EDEM1 reveals a quality control vesicular transport pathway out of the endoplasmic reticulum not involving the COPII exit sites

Abstract

Immature and nonnative proteins are retained in the endoplasmic reticulum (ER) by the quality control machinery. Folding-incompetent glycoproteins are eventually targeted for ER-associated protein degradation (ERAD). EDEM1 (ER degradation-enhancing ␣-mannosidaselike protein 1), a putative mannose-binding protein, targets misfolded glycoproteins for ERAD. We report that endogenous EDEM1 exists mainly as a soluble glycoprotein. By high-resolution immunolabeling and serial section analysis, we find that endogenous EDEM1 is sequestered in buds that form along cisternae of the rough ER at regions outside of the transitional ER. They give rise to Ϸ150-nm vesicles scattered throughout the cytoplasm that are lacking a recognizable COPII coat. About 87% of the immunogold labeling was over the vesicles and Ϸ11% over the ER lumen. Some of the EDEM1 vesicles also contain Derlin-2 and the misfolded Hong Kong variant of ␣-1-antitrypsin, a substrate for EDEM1 and ERAD. Our results demonstrate the existence of a vesicle budding transport pathway out of the rough ER that does not involve the canonical transitional ER exit sites and therefore represents a previously unrecognized passageway to remove potentially harmful misfolded luminal glycoproteins from the ER.electron microscopy ͉ protein misfolding ͉ protein quality control T he folding state of newly synthesized glycoproteins in the endoplasmic reticulum (ER) is monitored by a quality control machinery (1). Increased formation of misfolded proteins disturbs ER homeostasis resulting in protein degradation, as well as cell damage and death. This is the cause of many human diseases including cystic fibrosis, ␣-1-antitrypsin deficiency, renal diabetes insipidus, and congenital goiter (2, 3).Orderly occurring processes can be distinguished during the life and death of a folding-incompetent glycoprotein. The first involves recognition by the quality control machinery (4-6). The lectin chaperones calnexin/calreticulin retain nonnative conformers with monoglucosylated glycans (5, 7). Mannose-trimmed glycans generated by ER-mannosidases appear to represent a quality control tag for routing misfolded glycoproteins to the ER-associated degradation (ERAD) (8-12). The current view is that misfolded, mannosetrimmed glycoproteins are then retrotranslocated to the cytoplasm, where they are ubiquitinated and deglycosylated before proteasomal degradation (13).EDEM1 (ER degradation-enhancing ␣-mannosidase-like protein 1) and its yeast ortholog Htm1p/Mnl1p are putative mannosebinding proteins (14-18) that are transcriptionally induced by ER stress (14,17,18). They are believed to target misfolded glycoproteins for proteasomal degradation by removing them from the calnexin/calreticulin cycle (14-17). The overexpression of EDEM1 results in the accelerated proteasomal degradation of ERAD substrates such as the Hong Kong variant of ␣-1-antitrypsin (HK A1AT) and a luminal variant of beta-secretase, whereas the deletion or knockdown of EDEM1/Htm1p reduces the degradation rates of the ERAD substrates ...

Protein Folding and Quality Control in the ER

“…In support of this, electron microscopy showed that EDEM1 is primarily localized in double-membrane buds that form outside canonical ER exit sites, known as EDEMosomes (Zuber et al 2007;Le Fourn et al 2009;Reggiori et al 2010). At steady state, short-living ERAD components like EDEM1 and OS-9 appeared to be engulfed in these buds in a COPII-independent manner and degraded without the attachment of nonlipidated LC3.…”

Characterization of Early EDEM1 Protein Maturation Events and Their Functional Implications

“…1B, lane 1). As previously demonstrated (30), this heterogeneity was due to the inefficient recognition of its five glycosylation sites, as the protein doublet coalesced as a single band upon Endo H treatment (Fig. 1B, lane 2).…”

“…When the glucosidase inhibitor DNJ was added to remove any heterogeneity created by glucose trimming, EDEM1-S626A migrated with the same mobility as the lower band observed with the WT doublet that corresponded to EDEM1 containing four glycans (Fig. 1B, lanes 7 and 8) (30).…”

“…Plasmids-Wild type (WT) human EDEM1 cDNA expression plasmid, subcloned into p3XFLAG-CMV-14 vector (Sigma) and tagged with FLAG at its C terminus, was previously constructed and characterized (27,30). Point mutations at S626A and R14L were introduced by PCR to the WT plasmid as a template using the following set of primers : S626A, 5Ј-AGT-TAAAAGTCATCAACTCCGCCTCCAACTGCAATCGTG-TAC-3Ј and 5Ј-GTACACGATTGCAGTTGGAGGCGGAG-TTGATGACTTTTAACT-3Ј; R14L, 5Ј-GCTGGTGCTCCT-CCTGCTTGGCCTCCATGG-3Ј and 5Ј-CCATGGAGGCCA-AGCAGGAGGAGCACCAGC-3Ј.…”

“…The N-terminal hydrophobic domain was proposed to act as an integral membrane-spanning segment to create a type II membrane protein. More recent results have indicated that EDEM1 was, at least partially, present as a soluble protein (30,34). Analysis of signal sequence cleavage sites for EDEM1 using the SignalP 3.0 algorithm suggested three possible low probability cleavage sites (supplemental Fig.…”