…Hrd1 and gp78 are the two best-characterized mammalian ERAD E3 ubiquitin ligases and are responsible for ubiquitinating a subset of misfolded substrates (13-16). The interactions of VCP with Hrd1 and gp78 play a central role in ERAD (12,17,18). The proteasome inhibitors lactacystin and MG132 block neuroserpin degradation in the ER, suggesting that ERAD may be involved in neuroserpin degradation (19).…”