DROSOPHILA UDP-GLUCOSE-GLYCOPROTEIN GLUCOSYLTRANSFERASE - SEQUENCE AND CHARACTERIZATION OF AN ENZYME THAT DISTINGUISHES BETWEEN DENATURED AND NATIVE PROTEINS

A Drosophila UDP-glucose:glycoprotein glucosyltransferase was isolated , cloned and characterized. Its 1548 amino acid sequence begins with a signal peptide, lacks any putative transmembrane domains and terminat es in a potential endoplasmic reticulum retrieval signal, HGEL. The so luble, 170 kDa glycoprotein occurs throughout Drosophila embryos, in m icrosomes of highly secretory Drosophila Kc cells and in small amounts in cell culture media. The isolated enzyme transfers [C-14]glucose fr om UDP-[C-14]Glc to several purified extracellular matrix glycoprotein s (laminin, peroxidasin and glutactin) made by these cells, and to bov ine thyroglobulin. These proteins must be denatured to accept glucose, which is bound at endoglycosidase II-sensitive sites. The unusual abi lity to discriminate between malfolded and native glycoproteins is sha red by the rat liver homologue, previously described by A.J.Parodi and coworkers. The amino acid sequence presented differs from most glycos yltransferases. There is weak, though significant, similarity with a f ew bacterial lipopolysaccharide glycotransferases and a yeast protein Kre5p. In contrast, the 56-68% amino acid identities with partial sequ ences from genome projects of Caenorhabditis elegans, rice and Arabido psis suggest widespread homologues of the enzyme. This glucosyltransfe rase fits previously proposed hypotheses for an endoplasmic reticular sensor of the state of folding of newly made glycoproteins.