UDP-Glucose: Glycoprotein Glucosyltransferase (UGGT1) Promotes Substrate Solubility in the Endoplasmic Reticulum.

Abstract

Protein folding in the endoplasmic reticulum (ER) is error-prone, and ER quality control processes exist (ERQC) to ensure only correctly folded proteins are exported from the ER en route to the Golgi compartment. Glycoproteins with amino acid mutations can be retained in the ER by ERQC, and this retention contributes to multiple human diseases, termed ER storage diseases. UDP-glucose:glycoprotein glucosyltransferase (UGGT1, GT1) is proposed to be a central component of ER glycoprotein quality control. The mechanism of UGGT1-mediated monoglucosylation of deglucosylated N-glycans of incompletely folded glycoproteins, and the subsequent reassociation of these glycoproteins with lectin-like chaperones (calreticulin and calnexin) in the ER has been extensively described. However, the extent to which UGGT1 influences the folding of ER substrate proteins has only been investigated for a few selected substrates. Using mouse embryonic fibroblasts lacking UGGT1, or those with UGGT1 complementation, we investigated the effect of monoglucosylation on the soluble/insoluble distribution of two misfolded alpha-1 antitrypsin (AAT) variants responsible for AAT deficiency disease, null Hong Kong (NHK) and Z allele (ATZ). Our results indicate that, whereas substrate solubility increases directly with the number of N-linked glycosylation sites, additional solubility is conferred by UGGT1 enzymatic activity at the expense of the pool of insoluble protein complexes in the ER. Monoglucosylation-dependent solubility decreases both BiP association with NHK and unfolded protein response (UPR) activation, and the solubility increase is blocked in cells deficient for calreticulin. These results suggest that UGGT1-dependent monoglucosylation of N-linked glycoproteins promotes substrate solubility in the ER.