CLONING AND EXPRESSION OF GAL-BETA-1,3GALNAC-SPECIFIC GALNAC ALPHA-2,6-SIALYLTRANSFERASE

A cDNA clone encoding a new type of GalNAc alpha 2,6-sialyltransferase (ST6GalNAc II) with a structure similar to that of a previously clone d GalNAc alpha 2,6-sialyltransferase (ST6GalNAc I; Kurosawa, N., Hamam oto, T., Lee, Y.-C., Nakaoka, T., Kojima, N., and Tsuji, S. (1994) J. Biol. Chem. 269, 1402-1409) was obtained from chicken testes. The pred icted amino acid sequence of ST6GalNAc II encodes a protein with type II transmembrane topology, as found for other glycosyltransferases, an d showed 32% identity with that of ST6GalNAc I. Transfection of the fu ll length ST6GalNAc II gene into COS cells led to GalNAc alpha 2,6-sia lyltransferase activity with a different substrate specificity from th at of ST6GalNAc I. Moreover, asialofetuin after treatment with P-galac tosidase did not serve as an acceptor for this enzyme. C-14-Sialylated oligosaccharides obtained from resialylated asialobovine submaxillary mucin with this enzyme were identical to Gal beta 1,3([C-14]NeuAc alp ha 2,6)GalNAc-ol but not [C-14]NeuAc alpha 2,6GalNAc-ol. These results clearly show that the expressed enzyme is a novel type of sialyltrans ferase that requires beta-galactoside residues linked to GalNAc residu es, whereas sialic acid residues linked to galactose residues are not essential for the activity.