The UDP-glucose:Glycoprotein Glucosyltransferase Is Organized in at Least Two Tightly Bound Domains from Yeast to Mammals

The endoplasmic reticulum UDP-Glc:glycoprotein glucosyltransferase (GT) exclusively glucosylates nonnative glycoprotein conformers. GT sequence analysis suggests that it is composed of at least two domains: the N-terminal domain, which composes 80% of the molecule, has no significant similarity to other known proteins and was proposed to be involved in the recognition of non-native conformers and the C-terminal or catalytic domain, which displays a similar size and significant similarity to members of glycosyltransferase family 8. Here, we show that N- and C-terminal domains from Rattus norvegicus and Schizosaccharomyces pombe GTs remained tightly but not covalently bound upon a mild proteolytic treatment and could not be separated without loss of enzymatic activity. The notion of a two-domain protein was reinforced by the synthesis of an active enzyme upon transfection of S. pombe GT null mutants with two expression vectors, each of them encoding one of both domains. Transfection with the C-terminal domain-encoding vector alone yielded an inactive, rapidly degraded protein, thus indicating that the N-terminal domain is required for proper folding of the C-terminal catalytic portion. If, indeed, the N-terminal domain is, as proposed, also involved in glycoprotein conformation recognition, the tight association between N- and C-terminal domains may explain why only N-glycans in close proximity to protein structural perturbations are glucosylated by the enzyme. Although S. pombe and Drosophila melanogaster GT N-terminal domains display an extremely poor similarity (16.3%), chimeras containing either yeast N-terminal and fly C-terminal domains or the inverse construction were enzymatically and functionally active in vivo, thus indicating that the N-terminal domains of both GTs shared three-dimensional features.