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1-antitrypsin
1 Department of Molecular and Cellular Biology, Institute for Frontier Medical Sciences, Kyoto University, Kyoto 606-8397, Japan
2 CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
3 Department of Cell Sciences, Institute of Biomedical Sciences, Fukushima Medical University School of Medicine, Fukushima 960-1295, Japan
Misfolded glycoproteins are degraded by a mechanism known as ERAD (ER-associated degradation) after retrotranslocation out of the endoplasmic reticulum (ER). This mechanism plays an important role in ER quality control. We previously reported that an ER membrane protein, EDEM, accelerates ERAD of a misfolded 
1-antitrypsin variant, null (Hong Kong) (NHK), suggesting that EDEM may function as an acceptor of terminally misfolded glycoproteins. In this study, we constructed several genetically manipulated cell lines to test this hypothesis. EDEM expression did not alter the secretion rate of properly folded molecules and the forced retention of wild-type 1-antitrypsin in the ER did not cause its association with EDEM, suggesting that EDEM may function as a molecular chaperone. To examine this possibility, we analyzed the effect of EDEM over-expression on the structure of NHK, and found that the accumulation of covalent NHK dimers was selectively prevented by the over-expression of EDEM. Co-expression of NHK with two other ER membrane proteins, calnexin and H+/K+-ATPase (ß subunit), did not inhibit NHK dimer formation or accelerate NHK ERAD. These results indicate that EDEM may maintain the retrotranslocation competence of NHK by inhibiting aggregation so that unstable misfolded proteins can be accommodated by the dislocon for ERAD.
* Correspondence: E-mail: nobuko{at}frontier.kyoto-u.ac.jp
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