A developmentally regulated member of the sialyltransferase family (ST8Sia II, STX) is a polysialic acid synthase.

We found polysialic acid synthase activity of ST8Sia II (STX) in vitro and in vivo. Previously, we showed that mouse ST8Sia II exhibits alpha 2,3-sialylated N-glycan alpha 2,8-sialyltransferase activity, but the polysialic acid synthase activity of ST8Sia II was not detected at that time [Kojima, N. et al. (1995) FEBS Lett. 360, 1-4]. When fetuin was [14C]sialylated with ST8Sia II and then its N-linked oligosaccharides were analyzed, a part of the N-linked oligosaccharides was eluted in the void volume from a Sephadex G-50 column, and was eluted in the void volume from a Sephadex G-50 column, and was eluted from the DEAE-Toyopearl column at almost the same salt concentration as that where colomic acid was eluted. In addition, a series of 14C-labeled oligo-sialic acids were obtained from the oligosaccharides on partial mild acid hydrolysis. These results indicated that a part of N-linked oligosaccharides of fetuin were polysialylated with ST8Sia II. Transfection of ST8Sia II gene into several cell lines including NIH3T3 led to the expression of polysialic acids on the cell surface. Thus, ST8Sia II can directly synthesize polysialic acid chains on alpha 2,3-sialylated N-linked oligosaccharides of glycoproteins without any initiator sialytransferase.