Characterization of early EDEM1 protein maturation events and their functional implications.

The endoplasmic reticulum (ER) quality control factor EDEM1 associates with a number of ER proteins and ER-associated degradation (ERAD) substrates; however, an understanding of its role in ERAD is unclear. The early maturation events for EDEM1 including signal sequence cleavage and glycosylation were analyzed, and their relationship to the function of EDEM1 was determined. EDEM1 has five N-linked glycosylation sites with the most C-terminal site recognized poorly cotranslationally, resulting in the accumulation of EDEM1 containing four or five glycans. The fifth site was modified post-translationally when bypassed cotranslationally. Signal sequence cleavage of EDEM1 was found to be a slow and inefficient process. Signal sequence cleavage produced a soluble form of EDEM1 that efficiently associated with the oxidoreductase ERdj5 and most effectively accelerated the turnover of a soluble ERAD substrate. In contrast, a type-II membrane form of EDEM1 was generated when the signal sequence was uncleaved, creating an N-terminal transmembrane segment. The membrane form of EDEM1 efficiently associated with the ER membrane protein SEL1L and accelerated the turnover of a membrane-associated ERAD substrate. Together, these results demonstrated that signal sequence cleavage functionally regulated the association of EDEM1-soluble and membrane-integrated isoforms with distinct ERAD machinery and substrates.

EDEM1 recognition and delivery of misfolded proteins to the SEL1L-containing ERAD complex.

Terminally misfolded or unassembled secretory proteins are retained in the endoplasmic reticulum (ER) and subsequently cleared by the ER-associated degradation (ERAD) pathway. The degradation of ERAD substrates involves mannose trimming of N-linked glycans; however, the mechanisms of substrate recognition and sorting to the ERAD pathway are poorly defined. EDEM1 (ER degradation-enhancing alpha-mannosidase-like 1 protein) has been proposed to play a role in ERAD substrate signaling or recognition. We show that EDEM1 specifically binds nonnative proteins in a glycan-independent manner. Inhibition of mannosidase activity with kifunensine or disruption of the EDEM1 mannosidase-like domain by mutation had no effect on EDEM1 substrate binding but diminished its association with the ER membrane adaptor protein SEL1L. These results support a model whereby EDEM1 binds nonnative proteins and uses its mannosidase-like domain to target aberrant proteins to the ER membrane dislocation and ubiquitination complex containing SEL1L.

Sweet bays of ERAD.

Proteins that improperly mature in the endoplasmic reticulum (ER) are dislocated to the cytoplasm for proteasome-mediated destruction. A recent study provides insight into the incompletely understood processes for selection and targeting of aberrant proteins for ER-associated protein degradation. The identification of the ER chaperones GRP94 and BiP as binding partners for the mannose-binding proteins OS-9 and XTP3-B, indicates that these protein complexes bind to aberrant proteins and direct them to the Hrd1 dislocation and ubiquitylation complex in the ER membrane.

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

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 alpha-mannosidase-like 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 approximately 150-nm vesicles scattered throughout the cytoplasm that are lacking a recognizable COPII coat. About 87% of the immunogold labeling was over the vesicles and approximately 11% over the ER lumen. Some of the EDEM1 vesicles also contain Derlin-2 and the misfolded Hong Kong variant of alpha-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.

Yos9p: a sweet-toothed bouncer of the secretory pathway.

In this issue of Molecular Cell, Bhamidipati et al., (2005) and Kim et al., (2005), and Szathmary et al. (2005), and demonstrate that Yos9p selectively binds to aberrant glycoproteins in the endoplasmic reticulum (ER) and targets them for destruction through the ER-associated protein degradation pathway.