Glycoprotein reglucosylation and nucleotide sugar utilization in the secretory pathway: identification of a nucleoside diphosphatase in the endoplasmic reticulum

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Abstract
UDP is generated in the lumen of the endoplasmic reticulum (ER) as a product of the UDP‐glucose‐dependent glycoprotein reglucosylation in the calnexin/calreticulin cycle. We describe here the identification, purification and characterization of an ER enzyme that hydrolyzes UDP to UMP. This nucleoside diphosphatase is a ubiquitously expressed, soluble 45 kDa glycoprotein devoid of transmembrane domains and KDEL‐related ER localization sequences. It requires divalent cations for activity and hydrolyzes UDP, GDP and IDP but not any other nucleoside di‐, mono‐ or triphosphates, nor thiamine pyrophosphate. By eliminating UDP, which is an inhibitory product of the UDP‐Glc:glycoprotein glucosyltransferase, it is likely to promote reglucosylation reactions involved in glycoprotein folding and quality control in the ER.

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