Drosophila UDP-glucose:glycoprotein glucosyltransferase: sequence and characterization of an enzyme that distinguishes between denatured and native proteins

A Drosophila UDP glucose:glycoprotein glucosyl-transferase was isolated, cloned and characterized. Its 1548 amino acid sequence begins with a signal peptide, lacks any putative transmembrane domains and terminates in a potential endoplasmic reticulum retrieval signal, HGEL. The soluble, 170 kDa glycoprotein occurs throughout Drosophila embryos, in microsomes of highly secretory Drosophila Kc cells and in small amounts in cell culture media. The isolated enzyme transfers [l4C]glucose from UDP-[l4C]Glc to several purified extracellular matrix glycoproteins (laminin, peroxidasin and glutactin) made by these cells, and to bovine thyroglobulin. These proteins must be denatured to accept glucose, which is bound at endoglycosidase H-sensitive sites. The unusual ability to discriminate between malfolded and native glycoproteins is shared by the rat liver homologue, previously described by A.J. Parodi and coworkers. The amino acid sequence presented differs from most glycosyltransferases. There is weak, though significant, similarity with a few bacterial lipopolysaccharide glycotransferases and a yeast protein Kre5p. In contrast, the 56-68% amino acid identities with partial sequences from genome projects of Caenorhabditis elegans, rice and Arabidopsis suggest widespread homologues of the enzyme. This glucosyltransferase fits previously proposed hypotheses for an endoplasmic reticular sensor of the state of folding of newly made glycoproteins.