Two Paralogous Gb3/Cd77 Synthases In Birds Show Different Preferences For Their Glycoprotein And Glycosphingolipid Substrates

Most glycosyltransferases show remarkable gross and fine substrate specificity, which is reflected in the old one enzyme-one linkage paradigm. While human Gb3/CD77 synthase is a glycosyltransferase that synthesizes the Gal alpha 1 -> 4Gal moiety mainly on glycosphingolipids, its pigeon homolog prefers glycoproteins as acceptors. In this study, we characterized two Gb3/CD77 synthase paralogs found in pigeons (Columba livia). We evaluated their specificities in transfected human teratocarcinoma 2102Ep cells by flow cytofluorometry, Western blotting, high-performance thin-layer chromatography, mass spectrometry and metabolic labelling with C-14-galactose. We found that the previously described pigeon Gb3/CD77 synthase (called P) can use predominately glycoproteins as acceptors, while its paralog (called M), which we serendipitously discovered while conducting this study, efficiently synthesizes Gal alpha 1 -> 4Gal caps on both glycoproteins and glycosphingolipids. These two paralogs may underlie the difference in expression profiles of Gal alpha 1 -> 4Gal-terminated glycoconjugates between neoavians and mammals.