Redefined Substrate Specificity of ST6GalNAc II: A Second Candidate Sialyl-Tn Synthase

The acceptor substrate specificities of ST6GalNAc I and II, which act on the synthesis of O-linked oligosaccharides, were reexamined using ovine submaxillary mucin, [Ala-Thr(GalNAc)-Ala]n polymer (n = 7–11). It has been suggested that only ST6GalNAc I can synthesize carbohydrate structures of sialyl-Tn-antigen; i.e., NeuAcα2-6GalNAc-O-Thr/Ser [Kurosawa et al., J. Biol. Chem. 269, 19048–19053 (1994)] based on the result that ST6GalNAc I, not ST6GalNAc II, exhibited activity toward asialoagalacto-fetuin. In this study, we present evidence that both ST6GalNAc I and II exhibit activity toward asialo-OSM (ovine submaxillary mucin) and [Ala-Thr(GalNAc)-Ala]n polymer (n = 7–11) which have only the GalNAc-O-Thr/Ser-structures. These results strongly indicate that not only ST6GalNAc I but also II are candidates for sialyl-Tn synthases.