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. Highly expressed in fetal brain, lung and kidney and in adult heart, spleen and thymus. Present to a lesser extent in adult brain, placenta, lung, large and small intestine and peripheral blood leukocytes. Belongs to the glycosyltransferase 29 family. ST8Sia IV