Immunolocalization of UDP-glucose : glycoprotein glucosyltransferase indicates involvement of pre-Golgi intermediates in protein quality control

The UDP-glucose:glycoprotein glucosyltransferase (GT) is a protein folding sensor and glycosyltransferase that constitutes an important component of t he protein quality control machinery. With the use of quantitative immunogo ld electron microscopy, we established the subcellular distribution of GT i n rat liver and pancreas and Drosophila melanogaster salivary gland as well as cell lines and correlated it with that of glucosidase II, calreticulin, a nd pre-Golgi intermediate markers. Labeling for GT, as well as for gluco sidase II and calreticulin, was found in the endoplasmic reticulum (ER), in cluding nuclear envelope and pre-Golgi intermediates located between ER and Golgi apparatus, and in the cell periphery. In the rough ER, labeling for GT was inhomogeneous, with variously sized labeled and unlabeled cisternal regions alternating, indicative of a meshwork of quality control checkpoint s. Notably, labeling intensity for GT was highest in pre-Golgi intermediate s, corresponding to twice that of rough ER, whereas the Golgi apparatus exh ibited no specific labeling. These results suggest that protein quality con trol is not restricted to the ER and that the pre-Golgi intermediates, by v irtue of the presence of GT, glucosidase II, and calreticulin, are involved in this fundamental cellular process.