The interaction of the UDP-GLC:glycoprotein glucosyltransferase with the acceptor glycoprotein

The UDP-Glyc:glycoprotein glucosyltransferase is a soluble protein of the endoplasmic reticulum that in cell-free assays glucosylates denatured but not native glycoproteins. It has been postulated that the enzyme participates in the quality control mechanism of glycoprotein folding in the endoplasmic reticulum by which only properly folded species are transported to the Golgi apparatus. This paper reports studies on the protein determinants that upon recognition by the glucosyltransferase allow the selective glucosylation of denatured glycoproteins. It is shown here that extensive chemical modification of denatured soybean agglutinin does not modify its glucose acceptor capacity, thus suggesting that the protein determinants recognized by the glucosyltransferase are not formed by specific amino acids. Moreover, it is reported that the enzyme binds patches of hydrophobic amino acids exposed in denatured but not in native conformations. As exposure of such residues is the only feature common to all misfolded conformations, it is suggested that binding to hydrophobic amino acids forms part of the recognition mechanism of denatured conformations by the glucosyltransferase.