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

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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 α-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 ≈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 ≈11% over the ER lumen. Some of the EDEM1 vesicles also contain Derlin-2 and the misfolded Hong Kong variant of α-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.