Molecular cloning and expression of a sixth type of α2,8-sialyltransferase (ST8Sia VI) that sialylates O-glycans

Abstract

A novel member of the mouse α2,8-sialyltransferase (ST8Sia) family, designated ST8Sia VI, was identified by BLAST analysis of expressed sequence tags. The sequence of ST8Sia VI encodes a protein of 398 amino acids and shows 42.0 and 38.3% amino acid sequence identities to mouse α2,8-sialyltransferases ST8Sia I (GD3 synthase) and ST8Sia V (GD1c, GT1a, GQ1b, and GT3 synthases), respectively. The recombinant soluble form of ST8Sia VI expressed in COS-7 cells exhibited α2,8-sialyltransferase activity toward both glycolipids and glycoproteins that have the NeuAcα2,3(6)Gal sequence at the nonreducing end of their carbohydrate groups. This enzyme formed NeuAcα2,8NeuAc structures, but not oligosialic or polysialic acid structures. Analysis of the fetuin sialylated by ST8Sia VI indicated that ST8Sia VI prefers O-glycans to N-glycans as acceptor substrates. Substrate specificities and kinetic properties also showed that ST8Sia VI prefers O-glycans to glycolipids as acceptor substrates. ST8Sia VI also exhibited activity toward oligosaccharides such as sialyllactose and sialyllactosamine, and the structure of the minimal acceptor substrate for ST8Sia VI was determined as the NeuAcα2,3(6)Gal sequence. The expression of the ST8Sia VI gene was ubiquitous, and the highest expression was observed in kidney, with three major transcripts of 8.2, 3.8, and 2.7 kb. This is the first report of a mammalian α2,8-sialyltransferase that sialylates O-glycans preferentially.