We previously showed that mouse ST8Sia II (STX) exhibits polysialic acid (PSA) synthase activity in vivo as well as in vitro (Kojima, N., Yoshida, Y., and Tsuji, S. (1995) FEBS Lett. 373, 119-122, 1995). In this paper, we reported that the neural cell adhesion molecule (NCAM) was specifically polysialylated by a single enzyme, ST8Sia II. PSA-expressing Neuro2a cells (N2a-STX) were established by stable transfection of the mouse ST8Sia II gene. Only the 140- and 180-kDa isoforms of NCAM in N2a-STX cells were specifically polysialylated in vivo, although other membrane proteins of N2a-STX were polysialylated in vitro. A recombinant soluble mouse ST8Sia II synthesized PSA on a recombinant soluble NCAM fused with the Fc region of human IgG1 (NCAM-Fc) as well as fetuin. However, NCAM-Fc served as a 1500-fold better acceptor for ST8Sia II than fetuin. Treatment of NCAM-Fc with Charonia lampas alpha-fucosidase, which is able to cleave alpha1,6-linked fucose, clearly reduced the polysialylation of NCAM-Fc by ST8Sia II. PSA was not synthesized on the N-glycanase-treated NCAM-Fc polypeptide or the free N-glycans of NCAM-Fc. When fetuin and its glycopeptide and N-glycans of fetuin were used as substrates for ST8Sia II, PSA was found to be synthesized on native fetuin and its glycopeptide but not on free N-glycans. These results strongly suggested that core alpha1, 6-fucose on N-glycans as well as the antennary structures of N-glycans and the polypeptide regions are required for the polysialylation by ST8Sia II. Furthermore, oligo and single alpha2, 8-sialylated glycoproteins were no longer polysialylated by mouse ST8Sia II. Therefore, the single enzyme, ST8Sia II, directly transferred all alpha2,8-sialic acid residues on the alpha2,3-linked sialic acids of N-glycans of specific NCAM isoforms to yield PSA-NCAM. Polysialylation did not require any initiator alpha2, 8-sialyltransferase but did depend on the carbohydrate and protein structures of NCAM.