Cloning and expression of Galβ1,3GalNAc-specific GalNAc α2,6-sialyltransferase

A cDNA clone encoding a new type of GalNAc α2,6-sialyltransferase (ST6GalNAc II) with a structure similar to that of a previously cloned GalNAc α2,6-sialyl-transferase (ST6GalNAc I; Kurosawa, N., Hamamoto, T., Lee, Y.-C., Nakaoka, T., Kojima, N., and Tsuji, S. (1994) J. Biol. Chem. 269, 1402-1409) was obtained from chicken testes. The predicted amino acid sequence of ST6GalNAc II encodes a protein with type II transmembrane topology, as found for other glycosyltransferases, and showed 32% identity with that of ST6GalNAc I. Transfection of the full length ST6GalNAc II gene into COS cells led to GalNAc α2,6-sialyltransferase activity with a different substrate specificity from that of ST6GalNAc I. More-over, asialofetuin after treatment with β-galactosidase did not serve as an acceptor for this enzyme. ¹⁴C-Sialylated oligosaccharides obtained from resialylated asialobovine submaxillary mucin with this enzyme were identical to Galβ1,3([¹⁴C]NeuAcα2,6)GalNAc-ol but not [¹⁴C]NeuAcα2,6GalNAc-ol. These results clearly show that the expressed enzyme is a novel type of sialyltransferase that requires β-galactoside residues linked to GalNAc residues, whereas sialic acid residues linked to galactose residues are not essential for the activity.