Catalyzes the polycondensation of alpha-2,8-linked sialic acid required for the synthesis of polysialic acid (PSA), which is present on the embryonic neural cell adhesion molecule (N-CAM), necessary for plasticity of neural cells. Protein modification; protein glycosylation. Strongly expressed in lung, heart and spleen and weakly in brain. Expressed throughout development at a low level. Belongs to the glycosyltransferase 29 family. ST8Sia IV