Recognition of local glycoprotein misfolding by the ER folding sensor UDP-glucose : glycoprotein glucosyltransferase

The endoplasmic reticulum (ER) contains a stringent quality control system that ensures the correct folding of newly synthesized proteins to be export ed via the secretory pathway. In this system UDP-Glc:glycoprotein glucosylt ransferase (GT) serves as a glycoprotein specific folding sensor by specifi cally glucosylating N-linked glycans in misfolded glycoproteins thus retain ing them in the calnexin/calreticulin chaperone cycle. To investigate how G T senses the folding status of glycoproteins, we generated RNase B heterodi mers consisting of a folded and a misfolded domain. Only glycans linked to the misfolded domain were found to he glucosylated, indicating that the enz yme recognizes folding defects at the level of individual domains and only reglucosylates glycans directly attached to a misfolded domain. The result was confirmed with complexes of soybean agglutinin and misfolded thyroglobu lin.