The endoplasmic reticulum enzyme UDP-glucose glycoprotein:glucosyltransferase (UGGT) has the unique property of recognizing incompletely folded glycoproteins and, if they carry an N -linked Man(9)GlcNAc(2)oligosaccharide, of catalyzing the addition of a glucose residue from UDP-glucose. Using peptide sequence information, we have isolated the complete cDNA of rat liver UGGT and expressed it in insect cells. The cDNA specifies an open reading frame which codes for a protein of 1527 residues including an 18 amino acid signal peptide. The protein has a C-terminal tetrapeptide (HEEL) characteristic of endoplasmic reticulum luminal proteins. The purified recombinant enzyme shows the same preference for unfolded polypeptides with N -linked Man(9)GlcNAc(2)glycans as the enzyme purified from rat liver. A genetically engineered Saccharomyces cerevisiae strain capable of producing glyco-proteins with Man(9)GlcNAc(2)core oligosaccharides was constructed and secreted acid phosphatase (G0-AcP) was purified. G0-AcP was used as an acceptor glycoprotein for UGGT and found to be a better substrate than the previously used soybean agglutinin and thyroglobulin. Recombinant rat UGGT has a K (m) of 44 microM for UDP-glucose. A proteolytic fragment of UGGT was found to retain enzymatic activity thus localizing the catalytic site of the enzyme to the C-terminal 37 kDa of the protein. Using site-directed mutagenesis and photoaffinity labeling, we have identified residues D1334, D1336, Q1429, and N1433 to be necessary for the catalytic activity of the enzyme.